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Chromosome abberations in Galtonia Candicans Murphy, Mary 1943

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L £ 4 H 5=- n % c ^ CHROMOSOME ABERRATIONS I F GALTONIA QAMPI CANS Mary Murphy A T h e s i s s u b m i t t e d f o r the Degree o f MASTER OF ARTS i n the Department of BOTANY THE UNIVERSITY OF BRITISH COLUMBIA May, 1943-Table o f Contents. Acknowle dgenient s page I n t r o d u c t i o n 1 M a t e r i a l s ^ and Methods 3 O b s e r v a t i o n s 7 M e i o t i c chromosomes i n a n t h e r s 7 . Norma. 1 chromosome b e h a v i o u r 9 Metapha.se- 9 Anaphase and Telophs.se 11 Prophase 12 N u c l e o l i ^ . 1 2 A b e r r a t i o n s A b e r r a t i o n s i n r o o t t i p s F ragmentation A b e r r a t i o n s i n o v u l e s P a i r i n g T e r m i n a l f u s i o n s Chromatid f r a g m e n t a t i o n s * f o r m a t i o n . Polysomaty Review of L i t e r a t u r e and D i s c u s s i o n 22 H i s t o r i c a l 22 Chromosome s t r u c t u r e and s p i r a l s 25 N u c l e o l i • 26, Fragmentations and s t r u c t u r a l h y b r i d i t y 28 Chromosome p a i r i n g 31 T r a n s l o c a t i o n s and f u s i o n s 33 Somatic b r i d g e s -( " 34 Chromatid f r a g m e n t a t i o n 37 Diplochromosomes and polysomaty 37 12 13 13 15 and l o o p 18 19 C o n c l u s i o n s and Suggest i o n s f o r F u r t h e r Research 41 Summary E x p l a n a t i o n o f P l a t e s L i t e r a t u r e c i t e d P l a t e I P l a t e I I Page 43 44 46 51 52 Acknowledgements The w r i t e r w ishes,to thank Dr. A.H.Hutchinson, who suggested t h i s i n v e s t i g a t i o n and who gave i n v a l u a b l e a s s i s t a n c e I n the p r e p a r a t i o n of the manuscript and i n the photography. Thanks are due a l s o to Miss L o i s M. S t i l l , who prepared the anther s l i d e s , and to Mr. E. A. Schwahtje, c u r a t o r of the B o t a n i c a l Gardens of the U n i v e r s i t y of B r i t i s h Columbia, who s u p p l i e d the m a t e r i a l used i n t h i s i n v e s t i g a t i o n . CHROMOSOME ABERRATIONS IN GALTONIA CANDICANS I n t r o d u c t i o n The study of a b e r r a t i o n s i n chromosome s t r u c t u r e and behaviour has become an important f o u n d a t i o n f o r the understanding of the progress of e v o l u t i o n . S t r u c t u r a l h y b r i d i t y r e s u l t i n g from .the f r a g m e n t a t i o n of a s i n g l e chromosome to form two, or the t e r m i n a l f u s i o n of two chromosomes w i t h the accompanying l o s s of the centromere of one, may cause c o n s i d e r a b l e v a r i a t i o n I n the progeny. Each s p e c i e s i s c h a r a c t e r i z e d by i t s own genotype, and the gene- complement changes i n the course of e v o l u t i o may be caused by the i n h e r e n t p r o p e r t i e s of the genes themselves, or by t h e i r r e a c t i o n s to the environment. In c o n j u n c t i o n w i t h such genie or g e n o t y p i c changes, chromosomes are s u b j e c t t o another kind of changes d e s c r i b e d as s t r u c t u r a l or n u m e r i c a l a c c o r d i n g t o whether i t i n v o l v e s the s t r u c t u r e of the chromosomes or t h e i r number. In r e c e n t years a l a r g e volume of work has. been done on s t r u c t u r a l and n u m e r i c a l changes i n the chromosome complements of both p l a n t s and a n i m a l s . T h i s i n c l u d e s both s t u d i e s of changes o c c u r r i n g i n nature and those - 2 -Induced by ; . a r t i f i c i a l means. Induced a b e r r a t i o n s have been brought about by treatment w i t h X-rays, u l t r a - v i o l e t r a d i a t i o n , high or low temperatures, c e r t a i n drugs such as c o l c h i c i n e , and by a r t i f i c i a l h y b r i d i z a t i o n . A d i s t i n c t s e p a r a t i o n cannot be made between a r t i f i c i a l l y induced a b e r r a t i o n s and n a t u r a l l y o c c u r r i n g a b e r r a t i o n s because s e v e r a l f a c t o r s used i n e x p e r i m e n t a l work a l s o occur In, n a t u r e , namely - v a r i a t i o n s i n temperature and u l t r a - v i o l e t l i g h t . Age has a l s o been found to produce a l t e r a t i o n s i n chromosome s t r u c t u r e s . Moreover, induced mutations have the same phenotyplc e x p r e s s i o n s and chromosomal a b e r r a t i o n s as are found i n " n a t u r a l l y o c c u r r i n g " mutations. That I s , extreme c o n d i t i o n s change the time r a t e of mutation r a t h e r than a c t i n g as an I n i t i a l cause of m utation, The types of changes i n the chromosome complements found by r e c e n t workers i n c l u d e p o l y p l o i d y , polysomaty, d o u b l i n g or " p a i r i n g " of chromosomes, the f o r m a t i o n of l o o p s , f r a g m e n t a t i o n , and c a t e n a t i o n s . The purpose of t h i s paper i s to d e s c r i b e the chromosomal a b e r r a t i o n s found i n u n t r e a t e d p l a n t s of G a l t o n i a candicans, and t o c o r r e l a t e these, i f p o s s i b l e , w i t h the f i n d i n g s of o t h e r I n v e s t i g a t o r s . - 3 -M a t e r i a l s and Methods Galtonia- candicans, Decne. i s a n a t i v e of South A f r i c a , b e l o n g i n g to the f a m i l y L l l i a c e a e . The m a t e r i a l used f o r -this t h e s i s was c o l l e c t e d from p l a n t s growing i n the B o t a n i c a l Gardens a t the U n i v e r s i t y of B r i t i s h Columbia. Ovules of v a r i o u s stages were c o l l e c t e d from s e v e r a l j d a n t s . No attempt was made to separate and i d e n t i f y the ovules as coming from d i f f e r e n t p l a n t s as the r e were no m o r p h o l o g i c a l d i f f e r e n c e s n o t i c e d m the p l a n t s . C o l l e c t i o n s were made on J u l y 14th 1941, and at about the same date In 1942. A l l c o l l e c t i o n s were made between noon and thr e e o ' c l o c k . The ovules w e r e ' f i x e d I n B.C. f i x i n g s o l u t i o n ( 95 per cent a l c o h o l - 250 c c s . , g l a c i a l a c e t i c a c i d -5 c c s . , f o r m a l i n - 12 c c s . , water - 50 c c s . ) Thi,s i s a s u c c e s s f u l f i x a t i v e because the t i s s u e s may be l e f t i n I t almost i n d e f i n i t e l y without caus i n g excess d e h y d r a t i o n or mal f o r m a t i o n . The u s u a l d e h y d r a t i n g and embedding procedures were f o l l o w e d u s i n g methyl a l c o h o l and x y l o l as the c l e a r i n g agent. The m a t e r i a l was embedded In p a r a f f i n and s e c t i o n s were cut a t about 12 microns. ' - 4 -Egg albumen was used as an adhesive when the s e c t i o n s were.to be t r e a t e d w i t h Eeulgen's technique because I t was found t h a t the s e c t i o n s had l e s s tendency -to f l o a t o f f the s l i d e s d u r i n g the hydroiysing£ pr o c e s s than when the gum a r a b l e - potassium dichromate adhesive was used. However t h i s l a t t e r adhesive was found to be more s a t i s f a c t o r y than egg albumen f o r the other s t a i n i n g techniques used. S e v e r a l s t a i n combinations were t r i e d . F i r s t l y , Elemming's t r i p l e s t a i n ( s a f r a n i n i n 50 per cent a l c o h o l , g e n t i a n v i o l e t aqueous, orange G aqueous ) p l u s l i g h t green i n c l o v e o i l - t h i s combination s t a i n s the chromosomes p u r p l e , the n u c l e o l u s b r i g h t r e d , and the s p i n d l e f i b e r s green. I t i s not ve r y s a t i s f a c t o r y f o r d e t a i l e d work. The second s t a i n was iron-alum haematoxyiin w i t h or w i t h o u t a counter s t a i n of l i g h t green. The . chromosomes are s t a i n e d dark blue or b l a c k . The d i f f i c u l t y w i t h t h i s s t a i n i s t h a t i t s t a i n e the n u c l e o l u s the same as the chromosomes, and a l s o u n l e s s i t i s d e s t a i n e d e x t e n s i v e l y no d e t a i l s of the I n t e r n a l s t r u c t u r e of the chromosome can be seen. The t h i r d technique used was the Eeulgen method ( f u c h s i r : sulphurous a c i d ) f o l l o w e d by a l i g h t green n u c l e o l a r counter s t a i n . I t was the - 5 -most s a t i s f a c t o r y combination used, as i t d i f f e r e n t i a t e d the n u c l e o l a r m a t e r i a l from the chromatin and was best f o r showing the i n t e r n a l s t r u c t u r e of the chromosomes. The .root t i p s were o b t a i n e d by g e r m i n a t i n g G a l t o m a seeds which were c o l l e c t e d on October 1st 1941. Some of these seeds were incubated, i n p e t r i d i s h e s between moist p i e c e s of f i l t e r paper a t room temperature the week f o l l o w i n g the c o l l e c t i o n . These were f i x e d i n B.C. f i x i n g s o l u t i o n , embedded i n p a r a f f i n , s e c t i o n e d at 10 microns, and s t a i n e d by the Feulgen method. Other seeds c o l l e c t e d on the same day were incubated on March 12th 1943. These seeds were p l a c e d In constant temperature chambers a t 22 ;° C. and 29° ^. Root t i p s s u f f i c i e n t l y l o n g f o r the experiment were produced i n from s i x to t e n days and g e r m i n a t i o n was almost 100 per c e n t . These roo t t i p s were t r e a t e d f i r s t w i t h La Cour's f i x i n g s o l u t i o n ( a b s o l u t e a l c o h o l and g l a c i a l a c e t i c a c i d ) f o r about f i v e minutes and were then t r a n s f e r r e d t o B.C. f i x i n g s o l u t i o n . The Feulgen's squash technique f o r r o o t t i p s as d e s c r i b e d by H i l l a r y (1940) was used. S l i d e s of the anthers p r e p a r e d by Mi s s L o i s M. S t i l l f o r the Department of Botany were used f o r the m e i o t i c - 6 -d i v i s i o n s . These s l i d e s were s t a i n e d w i t h iron-alum haematoxyiln. • Observations M e i o t i c Chromosomes i n Anthers The stages of meiosis were examined c a r e f u l l y , but s i n c e the f i n d i n g s were s i m i l a r to those of Smith (1932), they w i l l not be repeated here. However, s i n c e the i n d i v i d u a l i t y of the chromosome p a i r s i s apparent a t d i a k i n e s i s these w i l l be recorded. U n f o r t u n a t e l y , s i n c e these drawings were made from s e c t i o n s and not smears, not a l l p a i r s were from one c e l l and I t i s p o s s i b l e t h a t they are not a l l i n the same stage of c o n t r a c t i o n . The separate p a i r s have been numbered f o r convenience ( F i g . 1) r a n g i n g from l o n g e s t to s h o r t e s t . The f i r s t p a i r are connected a t both ends and o f t e n show the t w i s t i l l u s t r a t e d ; t h i s dees not seem to be a chiasma however because the chromosomes are sometimes . found forming a c i r c l e l i k e the second p a i r ( F i g . 2 ) . Th i s i s the o n l y p a i r i n which the s p i n d l e attachment c o u l d be c l e a r l y seen. The second p a i r i s about the same l e n g t h as the f i r s t , the chromosomes are always arranged to form" an 0. The t h i r d p a i r Is s l i g h t l y s h o r t e r and has one t e r m i n a l chiasma, the chromosomes form the arms of a V. The f o u r t h p a i r I s about the same l e n g t h as the t h i r d , the chromosomes are closely-a s s o c i a t e d f o r about one t h i r d of t h e i r l e n g t h a t one end and then d i v e r g e , the o t h e r ends u s u a l l y approximate ag a i n to'fortn a "wish-bone". The f i f t h p a i r I s the - ; s a t e l l i t e p a i r , these chromosomes are r o d - l i k e , about h a l f the l e n g t h of the long chromosomes, and are u s u a l l y a t t a c h e d to the n u c l e o l u s . These chromosomes themselves are seldom j o i n e d , they may l i e f a r t h e r a p a r t or c l o s e r t o g e t h e r on the n u c l e o l u s than those shown. The homologous chromosomes of the s m a l l e r p a i r s are not i n c o n t a c t a t d i a k i n e s i s but are approximated a t r e g u l a r d i s t a n c e s . The s i x t h p a i r I s s i m i l a r i n s i z e and l o o k s l i k e two s h o r t rods. The chromosomes of the seventh p a i r are o v o i d and those of the e i g h t h p a i r are s p h e r i c a l . No d e t a i l s of i n t e r n a l s t r u c t u r e c o u l d be seen because of the v e r y dense haematoxyim s t a i n , even the. chromatids of the b i v a l e n t s were I n d i s t i n g u i s h a b l e i n most of the chromosomes. Digby (1910) and M o t t i e r (1907) both r e p o r t e d c a t e n a t i o n s or chains forming a t d i a k i n e s i s , but n o t h i n g of t h i s s o r t was" found i n t h e . m a t e r i a l used f o r t h i s paper. The o n l y arrangement which might have been mistaken f o r c a t e n a t i o n i s shown i n Figurer.2. This i s i I ! - 9 -• the f i r s t p a i r of chromosomes forming a 'wide r i n g . | The ot h e r chromosome p a i r , on the r i g h t s i d e , i s not i r connected t o the f i r s t p a i r but l i e s j u s t above i t . The f a c t t h a t the chromosomes are s i t u a t e d around the edge of the nucleus may malce them appear to be j o i n e d end to end. Smith (1932) a l s o f a i l e d to f i n d any c a t e n a t i o n s . Before s t u d y i n g the unusual m i t o t i c chromosome behaviour i n t h i s ' form, the normal occurrences s h o u l d be understood. Most of the r o o t t i p c e l l s show t y p i c a l m i t o s i s stages and these'stages w i l l be d i s c u s s e d f i r s t . Normal Chromosome Behaviour Metaphase: A p o l a r view of the metaphase p l a t e shows 16 . d i s t i n c t chromosomes. ( F i g . 3) Four p a i r of these are l o n g , two p a i r medium, and two. p a i r s h o r t . These can be I d e n t i f i e d a g a i n a t d i a k i n e s i s . Since some of the chromosomes are l o n g , they cannot l i e c o m p l e t e l y on the metaphase p l a t e , and the consequent c u r l i n g -;.makes i t v e r y d i f f i c u l t to measure them. In F i g u r e 3, s e v e r a l of the chromosomes t h a t appear to be of medium • l e n g t h are i n r e a l i t y long chromosomes c u r l e d up. Comparing a l a t e r a l view of the metaphase p l a t e ( F i g . 5) - 10 -w i t h the p o l a r view g i v e s a b e t t e r p i c t u r e of the comparative l e n g t h of the chromosomes. A l l the chromosomes have s u b - t e r m i n a l s p i n d l e attachments, and the l o n g chromosomes l i e w i t h t h e i r attachment p o i n t s towards the middle of the s p i n d l e . The s m a l l chromosomes are l o c a t e d c e n t r a l l y on the p l a t e . T h e i r i s no a s s o c i a t i o n of homologous or even s i m i l a r p a i r s of chromosomes a t any stage i n the r o o t t i p m i t o s i s , the 'chromosomes are d i s t r i b u t e d q u i t e a t random. One p a i r of medium s i z e d chromosomes has s m a l l t e r m i n a l l y a t t a c h e d s a t e l l i t e s . These s a t e l l i t e s may be seen best i n the prophases ( F i g . 8) because when the chromosomes are c o n t r a c t e d a t metaphase the s a t e l l i t e i s i n con t a c t w i t h the body of the chromosome and b e c o m e s ' i n d i s t i n g u i s h a b l e . At metaphase the chromosomes are s p l i t i n t o two chromatids but the s p i n d l e attachment i s s t i l l s i n g l e . The chromatids may be somewhat r e l a t l o n a l l y t w i s t e d a t t h i s phase, but the c o l l i n g i s not e x t e n s i v e . Those chromosomes i n E i g u r e 1 ?;hich do not show the d i v i s i o n are p r o b a b l y l y i n g so t h a t the s p i l t -is 'in the plane of the equator and would t h e r e f o r e not show m a p o l a r view. - 11 -Anaphase and t e l o p h a s e : Not a l l of the chromosomes s p l i t a t ' t h e same time, i n F i g u r e 5, the chromosome on the l e f t has s p l i t c o mpletely but has not separated whereas the upper one on the r i g h t s i d e has separated a c o n s i d e r a b l e d i s t a n c e . The s h o r t arm of the upper one of the two daughter chromosomes i s . s l i g h t l y e n l a r g e d and appears to be double. T h i s might i n d i c a t e t h a t the next s p l i t has taken p l a c e or i s t a k i n g p l a c e . No i n t e r n a l c o i l s c o u l d be seen i n the daughter chromosomes, and no f u r t h e r evidence of the next s p l i t c o u l d be found. In l a t e r anaphase ( F i g . 6) the chromosomes show a s t r u c t u r e which might be i n t e r p r e t e d e i t h e r as two r e l a t i o n a l l y c o l l e d s p i r a l s or as two Independent s p i r a l s l y i n g ' s i d e by s i d e . Since the telophase (Fig.7) shows d e f i n i t e l y i n t e r l o c k i n g s p i r a l s , i t i s l i k e l y t h a t the anaphase s t r u c t u r e i s s i m i l a r . . Evidence has been found t h a t the o r g a n i z a t i o n of the n u c l e o l u s commences before the end of the anax^hase. One anaphase, a l i t t l e l a t e r than the stage of F i g u r e 6, showed, when s t a i n e d w i t h Flemming's t r i p l e s t a i n , a s m a l l v a c u o l e - l i k e body, g i v i n g a decided n u c l e o l a r r e a c t i o n , a s s o c i a t e d w i t h one of the chromosomes. I t - 12 -was not p o s s i b l e t o see the chromosome to which t h i s was a t t a c h e d because of the c l o s e a s s o c i a t i o n of the chromosomes a t t h i s stage. Prophase: The e a r l y prophases i n the root t i p s show two d e f i n i t e s t r a n d s or chromonemata which u s u a l l y have a r e l i c c o l l . T h i s c o i l s t r a i g h t e n s out i n the middle prophase and the s t r a n d s l i e p a r a l l e l f o r most of t h e i r l e n g t h . ( F i g . 8) The chromonemata then shorten and t h i c k e n and a m a t r i x becomes e v i d e n t around each one. This dense m a t r i x obscures the i n t e r n a l s t r u c t u r e of the chromatids and may make the s p l i t l e s s apparent. N u c l e o l i : I n the r e s t i n g stages and In the e a r l y prophases t h e r e i s e i t h e r u s u a l l y one l a r g e .nucleolus, or o c c a s i o n a l l y two s m a l l n u c l e o l i . The l a r g e n u c l e o l u s I s u s u a l l y about t w i c e the s i z e of the s m a l l ones and p r o b a b l y r e s u l t s from a f u s i o n of the l a t t e r . A b e r r a t i o n s The a b e r r a t i o n s found i n the somatic c e l l s were of many types I n c l u d i n g f r a g m e n t a t i o n , p a i r i n g , f u s i o n s V - 13 -and' somatic b r i d g e s , chromatid fragmentations and loop f o r m a t i o n s , and polysomaty. A b e r r a t i o n s i n root t i p s - fr a g m e n t a t i o n : A b e r r a t i o n s of any k i n d were very uncommon i n the ro o t t i p s . Lagging chromosomes were found q u i t e f r e q u e n t l y ( F i g . 7) but these are probably simply the ends of some of the slower moving lon g chromosomes. The o n l y example of a d e f i n i t e a b n o r m a l i t y o c c u r r i n g i n the r o o t t i p s i s t h a t shown i n F i g u r e 9-. T h i s shows a stage between the anaphase and e a r l y t e l o p h a s e w i t h a chromosome s t i l l on the s p i n d l e . T h i s chromosome or chromosome fragment has two chromatids which have not • .-, sepa r a t e d . This i s pro b a b l y one of the medium lengthed chromosomes, but i t has become s t r e t c h e d by the lengthen-i n g of the s p i n d l e . The chromatids are narrower than tho those a t metaphase, but are not s u f f i c i e n t l y ,• narrow to be regarded as chromonemata of the anaphase chromosome. T h i s chromosome appears to be e i t h e r a t e l o c e n t r i c or an a c e n t r i c fragment r e s u l t i n g from the m i s d i v i s i o n of the centromere. ( D a r l i n g t o n , 1940) A b e r r a t i o n s i n ovule c e l l s : P a i r i n g . Taking the t y p i c a l r o o t m i t o s i s as the normal m i t o t i c d i v i s i o n , the f i r s t d i f f e r e n c e n o t i c e d i n - 14 -the ovule c e l l s i s the s i d e by s i d e p a i r i n g of the chromosomes. In the prophase the s p l i t bet?feen the two chromonemata i s evident o n l y In p a r t , there I s no complete s e p a r a t i o n of these s t r u c t u r e s s i m i l a r to t h a t found i n the r o o t t i p s . F i g u r e 10 shows an e a r l y prophase w i t h p a i r s of chromosomes c o l l e d l o o s e l y around each o t h e r . I have c a l l e d these threads separate chromosomes because they are much f a r t h e r a p a r t than any chromonemata (Sound i n the ovule c e l l s , and because t h e r e were so few threads p r e s e n t (probably e i g h t p a i r s ) . I f each chromosome c o n s i s t e d of two chromonemata as w i d e l y s s e p a r a t e d as these are, one would expect the nucleus to show many more threads than I t does, because t h e r e would be t h i r t y - s i x of them p r e s e n t . F i g u r e 4 i s a p o l a r view of a, metaphase p l a t e , i t shows d e f i n i t e p a i r i n g of s i m i l a r , i f not homologous chromosomes. The s i x t e e n chromosomes can be seen q u i t e p l a i n l y , two of the s m a l l ones are l o c a t e d under a c u r l e d p a i r . There are two l o n g chromosomes t h a t are not p a i r e d , and t h i s same c o n d i t i o n has been found i n other c e l l s In t h i s ovule t i s s u e . The chromosomes themselves do not show a s p l i t as d e f i n i t e as do those of the r o o t t i p s , ' - 15 -however some of them are s p l i t i n the short arm. Another XJOlar view of a s i m i l a r stage ( F i g . 11)" shows chromosomes t h a t l o o k c h a r a c t e r i s t i c a l l y m e i o t i c r a t h e r than m i t o t i c . They are more c o n t r a c t e d than o r d i n a r y m i t o t i c chromosomes - (compare F i g . 3.) The presence of n u c l e o l i a t t h i s stage a l s o makes i t more l i k e the m e i o t i c prophase, as the n u c l e o l u s i s o f t e n s t i l l present a t d l a k i n e s i s . The p a i r i n g here, as i n . F i g u r e 4, i s not complete; but i n t h i s i n s t a n c e , one of the medium s i z e d p a i r s , the s a t e l l i t e chromosomes, I s s e p a r a t e . This would seem to i n d i c a t e t h a t the p a i r i n g i s not s t r i c t l y s y n a p t i c , but I s r a t h e r a more loose a s s o c i a t i o n of chromosomes. Te r m i n a l fusions.- Terminal f u s i o n s of chromosomes were q u i t e common i n t h i s m a t e r i a l and seemed to be of two t y p e s . The f i r s t type shows anaattraotl'on between the ends of homologous chromosomes. The' a t t r a c t i o n may be between the ends of the lon g arms or the s h o r t arms but not both as r i n g f o r m a t i o n s were not found. F i g u r e 12 shows t e r m i n a l fusions- of chromosomes i n the prophase. F i g u r e 13 I l l u s t r a t e s the f u s i o n of the ends of the arms of two p a i r s of l o n g chromosomes. In F i g u r e 14 t h e r e appears to - 16 -be a j o i n i n g of two chromosomes i n the r e g i o n of the s p i n d l e attachment. Two rod chromosomes appear to be a t t a c h e d to a s i n g l e s p i n d l e attachment, forming a long chromosomeswith a median s p i n d l e attachment. T h i s . c o u l d not be v e r i f i e d because no fragments were seen and no M chromosomes were found i n any other anaphase. Th i s type of chromosome c o u l d have a r i s e n by the l o s s of the sh o r t arm on one chromosome and the sh o r t arm and centromere on the oth e r and a f u s i o n of the broken ends, i n other words, by a m i s d i v i s i o n of two chromosomes f o l l o w e d by the f o r m a t i o n of isochromosomes. ( D a r l i n g t o n , 1940.) The second type of t e r m i n a l f u s i o n s i n v o l v e s the s t i c k i n g t o g e t h e r of the chromosome sheaths or p e l l i c l e s . The chromosomes have become more v i s c o u s and tend to l o s e t h e i r c h a r a c t e r i s t i c shape when s u b j e c t e d t o the f o r c e s of the s p i n d l e . F i g u r e 15, a l a t e prophase, shows s e v e r a l chromosomes a t t a c h e d by processes extending from chromosome t o chromosome. The attachment i n t h i s case i s not n e c e s s a r i l y t e r m i n a l and does not appear to be an a s s o c i a t i o n of homologous chromatic u n i t s . The chromosomes here are c o n t r a c t e d to a g r e a t e r extent than i n o r d i n a r y m i t o t i c chromosomes. Some of these chromosomes may be - 1 7 -p a i r e d p a r a s y n a p t i c a l l y as w e l l as the obvious t e r m i n a l a s s o c i a t i o n ; the f a c t t h a t one chromosome a t the lower end of the group seems to be s i t u a t e d between two chromatids or chromosomes would I n d i c a t e t h a t the p a i r e d r e l a t i o n r e s u l t s from an approximation of chromosomes and not as a r e s u l t of the s e p a r a t i o n of chromonemata. F i g u r e 15 I s a . s i m i l a r stage a l s o showing s e v e r a l chromosomes a t t a c h e d to each other more or l e s s at random. The l e f t s i d e of t h i s group looks aaimost as though i t had been s e m i - f l u i d and the chromosomes had flowed t o g e t h e r . A metaphase i l l u s t r a t i n g t h i s v i s c o s i t y and the tendency of the chromosomes to adhere to each other i s shown i n F i g u r e 17. The chromosomes have become ve r y much lumped t o g e t h e r and non-homologous ones have become a t t a c h e d the one to the other a t the ends. The anaphases r e s u l t i n g from the s e p a r a t i o n of these chromosomes have i r r e g u l a r chromosome b r i d g e s . F i g u r e 18 p i c t u r e s an anaphase bef o r e the s p i n d l e has expanded f u l l y , the chromosomes are i r r e g u l a r but not g r e a t l y s t r e t c h e d . The j o i n i n g of the ends of non-homologous chromosomes may be seen v e r y d i s t i n c t l y here. A l a t e r anaphase, a f t e r the s p i n d l e has elongated ( F i g . 19) shows l o n g chromatin - 18 -br i d g e s which extend from one chromosome group t o the o t h e r . Very l i t t l e I n t e r n a l d e t a i l can be seen i n these chromosomes except t h a t they are double.;' A l t e r n a t e l y , I t i s p o s s i b l e t h a t t h e r e i s a f a i l u r e of s e p a r a t i o n of the chromatids . i n these n u c l e i , r e s u l t i n g i n an i r r e g u l a r s e g r e g a t i o n of genes. This would e x p l a i n the doubleness i n the anaphase chromosomes and. the appearance of so few chromosomes i n these d i v i s i o n s . The l o n g t hmgchromatin b r i d g e s are pr o b a b l y caused by the u n c o i l i n g of the chromonemata i n the anaphase chromosomes, when the s p i n d l e i s s t r e t c h e d . Chromatid fragmentations_and loop f o r m a t i o n s . Only one d e f i n i t e example of a loop f o r m a t i o n caused by a break In a s i n g l e chromatid was found. This ( F i g . 20) i s a chromosome from the nucleus shown i n f i g u r e 12.. The chromosome i s l o c a t e d underneath the n u c l e o l u s and seems to c i r c l e the base of t h i s s t r u c t u r e . I t was i m p o s s i b l e to be p o s i t i v e whether or not t h i s chromosome was a c t u a l l y a t t a c h e d t o the nucleolus;* A chromosome w i t h a s i m i l a r t e r m i n a l loop was found i n another nucleus, and t h i s second one was found t o be q u i t e separate from the n u c l e o l u s . IDf t h i s chromosome I s the second s a t e l l i t e chromosome i t e v i d e n t l y has another chromosome a t t a c h e d t e r m i n a l l y ; t h i s can be seen by comparing the l e n g t h of t h i s chromosome w i t h - 19 -the s a t e l l i t e chromosome a t t a c h e d to the l e f t s i d e of the n u c l e o l u s . I f t h i s c o n f i g u r a t i o n i s the r e s u l t of the f u s i o n of two chromosomes i t may have r e s u l t e d i n one of two ways: f i r s t l y , t h e r e may have been an i n t e r n a l -loss i n one chromatid by f r a g m e n t a t i o n , then the e x t r a p i e c e i n the other chromatid would form a l o o p , or secondly, i t may- have a r i s e n from the a d d i t i o n of a fragment forming a l o o p . I f on the other hand, t h i s i s a l l one chromosome, then one of the chromatids has fragmented and one end has f o l d e d back and become a t t a c h e d forming a l o o p . In t h i s event, t h e r e would be no l o s s or g a i n of chromatic m a t e r i a l i n the chromosome. T h i s second e x p l a n a t i o n seems to be more l i k e l y , s i n c e both ends of the chromosome are t h i c k and show a double s t r u c t u r e , whereas the middle p o r t i o n i s narrower and no s p l i t c o u l d be seen. T h i s cannot be decided f o r c e r t a i n t y because the chromosome i s s m a l l and i s l o c a t e d under other s t r u c t u r e s and i s i n consquence not e n t i r e l y d i s c r e t e ? i n appearance. ~ Polysomaty. Polysomaty i s the occurrence of n u c l e i c o n t a i n i n g m u l t i p l e s of the d i p l o i d number of chromosomes l o c a t e d In I n d i v i d u a l c e l l s or groups of c e l l s In a d i p l o i d organism. T h i s c o n d i t i o n was found i n two c e i l s l o c a t e d c l o s e to the o u t e r l a y e r of the o v u l e . These c e l l s were - 20 -. l a r g e r than most of the c e l l s showing d i v i s i o n s i n the ovule. Many of the surrounding c e l l s were about the same s i z e but they were a l l i n the r e s t i n g stage so there was no way to determine whether or not they were p o l y s o m a t i c . F i g u r e s 21a and 21b are two s e c t i o n s of the same nucleus showing t h i s c o n d i t i o n . Most of the chromosomes are arranged i n p a i r s , however the s m a l l e s t ones have e i t h e r s p l i t and separated or, e l s e they, have^ approximated. From the number of them pres e n t i t seems l i k e l y t h a t they have separated. The lo n g e r chromosomes have s p l i t c o m p l e t e l y but mosj; of them remain c l o s e t o g e t h e r . Some of them are i n contact o n l y at one end, oth e r s remain coi'&ed about each o t h e r . A l l chromosomes show a second p a r t i t i o n , but the chromatids have remained more c l o s e l y a s s o c i a t e d and c o i l e d than the chromosomes formed by the f i r s t p a r t i t i o n . The p a r t i t i o n i s not complete i n some chromosomes, but may be i n o t h e r s , t h e r e f o r e the nucleus cannot be s a i d to have o c t o p l o i d chromonemata. The f a c t t h a t most of the p a i r s of chromosomes are so c l o s e l y a s s o c i a t e d would suggest t h a t the t e t r a p l o i d c o n d i t i o n arose from a complete s p l i t i n the d i p l o i d chromosomes f o l l o w e d by a s p l i t of each daughter chromosome to form two chromatids. The s h o r t chromosomes may have separated - 21 -q u i t e e a s i l y because they can move i n a s m a l l e r space- and are l e s s l i k e l y t o remain c o i l e d t o g e t h e r than l o n g chromo somes. A l t e r n a t e l y , the nucleus o r i g i n a t e d from the f u s i o n of two d i p l o i d n u c l e i , the occurrence of p a i r s would' mean t h a t there i s no s y n a p t i c a t t r a c t i o n between the chromosomes; we would then expect the s m a l l e r chromosomes that, are ab l e t o move about most r e a d i l y to form the c l o s e s t a s s o c i a t i o n s . The f a c t t h a t some of the chromosomes are separated completely or i n t o wide V's would i n d i c a t e t h a t there i s no s y n a p t i c a t t r a c t i o n and t h a t the t w i s t i n g i s merely a r e l i c of the anaphase c o i l . However, s i n c e the s y n a p t i c a t t r a c t i o n I s j>resent c h i e f l y i n the e a r l y prophases of meaosls and seems to disappear to a c o n s i d e r a b l e extent by l a t e prophase or d l a k i n e s i s (see F i g . . 1) and s i n c e i t i s l e s s e v i d e n t I n the s m a l l chromosomes, the above statement must be m o d i f i e d . The q u e s t i o n of a t t r a c t i o n s and r e p u l s i o n s I s not y e t c l e a r l y understood and consequently no d e f i n i t e c o n c l u s i o n s can be reached. - 22 -Review of L i t e r a t u r e and D i s c u s s i o n H i s t o r i c a l The work done to date on the c y t o l o g y of G a l t o n i a candicans..'has been mainly t h a t of t r a c i n g the chromosomes „ through the v a r i o u s stages of m i t o s i s and m e l o s i s . Digby (1910) has g i v e n an extended d e s c r i p t i o n of the minute s t r u c t u r e of the chromosomes e s p e c i a l l y through t e l o p h a s e s and prophases. However her work has been m o d i f i e d c o n s i d e r a b l y s i n c e then by l a t e r workers a p p l y i n g more recent conceptions of chromosome s t r u c t u r e and p o s s i b l y u s i n g improved f i x a t i o n and s t a i n i n g methods on t h e i r m a t e r i a l . The o n l y unusual occurrence mentioned by Miss Digby i n t h i s paper i s the o c c a s i o n a l f o r m a t i o n of c h a i n s or r i n g s of chromosomes at d i a k i n e s i s . An e a r l i e r paper by t h i s worker (Digby, 1909) r e p o r t s the f o r m a t i o n of chromatin bodies produced at. s y n a p s i s In the p o l l e n mother c e l l s . ' These she r e p o r t s as a r i s i n g from the n u c l e o l u s or the n u c l e o l a r framework.. No t h i n g of t h i s nature has been found m the p r e s e n t study. Newton (1924) i n s t u d y i n g the somatic chromosomes i n G a l t o n i a found a l o o s e p a l r l n g of ;somatic chromosomes at e a r l y metaphase and a t r u e p a i r i n g a t e a r l y anaphase. Examining the somatic c e l l s of the r o o t t i p and - 23 -anther, Smith (1932) made a comprehensive study of the chromonemata i n the chromosome a t mei o s l s and m i t o s i s . He found t h a t the chromonemata were c l o s e l y a s s o c i a t e d m the anaphase of the r o o t t i p s , but ?<rere more w i d e l y . separated i n the same stage i n the somatic c e l l s of the ant h e r . My r e s u l t s i n d i c a t e t h a t there are two d e f i n i t e s t r a n d s present, even In the root' t i p anaphases. Smith found chromosome l a g g i n g but i t was not c h a r a c t e r i s t i c of any p a r t i c u l a r chromosomes, he found t h a t e i t h e r l o n g or s h o r t ones might l a g . He a l s o r e p o r t s t h a t t h e r e i s no c h a i n f o r m a t i o n at d i a k l n e s i s . The same author (Smith, 1932) working on the r e l a t i o n of the n u c l e o l u s and s a t e l l i t e s found one or two n u c l e o l i p r e s e n t i n somatic c e l l s w i t h s a t e l l i t e chromosomes.attached. In m e i o t i c stages he f i n d s j t f t t s t one n u c l e o l u s on which t h e r e may be buds g i r d l e d by the f i b e r s of the s a t e l l i t e attachments. Ghromosome S t r u c t u r e and S p i r a l s Some of the re s e n t workers have devoted themselves almost e n t i r e l y t o d i s c o v e r i n g the I n t e r n a l s t r u c t u r e and s p i r a l i z a t l o n of - chromosomes. I t I s now g e n e r a l l y ' a c c e p t e d t h a t the somatic anaphase chromosomes c o n s i s t of a t l e a s t two s p i r a l l y wound chromonemata. Abraham (1939) r e p o r t s t h a t the somatic anaphase chromosome shows two s p i r a l - 24 -chromonemata embedded m a common m a t r i x , these c o i l s are e n t i r e l y f r e e from each other and u s u a l l y run p a r a l l e l . The f i g u r e s he-shows i n support of t h i s i d e a are not very c i e a r , and i t seems he may have m i s i n t e r p r e t e d what h i s s l i d e s show. He concludes t h a t the mechanism of s p i r a l l z a a t i o n i s a s s o c i a t e d w i t h a compensating i n t e r n a l t w i s t i n the'chromonemata.. D a r l i n g t o n (1932) r e f e r s to t h i s t w i s t as a.molecular s p i r a l . . Abraham says t h a t an i n t e r l a c e d s p i r a l i s not a r e s u l t of the cleavage of a s p i r a l chromatid but t h a t the t w i s t s are formed l a t e r . Coleman (1940) on the other hand agrees w i t h Sax, U p c o t t , and D a r l i n g t o n i n r e g a r d i n g the somatic anaphase and prophase chromosomes as s i n g l e c o i l e d s t r u c t u r e s . He s t a t e s t h a t the s o - c a l l e d i n t e r l o c k i n g double c o i l e d s t r u c t u r e I s a f i x a t i o n a r t i f a c t . He f u r t h e r s t a t e s to t h a t t h i s c o n c l u s i o n does not touch fthe q u e s t i o n a s A whether the anaphase chromosomes are b i p a r t i t e e or. not. prom t h i s I gathered he means t h a t there i s only one s p i r a l but i t might be double. The g r e a t m a j o r i t y of c y t o l o g l s t s c o n s i d e r t h a t the anaphase chromosomes possess a v i s i b l y b i p a r t i t e s t r u c t u r e and t h a t they c o n s i s t of two chromonemata c o i l e d about each o t h e r so t h a t the gyres i n t e r l o c k . - 25 -W i l s o n and Buskins (1939) wording w i t h T r i l l i u m propose the t h e o r y t h a t the m e i o t i c s p i r a l i s caused by the e l o n g a t i o n of the chromonemata w i t h i n an enveloping- sheath or p e l l i c l e , the nature-. of which remains to be determined. Sparrow, H u s k i n s , and W i l s o n (1941) add t h a t the h a l f chromatids a t metaphase are"wound i n the form of a plectonemlc s p i r a l . T h i s i s a s p i r a l arranged so t h a t the chromatids can separate by l a t e r a l movement and not by u n r a v e l l i n g from the ends. Nebel (1941) p r e s e n t s \ i n t e r e s t i n g o p t i c a l evidence of the types of c o i l s found I n T r i l l i u m chromosomes, he photographs l i g h t t r a n s m i t t e d through g l a s s s p i r a l s and compares the r e s u l t s w i t h the Images formed by the chromonemata c o i l s . Huskins (1941) d e s c r i b e s the l a r g e gyred major c o i l s i n m e l o s i s , and s i m i l a r c o i l s i n m i t o t i c chromosomes. The evidence p r e s e n t e d i n these papers was taken from m a t e r i a l prepared e s p e c i a l l y to show these s t r u c t u r e s . The s p i r a l s t h a t show i n my m a t e r i a l are not n e a r l y so c l e a r as the I l l u s t r a t i o n s shown by these men, but they do i n d i c a t e a s i m i l a r s t r u c t u r e , namely, a ;pair of s p i r a l chromonemata i n the anaphase chromosomes. The time of the somatic s p l i t has been found to v a r y spa i n d i f f e r e n t forms. Nebel and R u t t l e (1936) d e s c r i b e d the A - 26 -i n Trad.escant.ia t a k i n g p l a c e a t metaphase, two m i t o t i c c y c l e s p r i o r t o ' t h e i r s e p a r a t i o n . Smith (1932) on G a l t o n i a found the s p l i t o c c u r r i n g d u r i n g the prophase of the d i v i s i o n p r e v i o u s t o s e p a r a t i o n . N u c l e o l i The s i z e , number, and f o r m a t i o n of n u c l e o l i have been g i v e n prominence i n recent c y t o l o g i c a l works. Digby (1910) r e p o r t e d t h a t two s m a l l chromosomes were a t t a c h e d to the n u c l e o l u s i n G a l t o n i a , however, i t has s i n c e been shown t h a t a medium-lengthed p a i r i s a t t a c h e d t o the n u c l e o l u s (Smith,1933). L e s l e y (1938) d i s c o v e r e d t h a t i n tomato c e l l s the n u c l e i w i t h chromosomes t h a t had s a t e l l i t e s • i n c r e a s e d i n s l z e i , had l a r g e r n u c l e o l i than the' c e l l s w i t h o r d i n a r y s a t e l l i t e s . Pathak (1940) working w i t h c e r e a l s , f i n d s the n u c l e o l i are o r g a n i z e d i n the te l o p h a s e by a p a i r of chromosomes w i t h secondary c o n s t r i c t i o n s . The n u c l e o l i u s u a l l y fuse and the two chromosomes may be seen a t t a c h e d to the n u c l e o l u s . The n u c l e o l u s i s o r g a n i z e d around the t h r e a d of the secondary c o n s t r u c t i o n . The number of n u c l e o l i i s p r o p o r t i o n a l to the number of chromosomes w i t h secondary c o n s t r i c t i o n s . The t e t r a p l o l d and h e x a p i o i d T r i t i c u m and A e g i l o p s s p e c i e s have f o u r and s i x n u c l e o l i r e s p e c t i v e l y . - 27 -Berger (1940) working w i t h p o l y s o m a t l c c e l l s i n S p i n a c l a , found t h a t the d i p l o i d c e l l s always have e i t h e r two s m a l l or one l a r g e n u c l e o l u s . T e t r a p l o i d c e l l s have f o u r s m a l l n u c l e o l i , one l a r g e , or an i n t e r m e d i a t e number i n the .process of f u s i o n . Bhaduri (1939) found t h a t the presence of u n p a i r e d n u c l e o l i m Oenothera I n d i c a t e d h e t e r o z y g o s i t y o f ' t h e s p e c i e s . . Mensinkai (1940) from a b n o r m a l i t i e s found I n the s a t e l l i t e s of v a r i o u s s p e c i e s of A l l i u m concluded t h a t the s a t e l l i t e i s o n l y the r o l l e d - u p end of the chromosome-helix. Gave and B r a d l e y (1943) found as many as s i x chromosomes a s s o c i a t e d w i t h the n u c l e o l u s of Miersia c h l l e n s i s at pachytene. The n u c l e o l u s forming r e g i o n s are a p p a r e n t l y I n c o r p o r a t e d I n the s h o r t arms of c e r t a i n r o d chromosomes, " i n t h i s s p e c i e s no s a t e l l i t e s have been found unless i t may be c o n s i d e r e d t h a t the s h o r t arms of c e r t a i n rod chromosomes have become,, so reduced t h a t they c o n s i s t of n o t h i n g more than s a t e l l i t e s and n u c l e o l u s .forming r e g i o n s . T h i s l a c k of r e s t r i c t i o n of n u c l e o l u s f o r m a t i o n has a l s o been found i n T r i l l i u m . Warmke (1941) s t a t e s t h a t i t i s pro b a b l y c o r r e c t t o c o n s i d e r the s a t e l l i t e s as separated from the main body of the chromosome by a secondary c o n s t r i c t i o n . These secondary c o n s t r u c t i o n s are l i n k e d w i t h n u c l e o l u s f o r m a t i o n . - 2 8 -The b e g i n n i n g of n u c l e o l u s f o r m a t i o n a t the end of anaphase was recorded i n t h i s paper. I t was a l s o n o t i c e d t h a t a l l gametophyte.t c e l l s had but one n u c l e o l u s , the sporophyte c e l l s had e i t h e r one l a r g e n u c l e o l u s or two s m a l l ones, and the endosperm c e l l s a l l showed three s m a l l n u c l e o l i . Fragmentations and S t r u c t u r a l H y b r i d i t y Fragmentation.-; i s the t r a n s v e r s e s p l i t t i n g of the chromosomes. I t has been found by s e v e r a l workers t h a t fragmentations may be caused by X-rays but t h a t the broken ends tend t o r e j o i n more r e a d i l y a t high temperatures. Sax and Enzmann (1939) found t h i s o c c u r r i n g - i n /(mo -) Trades c a n t i a microspores. Fabergen a l s o working w i t h p o l l e n g r a i n s i n T r a d e s c a n t i a concluded t h a t heat caused the broken ends to j o i n up more r e a d i l y . D a r l i n g t o n (1940) r e p o r t e d a t r a n s v e r s e f r a g m e n t a t i o n ' o f the centromere. The new t e l o c e n t r i c chromosome so formed i s s u b - e f f i c i e n t J i t reproduces.and d i v i d e s w i t h l e s s r e g u l a r i t y than the complete chromosomes. He s t a t e s t h a t i n the d i p l o i d f r i t l l l a r y I t i s a consequence of the n o n - c o - o r i e n t a t l o n of b i v a l e n t s as we&l as u n i v a l e n t s , there must be a s p e c i f i c and e x c e p t i o n a l c a p a c i t y f o r m l s d i v l s i o n or i n c a p a c i t y f o r c o r r e c t d i v i s i o n i n h e r e n t e i t h e r i n the centromeres or i n - 29 -the o r g a n i z a t i o n of m e i o s i s . These t e l o c e n t r i c s l a g and o f t e n become e x t r a n u c l e a r b o d i e s . M i s d i v i s i o n i s the o n l y known means of adaxotation i n the s i z e and f u n c t i o n s of the centromere. The t e l o c e n t r i c chromosome can r e s t o r e the i n t e r c a l a r y p o s i t i o n of i t s centromere by a secondary change, the two s i s t e r chromatids becoming concur r e n t arms. The new chromosome w i t h two i d e n t i c a l arms may then be d e s c r i b e d as an isochromosome. Rhoades (1940) examining the t e l o c e n t r i c chromosomes i n maize found t h a t they were u n s t a b l e and formed isochromosomes, s i m i l a r to those r e p o r t e d by D a r l i n g t o n . Bhaduri (1939) found t h a t m Oenothera b i a n d l n a i n s t e a d of the normal complement of 2n = 14 chromosomes, some c e i l s had 15 chromosomes. T h i s has been a s c e r t a i n e d to be due to r e a l f r a g m e n t a t i o n . T h i s fragment belongs to one of the l o n g chromosomes to which i t i s normally a t t a c h e d by a secondary c o n s t r i c t i o n . Levan and Emsweller/\ s t u d y i n g Nothoscordum r e p o r t e d t h a t e a r l y s t u d i e s showed one f r a g r a n s form w i t h 2n = 16 chromosomes, a l l w i t h median attachments. A c l o s e l y relateo. form had 2n = 18 chromo-somes, f o u r of which had t e r m i n a l attachments. Since these t e r m i n a l l y a t t a c h e d chromosomes were about the same l e n g t h as one arm of the l o n g e s t bi-armed chromosomes - 30 -p r e s e n t , t h e i r o r i g i n has been e x p l a i n e d by fra g m e n t a t i o n of one long f cbi-armed chromosome i n the e i g h t chromosome genora. The new form d i s c o v e r e d by these authors had n i n e t e e n somatic chromosomes, t h i r t e e n w i t h median a t t a c h -m e n t s and s i x w i t h t e r m i n a l . A second tjipe of s t r u c t u r a l h y b r i d i t y was r e p o r t e d by Gave and B r a d l e y (1943) r e s u l t i n g from the f u s i o n of t?i?o chromosomes w i t h t e r m i n a l attachments to form a V chromosome. They account f o r the o r i g i n of these V chromosomes i n one of two ways: "(a) r e c i p r o c a l t r a n s l o c a t i o n s between two non-homologous rod chromosomes and consequent l o s s of one centromere and both s h o r t arms, or (b) the f u s i o n of the centromeres of two non-homologous rod chromosomes! Evidence was found of changes i n chromosome of s t r u c t u r e s i n the somatic c e i l s , G a l t o n i a s i m i l a r t o those ^ — : -r e p o r t e d above. The t e l o c e n t r i c chromosome found In the r o o t t i p anaphase ( F i g . 9) behaves i n the same manner as those r e p o r t e d by D a r l i n g t o n . There was a l s o some evidence found of the f o r m a t i o n of median chromosomes from two t e r m i n a l l y a t t a c h e d chromosomes. However both these types of a b n o r m a l i t y were v e r y uncommon, and no c o n c l u s i o n c o u l d be made as t o t h e i r g e n e t i c s i g n i f i c a n c e In t h i s form. - 31 -Chromosome P a i r i n g The p a i r i n g " o f homologous chromosomes i n the somatic d i v i s i o n s has been r e p o r t e d by s e v e r a l workers, but the s u b j e c t i s s t i l l c o n t r o v e r s i a l . I n G-altonia candicans, somatic p a i r i n g a t e a r l y metaphase and a t anaphase has been r e p o r t e d by Newton (1924) He co n s i d e r e d the metaphase p a i r i n g to be a l o o s e arrangement and the anaphase a s s o c i a t i o n to be t r u e p a i r i n g . The m a t e r i a l s t u d i e d i n t h i s paper showed no s i g n s of any p a i r e d arrangement of the chromosomes i n the r o o t t i p s a t any stage, but q u i t e d e f i n i t e p a i r i n g of the chromosomes i n the somatic c e i l s of the ovule. The f a c t t h a t the ovule mitoses resemble m e i o s i s i n s e v e r a l ways, namely -the p a i r i n g of chromosomes, the extreme c o n t r a c t i o n of the chromosomes, and the o c c a s i o n a l presence of n u c l e o l i at l a t e prophase, would suggest t h a t the c o n d i t i o n s which b r i n g about the m e i o t i c d i v i s i o n s have some e f f e c t on the m i t o t i c chromosomes. What these c o n d i t i o n s are has not been determined, some workers suggest t h a t they are caused by age, some suggest the presence of necrohormones (see G ustafsson (1939} who suggests t h a t they are r e l a t e d t o a sap i n t a k e and growth p e r i o d , and D a r l i n g t o n (1936) who e x p l a i n s m e i o s i s as the consequence of a s h i f t i n - 32 -time c o o r d i n a t i o n between e x t e r n a l and I n t e r n a l f a c t o r s i n the development of the chromosomes. The prophase begins before the chromosomes have d i v i d e d , t h a t i s , I t i s p r e c o c i o u s . A l l of these t h e o r i e s seem to suggest .that the change from m i t o t i s t o meiosis i s caused by some e x t e r n a l ^ p h y s i o l o g i c a l c o n d i t i o n about which v e r y l i t t l e Is known., i f i t i s p h y s i o l o g i c a l , I can see no reason why I t c a n n o t . e f f e c t the c e l l s s u r r o u n d i n g the germ c e l l s , t o some e x t e n t . Gustafsson (1939) c i t e s F a g e r l m d who found i n h i s review of the c y t o l o g y of Rublaceae found i n s p e c i e s w i t h pronounced degeneration of the m e i o t i c c e l l s and n u c l e i , somatic c e l l s can be changed i n a m e i o t i c d i r e c t i o n , showing chromosome c o n t r a c t i o n and s y n e s e s i s s stages. T h i s siiaggests t h a t some substance i s r e l e a s e d by the d i s i n t e g r a t i n g c e l l s and t h a t t h i s substance a f f e c t s a d j a c e n t c e l l s . G u stafsson (1939) found t h a t the Intake of sap and growth of a nucleus changes to me i o s i s t o m i t o s i s , " i f no growth and v a c u o l i z a t i o n p e r i o d s e t s i n d i v i s i o n i s r e d u c t i o n a l . A f t e r a c e r t a i n amount of h y d r a t i o n and v a c u o l i z a t i o n d i v i s i o n becomes e q u a t i o n a l , but I s s t i l l .meiotic I n c h a r a c t e r , and o n l y when growth and h y d r a t i o n have reached another t h r e s h o l d i s d i v i s i o n m i t o t i c ! - 33 -Bhaduri (1939) c i t e s Marquardt as o b s e r v i n g cases of apparent p a i r i n g of somatic chromosomes i n the anther archesporium. Marquardt suggested t h a t t h i s ma-y r e p r e s e n t a step towards c o n j u g a t i o n , which w i l l take p l a c e In the f o l l o w i n g n u c l e a r d i v i s i o n . Bhaduri does not agree w i t h t h i s view. Very d e f i n i t e p a i r i n g of somaticichromosomes i s found In a l l the t i s s u e s of the D i p t e r a (Metz, 1916). This has a l s o been r e p o r t e d m some of the o t h e r I n s e c t s . Since t h i s c o n d i t i o n i s so constant In the t i s s u e s , i t i s u n l i k e l y t h a t i t i s the same time of p a i r i n g as t h a t found i n the ovule c e l l s of G a l t o n l a . T r a n s l o c a t i o n s and Fusions Types of t r a n s l o c a t i o n s and t e r m i n a l f u s i o n s , both Induced and those o c c u r r i n g i n nature, have been s t u d i e d by many c y t o i o g i s t s . P r o b a b l y the most tho r o u g h l y i n v e s t i g a t e d phenomenon Is the r i n g sBo'rmation or c a t e n a t i o n i n Oenothera. S t u r t e v a n t and Dobzhansky (I93(li) drew a p a r a l l e l between the p a i r i n g s found i n Dros°phila and the c a t e n a t i o n s found, i n Oenothera. They found t h a t the p a i r i n g was between l i k e r e g i o n s on the chromosomes, and not the whole chromosome. The t h e o r y was advanced t h a t t r a n s l o c a t i o n between non-homologous chromosomes caused r i n g f o r m a t i o n s - 34 -i n Oenothera. They found c y t o l o g i c a i evidence f o r such t r a n s l o c a t i o n s i n D r o s o p h i l a . Bhaduri (1939) agrees w i t h the"above e x p l a n a t i o n of the o r i g i n of t h e s e . c h a i n s , S l k k a (1940) g i v e s a review of the work done on Oenothera. He f u r t h e r suggests t h a t s i n c e the f o r c e which holds the chromosomes t o g e t h e r i s s y n a p t i c , and s i n c e , i n a d i p l o i d , s y n a p s i s i s between one maternal and one p a t e r n a l chromo-some, I t f o l l o w s t h a t these chromosomes i n the r i n g are a l t e r n a t e l y p a t e r n a l and maternal In o r i g i n . Both SIkka and Jacob (1940) r e c o g n i z e d t h a t there i s a p a r t i c u l a r arrangement of the chromosomes f o r each s p e c i e s or t y p e . M e n s l n k a i (1940) found r i n g s and chains formed i n A l l i u m by the same type of r e c i p r o c a l t r a n s l o c a t i o n s between non-homologous chromosomes. A s i m i l a r c o n d i t i o n was r e p o r t e d i n P r i m u l a Kewensis by UpcotT (1940).. Somatic B r i d g e s The Chromosome b r i d g e s formed at anaphase i n . some abnormal n u c l e i may be caused by s e v e r a l d i f f e r e n t c o n d i t i o n s , namely - the f o r m a t i o n of d i c e n t r i c chromosomes, the l a g g i n g of a c e n t r i c chromosomes, or by the adhesion of chromosomes. Upcott (1936) working w i t h t r i p l o i d t u l i p s r e p o r t s the f i r s t type of chromosome b r i d g e s . In a l l p l a n t s s t u d i e d , d i c e n t r i c chromatids are formed at - 35 -me i o s i s as a r e s u l t of c r o s s i n g over between r e l a t i v e l y i n v e r t e d segments of chromosomes p a i r i n g d u r i n g prophases. These chromatids form b r i d g e s at f i r s t and second anaphases. 'rThe s t r e t c h i n g of the b r i d g e s a t anaphase bears out the view of B e l a r and D a r l i n g t o n (1932) t h a t the anaphase s e p a r a t i o n I s determined by two agents, f i r s t , by the mutual r e p u l s i d n n of the centromeres, and l a t e r by the s t r e t c h i n g of the c e n t r a l r e g i o n of the s p i n d l e which completes the process ^y:i<. hushing the chromosomes f a r t h e r toward the p o l e s . Observations show t h a t the s p i n d l e expansion takes p l a c e i n two zones - between the equator and the poles." The r e g i o n a d j a c e n t to the centromere I s more s u s c e p t i b l e to u n c o i l i n g . "The f a c t t h a t the s t r a i n c a u s i n g the u n c o i l i n g i s not t r a n s m i t t e d e q u a l l y a c r o s s the b r i d g e but remains l o c a l i z e d suggests t h a t there i s c o n s i d e r a b l e f r i c t i o n between the chromosomes and the s p i n d l e m a t e r i a l . M e n s i n k a i (194-0) r e p o r t s a somatic b r i d g e i n a r o o t t i p of A l l i u m . He c o n s i d e r s t h i s b r i d g e f o r m a t i o n to be "due to some adverse e x t e r n a l circumstances i n f l u e n c i n g the p h y s i o l o g y of the p a r t i c u l a r nucleus p r o d u c i n g d e n a t u r a t i o n .of chromatin i n the p a r t i c u l a r chromosome 1' He f u r t h e r says t h a t the l a g g i n g of chromosomes and b r i d g e formations i n - 36 -somatic m i t o s i s i s not of s i g n i f i c a n c e unless m the germ t r a c k , when they a f f e c t the germ c e l l s and induce t r a n s m i s s i b l e v a r i a t i o n s . The t h i r d type of somatic b r i d g e i s not caused by t r a n s l o c a t i o n s but i s merely adhesions of the chromosomes. White (1936) t r e a t e d O r t h o p t e r a m a t e r i a l w i t h X-rays. He fomid t h a t a f t e r i r r a d i a t i o n the chromosomes appear to have become more v i s c o u s , so t h a t they only separate w i t h g r e a t d i f f i c u l t y . ,The two chromosomes adhere i n s p i t e of the r e p u l s i o n between the s p i n d l e attachments which i s p u l l i n g them a p a r t . The r e s u l t i s t h a t the two groups of anaphase, chromosomes are h e l d t o g e t h e r by a " b r i d g e " . White suggests t h a t t h i s and other occurrences i n i r r a d i a t e d m a t e r i a l i s p r o b a b l y due to a g e n e r a l d i s i n t e g r a t i o n of the chromosomes. The b r i d g e s form i n the ovule c e l l s of G a l t o n i a i n t h i s l a s t manner. ( F i g s . 17 and 18) The chromosomes appear to have become v i s c o u s ; they have l o s t t h e i r i n d i v i d u a l shape; they adhere i n s t e a d of s e p a r a t i n g n o r m a l l y . Sax (1938) a l s o r e p o r t e d the clumping of the chromo-somes as the f i r s t e f f e c t of i r r a d i a t i o n . The treatment a p p a r e n t l y a f f e c t s the chromosome envelope. The f u s i o n s r e s u l t i n g from the clumping of chromosomds may a l s o be produced by heat and age. - 37 -Chromatid Fragmentation Sax (1938) s t a t e d t h a t X-rays may break both chromatids w h i l e u l t r a - v i o l e t rays u s u a l l y break only one of the two chromatids. A c c o r d i n g to t h i s author, the chromosomes are I n the form of r e l a x e d c o l l e d chromonemata d u r i n g the r e s t i n g s t a g e . An X-ray h i t t i n g a chromosome can break two adjacent gyres and the reunion of broken ends i n new a s s o c i a t i o n s w i l l produce s m a l l d e f i c i e n c i e s and I n v e r s i o n s . The s i z e of the d e f i c i e n c y or i n v e r s i o n w i l l depend on the stage at which the chromosome was h i t , s i n c e the s i z e and number of the gyres vary at d i f f e r e n t s t a g e s . The same worker (Sax, 1941) has done f u r t h e r work on chromosome and chromatid f r a g m e n t a t i o n induced by X-rays In r e g a r d to dosages and time r e l a t i o n s . U l t r a - v m o l e t rays from s u n l i g h t or changed temperatmre c o n d i t i o n s might p o s s i b l y have caused the s i n g l e , chromatid break shown I n F i g u r e 19 and the r e s u l t i n g loop f o r m a t i o n . Diplochromosomes and Polysomaty I have grouped these two types of a b n o r m a l i t y t o g e t h e r because both of them seem to I n v o l v e a s p l i t of the chromosome or chromatids and a d e l a y or f a i l u r e of d i v i s i o n . - 38 -Diplochromosomes were f i r s t r e p o r t e d by White (1935). He found these V-shaped chromosomes present i n approximately the d i p l o i d number.• Each arm of the V i s l o n g i t u d i n a l l y s p l i t i n t o two chromatids. Each chromosome i s a compound s t r u c t u r e w i t h two g e n e t i c a l l y homologous arms and a s p i n d l e attachment In the middle. White Says t h a t diplochromosomes have e i g h t chromatids. He a p p a r e n t l y c o n s i d e r s the arms on each s i d e of the. centromere as separate chromatids. Barber (1939) spealcs of diplochromosomes as f o u r chromatids p a s s i n g through one centromere. These are formed by repeated d i v i s i o n of the chromosomes without m i t o s i s , t h i s may be d e s c r i b e d as progeny p a i r i n g , t h a t i s , i t m a i n t a i n s the s t a t u s q_uo i n c o n t r a s t to pachytene p a i r i n g w h i c h i l s :,an a c t i v e type of p a i r i n g ! R e d u p l i c a t i o n of chromonemata d u r i n g r e s t i n g stages has been found q u i t e f r e q u e n t l y in- both p l a n t s and a n i m a l s . Bauer (1936) p o s t u l a t e d t h a t the l a r g e s a l i v a r y gland chromosomes of D r o s o p h i l a were composed of a a l a r g e number of c l o s e l y u n i t e d chromonemata. P a i n t e r (1940) d e s c r i b e s the s a l i v a r y gland chromosomes as o r i g i n a l l y having f o u r chromatids. A f t e r s y n a p s i s the compound chromosome grows i n diameter and l e n g t h . The growth i n s i z e of the chromo-meres i s presumably due to r e d u p l i c a t i o n w i thout a - 39 -v i s i b l e d i v i s i o n of p a r t s . The u n c o i l i n g which p a r t l y causes the i n c r e a s e i n l e n g t h would a l s o e x p l a i n why we see a more complex p a t t e r n of bands. Berger (1938) found the c e l l s of the l a r v a l ileum of Gulex i n c r e a s i n g i n s i z e -but not i n number. The. Increase In s i z e i s p a r a l l e l e d by an Increase i n chromosome number. A c c o r d i n g to Berger, "the nucleus r e t a i n s the t y p i c a l r e s t i n g stage c o n d i t i o n throughout the growth and chromosome m u l t i p l i c a t i o n p e r i o d . Subsequent d i v i s i o n s show o r d e r l y somatic s y n a p s i s and are a p p a r e n t l y somatic r e d u c t i o n d i v i s i o n s g i v i n g s m a l l e r c e l l s w i t h the reduced chromosome number'. Gentcheff and Gustafsson (1939) d e s c r i b e what they c a l l e d "polysomatic c e l l s " i n S p i n a c i a . They found t h a t i n most of these c e l l s the chromosomes are c l o s e l y a s s o c i a t e d i n p a i r s . They a l s o found t h a t these c e l l s were l a r g e r than those w i t h a normal chromosome complement. Some n u c l e i as soon as the chromosomes show up In e a r l y prophase may have a q u a d r i p a r t i t e 1 e s t r u c t u r e . Two major t h r e a d s , each c o n s i s t i n g of two chromatids, l i e separate from each othereexcept f o r a marked t w i s t i n g . T his t w i s t i n g I s pr o b a b l y I d e n t i c a l w i t h the r e l a t i o n a l c o i l i n g of chromatids i n m i t o s i s . F i g u r e 7 shown by these workers l o o k s v e r y s i m i l a r t o F i g u r e s 21a and 21b of G a l t o n i a . - 40 -From the s t r u c t u r e and r e l a t i o n s of the chromosomes I would assume t h a t they o r i g i n a t e m a s i m i l a r manner. Eerger (1940) r e i n v e s t i g a t e d t h i s c o n d i t i o n i n S p i n a c i a and added t h a t p o l y p l o i d c e l l s w i t h unpaired chromosomes have undergone at l e a s t one fflatotic d i v i s i o n s i n c e the time of double r e p r o d u c t i o n because the chromosomes remain p a i r e d u n t i l the nucleus d i v i d e s , at which time the p a i r e d r e l a t i o n i s l o s t . • Erwin (1939) r e p o r t s a s i m i l a r c o n d i t i o n i n the r o o t s of Cuoumis melq. A l l workers agree t h a t t h i s r e d u p l i c a t i o n of the chromosomes occursj; d u r i n g the stages when the chromosomes are non-mobile. - 41 -Con c l u s i o n s and Suggestions f o r F u r t h e r Research The a b n o r m a l i t i e s found In the ovule c e l l s of G a l t o n i a p r o b a b l y r e s u l t e d from a combination of f a c t o r s such as: u l t r a - v i o l e t r a y s from s u n l i g h t , heat, age, and p o s s i b l y hormones or p h y s i o l o g i c a l c o n d i t i o n s which are o r d i n a r i l y a s s o c i a t e d w i t h m e i o s i s . Whatever c o n d i t i o n or c o n d i t i o n s caused the a b e r r a t i o n s i n the somatic c e l l s of the ovule a p p a r e n t l y d i d not a f f e c t a d v e r s e l y m e i o s i s or the develop-ment of the germ c e l l s . Since the f o r c e s a f f e c t i n g the su r r o u n d i n g c e l l s a c t i n t h i s d i r e c t i o n the normal behaviour d u r i n g m e i o s i s I s to be expected. The r e s u l t s found c o n f i r m t h i s e x p e c t a t i o n . The seeds used f o r germination purposes were taken from the same p l a n t s and the r e s u l t i n g t o o t t t i p s showednnormal m i t o t i c d i v i s i o n s . The f a c t t h a t the seeds' germinated s u c c e s s f u l l y even a f t e r a year and a h a l f , p r o d u c i n g normal r o o t t i p s , would i n d i c a t e q u i t e d e f i n i t e l y t h a t none of these a b n o r m a l i t i e s are c a r r i e d I n t o the germ c e l l s . The developmental stages of the female gametophyte are a l s o seen t o be q u i t e normal. There are s e v e r a l p o i n t s touched on i n t h i s paper"' which suggest f u r t h e r i n v e s t i g a t i o n . I t would be i n t e r e s t -i n g t o d i s c o v e r j u s t when the a b n o r m a l i t i e s begin to occur and whether or not they are c o n f i n e d to the f l o w e r i n g p a r t s . - 42 -I t was noted d u r i n g the i n v e s t i g a t i o n t h a t the somatic c e l l s i n young ovules showed more t y p i c a l l y m i t o t i c d i v i s i o n s . The q u e s t i o n of polysomaty might a l s o be i n v e s t i g a t e d f u r t h e r to a s c e r t a i n i f t h i s were'merely a chance occurrence or whether i t were, common i n c e r t a i n t i s s u e s . The chromosome behaviour at the time of syngamy has not been observed and the a n a l y s i s of these stages i n the l i f e h i s t o r y should be s i g n i f i c a n t . - 43 -Summary 1. There are s i x t e e n somatic chromosomes and although complete. I n d i v i d u a l i t y e x i s t s , i t i s convenient to d i v i d e them i n t o t h r e e s i z e groups: f o u r l a r g e p a i r s , two medium p a i r s , and two s m a l l p a i r s . These were i d e n t i f i e d v e r y .markedly a t d i a k i n e s i s . 2. Normal m i t o t i c stages are d e s c r i b e d . 3. The n u c l e o l i are a s s o c i a t e d w i t h a p a i r of medfum-lengthed chromosomes w i t h s a t e l l i t e s . 4. A s i n g l e i n s t a n c e of a t e l o c e n t r i c chromosome i s the o n l y a b n o r m a l i t y found i n the roo t t i p s . 5. A b e r r a t i o n s i n ovule c e l l s i n c l u d e p a i r i n g , .fusions, chromatid f r a g m e n t a t i o n , and polysomaty. ^ 6. The c o n d i t i o n s c a u s i n g these a b e r r a t i o n s do not a f f e c t germ c e l l s a d v e r s e l y . I t may be t h a t the f o r c e s promoting m e i o s i s and germ c e l l development are potent to the extent of having an a p p r e c i a b l e e f f e c t upon the su r r o u n d i n g t i s s u e s thereby c a u s i n g the a b e r r a t i o n s p r e s e n t , m t h e . d i r e c t i o n of p a i r i n g , s y n a p s i s , and-precocious s p l i t t i n g . E x p l a n a t i o n of p l a t e s A l l drawings were made w i t h the a i d of a camera l u c i d a . The m a g n i f i c a t i o n a t ,table l e v e l was about 4800 diameters. The drawings have been reduced one h a l f i n r e p r o d u c t i o n . A l l drawings are a t the same m a g n i f i c a t i o n . P l a t e _ l P i g . 1. I n d i v i d u a l i t y of chromosomes at d i a k i n e s i s . • P i g . 2. Ring arrangement of p a i r No. 1. Fig.- 3. P o l a r view of metaphase p l a t e In r o o t t i p , showing-i n d i v i d u a l i t y of the chromosomes and the random arrangement F i g . 4. P o l a r vie?/ of metaphase p l a t e i n ovule c e l l m i t o s i s showing p a i r e d arrangement- of the chromosomes. F i g . 5. L a t e r a l view-of metaphase i n root t i p m i t o s i s . F i g . 6. Chromosomes a t middle anaphase i n r o o t t i p s . F i g . 7. Telophase chromosomes i n r o o t t i p s sho?>ring s p i r a l chromonemata. F i g . 8. Prophase chromosomes having two-more or l e s s chromonemata. P l a t e I I F i g . 9. T e l o c e n t r i c fragment i n r o o t t i p t e l o p h a s e . F i g . 10. Prophase i n ovule c e l l sho?/Ing p a i r s of t h r e a d -l i k e chromosomes. F i g . 11. P o l a r view of m i t o t i c d i v i s i o n In ovule c e l l , showing d i s t i n c t m e i o t i c t e n d e n c i e s of the chromosomes. - 45 -F i g . 12. Terminal f u s i o n s of m i t o t i c chromosomes i n ovules d u r i n g the prophase. F i g . 13. Terminal f u s i o n of homologous chromosomes i n ovule c e l l s a t metaphase. F i g . 14. Two chromosomes a t t a c h e d to one s p i n d l e attachment a t e a r l y anaphase i n ovule c e l l s . F i g s . 1 5 , 16, 17. Fusions of non-homologous chromosomes at prophase and metaphase. F i g s . 18, 19 .Somatic, b r i d g e s a t middle and l a t e anaphase. F i g . 20, Chromatid break forming a loop i n a m i t o t i c chromosome. F i g s . 21a, 21b. Two s e c t i o n s of po l y s o m a t i c c o n d i t i o n , grouped i n p a i r s . the same nucleus The chromosomes showing are - 46 -L i t e r a t u r e C i t e d Abraham, A. (1939): "Chromosome S t r u c t u r e and Mechanics of M i t o s i s and M e i o s i s . " Annals Bot. N.S. 3: 5.46-65. Barber, H.M. (1939): "The su p p r e s s i o n of M e i o s i s and the O r i g i n of.Diplochromosomes." • P r o c . Roy. Soc. Lond. Ser. B. 128: 170 Bauer, Hans (1936): " S t r u c t u r e and Arrangement of S a l i v a r y Chromosomes i n D r o s o p h i l a S p e c i e s . " P r o c . Nat. Acad. S c i . 22. Berger, G.A. (1938): "Prophase Chromosome Behaviour i n the D i v i s i o n of C e l l s w i t h M u l t i p l e Chromosome Complexes." j£HIi_££_S£££^' 29"• 351-57. Berger, C.A.(1940): " R e i n v e s t i g a t i o n of polysomaty i n Sumac l a . " Bot. Gaz. 102: 759-69. Bh a d u r i , P.N. (1939): " C y t o l o g i c a l S t u d i e s i n Oenothera w i t h s p e c i a l r e f e r e n c e t o ' t h e r e l a t i o n of Chromosomes to N u c l e o l i . " P r o c . Roy. Soc. Lond. Ser. B. 128: 353-78. Cave, M.S. and B r a d l e y , M.V.(1943): "Chromosome^ Number i n M i e r s i a C h i ' l e n s i s . " Amer. J o u r . Bot. 30: 142. Coleman, L.C. (1940): "The s t r u c t u r e of Homotypic and Somatic Chromosomes." Amer. ffour. Bot. 27: 683-86. D a r l i n g t o n , C D . (1932): Recent Advances i n Cyt o l o g y P h i l a d e p h i a : P. B i a k i s t o n ' s Son & Co. I n c . 1932. D a r l i n g t o n , C D . (1936): "The E x t e r n a l Mechanics of the Chromosomes.".. P r o c . Roy. Soc. Lond. Ser. B. 121: 265-319. - 47 -D a r l i n g t o n , C D . (1939): " M i s d i v i s i o n and Genetics of the Centromere." - - . Jour , of Genet.- 37: 341-64. D a r l i n g t o n , C D . (1940): "The O r i g i n of Isochroraosomes." A J o u r , of Genet. 39. Digby, L. (1909): "Observations on 'Chromatin Bodies' anci t h e i r - R e l a t i o n to the N u c l e o l u s i n G a l t o n i a Gandicans, Decne." Annals: Bot. 23. Digby, L. (1910): "The Somatic, P r e m e i o t i c , and M e i o t i c N u c l e a r D i v i s i o n s of G a l t o n i a Candicans." Annals Bot. 24: 727-67. E r w i n , C D . (1939): Polysomaty i n Cucumis Melo." S l H i j L - N a ^ J i c a d ^ S c i . 25: 335- 38. Faberge, A.C (1940): "An Experiment on Chromosome Fragmentation i n T r a d e s c a n t l a by X-rays." J o u r , of Genet. 39. Gust a f s s o n , Ake (1939):"The I n t e r r e l a t i o n Of M e i o s i s amd M i t o s i s . " . Hereditas.25:287 -322 H i l l a r y , B.B. (1940): "The Use of Feulgen's S t a i n . " Bot. Gaz. 102. Huski n s , C L . (1941):"The Coining of Chromonemata" Symp. quant.' B i o l . 9: 13-18. Jacob, K.T. (1940): "Chromosome c a t e n a t i o n In Oenothera." B o t ^ G a z . 102: 143-55. L e s l e y , M.M. (1938): "The r e l a t i o n between s a t e l l i t e s i z e and n u c l e o l u s s i z e i n th r e e races of Solanum Lypersicum." Genetics 23: 485. Levan, A. and S. L. Emsweller (1938): " S t r u c t u r a l H y b r l d i t y i n Nothoscordum f r a g r a n s . " J o u r . Hered. 29: 291 -4. - 48 -M e n s i n k a i , S.W. (1940): "Cytogenetic S t u d i o i n the Genus A l l i u m . " - -Jour of Genet. 39. Metz, C.W. (1916) : "Pairing; of Chromosomes i n D i p t e r a . " " Jour- Exper. Z o o l . 21: 213 -80. M o t t i e r , E.M.(1907): "The Development of the H e t e r o t y p i c Chromosomes i n P o l l e n Mother C e l l s . " Annals Bot. 21:309 -47. Nebel, B.R.- (1941): " S t r u c t u r e of T r a d e s c a n t i a and T r i l l i u m Chromosomes w i t h p a r t i c u l a r emphasis on Number of Chromonemata." Symp. quant. B i o l . 9:7-12. l e b e l , B.R. ^nd M.E. R u t t l e (1936): "Chromosome S t r u c t u r e , 9,-1 x T r a d e s c a n t i a R e f l e x a and T r i l l i u m Erectum." Amer. Jo u r . Bot. 23:653 -63. Newton, W.CP. (1924) : " S t u d i e s on Somatic Chromosomes No. 1. P a i r i n g and. Segmentation m G a l t o n l a . " Annals Bot. 38: 197 -206. P a i n t e r , T.S. (1941): "An e x p e r i m e n t a l study of S a l i v a r y Chromosomes." Symp. C^uant. B i o l . 9 : 4 7 - 5 4 . Pathak, G.N. (1940): " S t u d i e s m the C y t o l o g y of C e r e a l s . " Jour Genet. 39:, 437. / Hhoades, M.M. (1940): " T e l o c e n t r i c Chromosomes i n Maize w i t h r e f e r e n c e to the s t a b i l i t y of i t s centromere." G e n e t i c s . 25:483-520. Sax, C a r l (1938) : "Chromosome a b e r r a t i o n s induced by X-rays." Genetics,, 23: 494. Sax, C a r l (1941): "Types and f r e q u e n c i e s of chromosomal a b e r r a t i o n s indmced by X-rays." Symp. quant. B i o l . 9: 93-103. -- %2 -Sax, K a r l and E.V. Enzmann (193S): "The e f f e c t of temperature on X-ray induced chromosome a b e r r a t i o n s . " P r o c . Mat. Acad. S c l . 25: 397-405. S i k k a , 8.M. (1940): "A study of chromosome c a t e n a t i o n - i n Oenothera - h y b r i d s . " J o u r . Genet. 39: 309. Smith, F.H. (1932): "The s t r u c t u r e of somatic and m e i o t i c chromosomes of G a l t o n i a candlcans." La C e l l u l e . 31: 243-63. Smith, F.H. (1932): "The r e l a t i o n of the s a t e l l i t e s to the n u c l e o l u s . i n G a l t o n i a can/dicans." Amer. J o u r . Bot. 20: 188-95. Sparrow, A.H., C . l . Huskins and G.B.Wilson (1941): " S t u d i e s on the Chromosome s p l r a l i z a t i o n c y c l e i n T r i l l i u m . " Can.Jour. Research, 19. Sec.C: 323-49. S t u r t e v a n t , A.H. and T. Dobzhansky (1931): "Chromosome anomalies In D r o s o p h i l a melanogaster. U p c o t t , M. (1936): "The e x t e r n a l mechanics of the chromosome, 6 v 1 - the behaviour of the centromere a t m e i o s i s . " P r o c . Roy. Soc. Lond. Ser. B.. 124: 336. U p c o t t , M. (1940):."The nature of t e t r a p l o i d y i n P r i m u l a Kewensis." J o u r . Genet. 39: 79-100. Warmke, H.E. (1941): "Chromosome c o n t i n u i t y and i n d i v i d u a l i t y . " . Symp. Q,uant. B i o l . 9: 1-6. W i l s o n , G.B. and C.L.Huskins (1939):"Chromosome and Chromonema l e n g t h d u r i n g M e i o t i c c o i l i n g — " Annals Bot. N.s.3: 257-69. White, M.J.D. (!935):"The e f f e c t of X-rays on M i t o s i s " P r o c . Roy. S o c . L o n d . S e r . B . 119. 50 -White, M.J.D. (1936): "The e f f e c t of X-rays on the f i r s t m e i o t i c d i v i s i o n i n three s p e c i e s of Othoptera." P r o c . Roy.-Soc. Lond. Ser. B. 124: 183-96. - 51 -P l a t e I $ OA, A4 ^ 5 II 6 t l 7 • • 8 

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