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A study of the partial sterility of certain alfalfa strains Farley, Helen Mary 1936

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A STUDY OF THE PARTIAL STERILITY OP CERTAIN ALFALFA STRAINS. fry Helen M. P a r l e y . A Thesis submitted f o r the Degree of MASTER OP SCIENCE I F AGRICULTURE i n the department of BOTANY. THE UNIVERSITY OF BRITISH COLUMBIA Octoberj 1936. Acknowledgements I wish to acknowledge with sincere a p p r e c i a t i o n my i n -debtedness to Dr. A.H. Hutchinson f o r h i s help, i n s t r u c t i o n , and encouragement throughout t h i s work. To Dr. G.G. Moe under whose suggestion and i n s p i r a t i o n t h i s work began, I give thanks. Appreciation i s given to a l l those upon whom t h i s study depended - Dr. G.G. Moe, P r o f . P. Boving, and President u l i n c k . I am g r a t e f u l to Dean P.M. Clement f o r the i n t e r e s t he has taken. Table of Contents Page-Acknowledgements:. Introd u c t i o n . 1 Experimental Methods. 4 Re s u l t s . Development of the male and female, gametophyte 5 of a l f a l f a . . Male gametophyte. F'emal. e game tophyte. . Seed production as i n t e r p r e t e d from data 16 represented i n Charts: L and 2. Comparison of H 7-7, a. Mgh-seed-producer, 2G and H 68-5 a low->s:eed-producer at 2-, 6-p and 9-days a f t e r t r i p p i n g . Comparison of Ii 7-7, H 68-5 and Parent Grimm 22 at 2-days a f t e r t r i p p i n g . Review of L i t e r a t u r e . Discussion and Conclusiona Bibliography. Plates:. Charts?* 23 26 m 32 A STUDY OF THE PARTIAL STERILITY OP CERTAIN ALPALPA STRAINS Introduction The purpose of t h i s study was to i n v e s t i g a t e the var-i a t i o n i n seed production, f i r s t noticed by Dr. Moe i n h i s p£ hybrid a l f a l f a s . . These plants were a r e s u l t of a cross "between the variegated a l f a l f a "iledicago media" and the yellow flowered "Medicago falcata"'. Cuttings from one plant of f a l c a t a ? common-l y known as Don, were brought to the U n i v e r s i t y of B r i t i s h Columbia Farm, by Dr. K l i n c k formerly of Mac. Lonala College and now President of the U n i v e r s i t y of B r i t i s h Columbia. I t i s important to know that a l l Don m a t e r i a l on the U n i v e r s i t y farm came o r i g i n a l l y from one parent. Professor Boving knowing the almost complete s t e r i l i t y of the Don, attempted i n 1917 to obtain crosses by allowing the Grimm a l f a l f a , growing alongside i t to bloom simultaneously and thus f a c i l i t a t e f e r t i l i z a t i o n of the Don v a r i e t y . Out of some 250 plants produced from seed gathered from the Don plants i n 1917, 6 plants showed hybrid characters. In 1920 a l l other a l f a l f a s were cut back except the s i x hybrid p l a n t s . These were l e f t untented, but the flowers were manipulated. A heavy y i e l d of seed was produced by these hybrids. .In 1921, a row of 33. plants was set out from the seed of each hybrid. One hundred and seventy plants grew to maturity. A l l hybrid plants ( f i r s t growth) were manipulated by squeezing w i t h the hands ? and the weight i n grams of seed was taken. A heavy y i e l d was produced from some p l a n t s , a medium from others., while again other rjlants produced only a few seeds or were completely s t e r i l e . This cross has combined i n one i n d i v i d u a l the spread-ing root system of the Don and the erect habit of the Grimm. Rhizomes of the Don develop approximately 2 inches,below the surface of the ground. These break through the s o i l at varying distances from the centre of the p l a n t , thus gradually i n c r e a s -ing the circumference. The Grimm while being c h a r a c t e r i s t i c a l l y tap rooted, and tiier e f o r e subject to heaving by f r o s t , nas long erect stems, making i t s u i t a b l e f o r hay. The combination i n the hybrids, of spreading root system and t a l l erect stems produces.j a valuable hay crop, Ysfhich i s more r e s i s t a n t to heaving. Since i t has been shown tnat there i s considerable v a r i a t i o n i n seed production as between the two-parents Grimm and Don i n the f i r s t place, and between the d i f f e r e n t hybrids i n the second place, the problem a r i s e s as to the nature of i n c o m p a t i b i l i t y between these low seed producing froms, and whether t h i s i n c o m p a t i b i l i t y i s i n h e r i t e d or not, and i f i n -h e r i t e d upon what genes the i n c o m p a t i b i l i t y depends. The quest-ion a r i s e s as to whether low seed production i s due to p o l l e n -s t e r i l i t y , n o n - d i s t r i b u t i o n of p o l l e n , or i n c o m p a t i b i l i t y , the l a t t e r r e s u l t i n g i n f a u l t y development of p o l l e n tubes, f u s i o n of gametes, or development of the embryo and endosperm. As a r e s u l t of p o l l i n a t i o n studies i n c l u d i n g morphological studies of preserved p o l l e n and p o l l e n tube growth i n sugar s o l u t i o n c u l t u r e s , studies i n t r i p p i n g of plants exposed to the a i r , enclosed i n cellophane sacs, and brought i n the laboratory, as w e l l as h i s t o l o g i c a l studies of carpels c o l l e c t e d 3 days and 6 days a f t e r t r i p p i n g , evidence i n d i c a t e d 6 conclusions; (Helen M. F a r l e y , A Study of the P a r t i a l S t e r i l i t y of C e r t a i n A l f a l f a S t r a i n s I , 1934 - 1935) 1. High seed production i s not r e l a t e d to t r i p p i n g . 2. High seed production i s g e n e r a l l y r e l a t e d to the r a t e of p o l l e n tube growth. 3. Where there i s low seed production there i s poor development of gland c e l l s l i n i n g the duct of the c a r p e l . 4. When ovules degenerate, t h i s begins immediately a f t e r f e r t -i l i z a t i o n would normally take place. 5. The ovules more remote from the s t y l e are most fre q u e n t l y a b o r t i v e , or seed development takes place from the ovules s i t u a t e d i n the upper part of the ovary i n contrast to the l e s s frequent development i n the lower p o r t i o n . This would seem to be dependent upon the rate of growth of the p o l l e n tubes i n r e l a t i o n to the maturity of the egg. 6. Forms with low seed production generally show poor endosperm development at l e a s t i n the e a r l y stages, even i n ovules where f u s i o n occurs and the endosperm begins to develop normally. Experimental methods The j? 2 m a t e r i a l f o r t h i s study was c o l l e c t e d i n J u l y and August of 1935 on the U n i v e r s i t y of B r i t i s h Golumhia Farm. The nature of this, hybrid m a t e r i a l lias already been given ( I n t r o d u c t i o n ) . Eight stages, of H 7-7, a high Grimm seed pro-ducer , were taken beginning before reduction d i v i s i o n of the p o l l e n mother c e l l s to two days a f t e r t r i p p i n g . The stage two days a f t e r t r i p p i n g was also c o l l e c t e d i n the case of H 68-5, low Grimm s.eed producer, H 7-33, high D Q n seed producer,, H 71-26, low Don seed producer, and Parent Grimm, i . e . Hedicago media. In the abo.ve eases where m a t e r i a l was c o l l e c t e d -after t r i p p i n g , or at time of t r i p p i n g s t r i p p i n g was a r t i f i c i a l l y e f f e c t e d . In a d d i t i o n to. the above a r t i f i c i a l l y t r i p p e d stages of H 7-7, one stage was c o l l e c t e d w i t h i n twenty four hours f o l l o w i n g the time at which n a t u r a l t r i p p i n g occured* B.C. F i x i n g s o l u t i o n , a formal acetoalcohol f l u i d was used- i n k i l l i n g and f i x i n g . L o n g i t u d i n a l sections were cut ten microns i n thickness from p a r a f i n preparations. L i g h t green and s a f r a n i n were used i n staining.. Development of the male and female gametophytes of a l f a l f a . Male gametophyte. The microsporocytes as they l i e i n the anther f l u i d a f t e r rounding up, undergo two d i v i s i o n s to form quartets of microspores. Cytokinesis does not occur u n t i l a f t e r the second meiosis, when the c e l l i s divided i n t o four spores ( P l a t e I , f i g s . I & 2). Each microspore develops a t h i c k w a l l enclosing the male gametophyte ( P l a t e I , f i g . 3 ) . At t h i s stage some of the p o l l e n grains begin to appear shrunken or aborted (Pl a t e I , f i g . 4 ) . In the other p o l l e n grains the microspore nucleus which has the reduced number of chromosomes d i v i d e s i n t o two, around •one of which a membrane cuts out a generative c e l l (Plate I , f i g . 3 ) . The other nucleus and the r e s t of the microspore cyto-plasm c o n s t i t u t e the tube c e l l , which l a t e r grows out to form the p o l l e n tube. The generative c e l l d i v i d e s to form two male gametes. This d i v i s i o n occurs i n the growing p o l l e n tube a f t e r p o l l i n a t i o n , as i t i s making i t s way down the s t y l e (Plate I , f i g . 5). Ordin-a r i l y only two sperms enter the embryo sac. One unites w i t h the egg, the other with the f u s i o n nucleus. In a number of embryo sacs however, s p i r a l or vermiform sperms were seen a f t e r f e r t i l -i z a t i o n . ( P l a t e I I , f i g . 14). Circumstances i n d i c a t e d that these were supernumerary sperms. According to Sharp the male n u c l e i i n many species become vermiform, p a r t i c u l a r l y a f t e r entering the embro sac. This change may occur xore r a p i d l y i n one nucleus than i n the other, the two thus being u n l i k e at l a t e stages. The male gametophyte of a l f a l f a shows no marked modi f i c a t i o n s from the common development found i n Angiosperms. P o l l e n tubes were more numerous i n the s t y l e s and ovaries of high seed producers than of low, Eh?-7 c o l l e c t e d w i t h i n 2.4 hours a f t e r n a t u r a l t r i p p i n g shows, numerous, p o l l e n tubes i n the upper p o r t i o n of the s t y l e (Chart I , row 2)„ Plate VT, f i g . I shows two p o l l e n tubes passing down the "style before reaching the ovules. P l a t e I , f i g * 6 shows a p o l l e n tube w i t h i t s tube nucleus and two: sperms, entering the micropyle of the t h i r d ovule. E 7-7 at time of a r t i f i c i a l t r i p p i n g , a stage a l i t t l e e a r l i e r than the above, does not show p o l l e n tubes (Chart I* row I ) . However i t would seem that f e r t i l i z a t i o n has taken place since a sperm appears to be f u s i n g w i t h the egg. I f this. Is a sperm, then s e l f f e r t i l i z a t i o n has taken place. In H 7-7 at two. days a f t e r a r t i f i c i a l t r i p p i n g , p o l l e n tubes ?/ere num-erous:,, and l a t e p o l l e n tubes were s t i l l l making t h e i r way down the s t y l e (Chart I , row 5 ) . P o l l e n tubes are fewer i n the lower regions of the ovary and none were seen i n the passages, below the 9th ovule. I t is. evident however that they had reached th 14th ovule since a sperm i s i n the process, of f e r t i l i z i n g the egg, and a second, near the micropylar reg i o n , i s advancing t o -wards the f u s i o n nucleus w i t h which i t fuses to form endosperm (Plate I I I , f i g s . 21-23). Mo p o l l e n tubes were seen l a t e r than 2 days a f t e r a r t i f i c i a l t r i p p i n g . - 7 -P o l l e n tubes i n H 6-8-5, low seed producer of Grimm type, are few i n number at two days: a f t e r t r i p p i n (Chart I , row 4 ) . Only around the f i r s t ovule were they observed., although glandular hairs, were found to the f o u r t h ovule. The c a r p e l stud-led, contained 8 ovules;, the second to the l a s t being decidedly shrunken. P o l l e n tubes had reached the l a s t ovule, although too l a t e to e f f e c t proper f e r t i l i z a t i o n . The egg had become over mature, and the sperm maintained i t s membrane whiie entering the egg, whereas: normally the membrane, would disappear. (Plate IV, f i g s . 24-26)., Ho p o l l e n tubes were observed near any of: the ten ovules of Parent Grimm, 2 days a f t e r a r t i f i c i a l t r i p p i n g . (Chart I, row 5). This i s a r e s u l t of the advanced, development found here. Young embryos and endosperm had formed i n the f o u r t h and s i x t h ovules. (Plate I I , f i g . 16). Apparently p o l l e n tubes had not. reached the tenth ovule where the egg s t i l l appears unfer-t i l i z e d . P o l l e n tubes found i n H. 7-33, high seed producer of the Don type at two days, a f t e r t r i p p i n g , were, considerably fewer than i n H 7-7, and were again i n the upper regions of the ovary only. JTone were seen i n H 71-26, low seed producer of Don type. I t appears that i n this, hybrid development took place normally up u n t i l the time p o l l i n a t i o n would normally take place and then d i s i n t e g r a t i o n began. I t i s evident that the seed prod-u c t i o n of the plants, concerned has decreased d i r e c t l y as the number of p o l l e n tubes. -8-The p o l l e n tubes are"dependent upon glandular h a i r s f o r food supply, and i n the m a t e r i a l studied there was a def-i n i t e c o r r e l a t i o n between them. The glandular h a i r s are several c e l l s i n length, and are attached to the w a l l of the ovary at the hase of the f u n i c u l u s as w e l l as along the s t y l a r passage. The p o l l e n tubes grow along the surface of the h a i r s , along the f u n i c u l u s and Into the micropyle (Plate VI,, f i g . 2 ) . Glandular h a i r s are most abundant i n the upper regions of the ovary, and do not grow to any great extent below the point where p o l l e n tubes were observed i n the ovary* These h a i r s are more prevalent In some hybrids than others. In H 7-7 a high seed producer of Grimm type, they are much more vigorous, and are more abundant and l a r g e r i n s i z e than i n H 68-5, a low seed producer of Grimm type, or H 7-33, a high seed producer of Don type. In H 71-26 development of h a i r s i s very poor. Hence there appears to be a double r e l a t i o n s h i p between development of glandular h a i r s and p o l l e n tubes - the r e l a t i o n s h i p of t h e i r p o s i t i o n i n the ovary, and the r e l a t i o n s h i p of t h e i r frequency i n low and high seed producers r e s p e c t i v e l y . A second r e l a t i o n s h i p Is found between development of vascular bundles, and p o l l e n tubes, p a r t i c u l a r l y i n the s t y l e , where vascular bundles are I n v a r i a b l y found i n the v i c i n i t y of • p o l l e n tube's. To sum up i t may be s a i d that p o l l e n tubes are more numerous i n the upper regions of the ovary than i n the lower regions. Some have more r a p i d growth and f e r t i l i z e the upper ovules, whereaa those of slower growth f e r t i l i z e the lower ovules or f a i l to f u n c t i o n i n cases where the female gametophyte has passed maturity a t the time of t h e i r a r r i v a l . In other cases where the p o l l e n tubes have not been f u l l y nourished, that i s there i s an i n s u f f i c i e n c y of glandular h a i r s and vascular bundles, the p o l l e n tubes may discontinue growth before they a r r i v e at an u n f e r t i l i z e d gametophyte. Female gametophyte In d e a l i n g w i t h the female gametophyte of the a l f a l f a studied f o r t h i s t h e s i s a comparison might be drawn between H 7-7, a r e s u l t of the cross Medicago s a t i v a X Medioago f a l c a t a , w ith the work done by Reeves, and that done by Cooper on Medicago s a t i v a . Each,ovule of M. s a t i v a as described by Reeves contains I to 3 primary sporogenous c e l l s . Where more than one, these l i e i n a plane transverse to the l o n g i t u d i n a l a x i s of the ovule i n some instances, but o c c a s i o n a l l y they are i n the plane of the l o n g i t u d i n a l a x i s , while i n H 7-7 not more than a s i n g l e sporogenous c e l l i n any ovule was found. As i n s a t i v a (Cooper & Reeves) the primary sporogenous c e l l of H 7-7 functions d i r e c t l y as the macrospore mother c e l l . In consequence of the two meiotic d i v i s i o n s of the si n g l e macrospore mother c e l l , four macrospores only are present i n H 7-7, as compared with as mahy as 8 i n s a t i v a (Cooper & Reeves). "In s a t i v a the c h a l a z a l megaspore develops i n t o an 8 nucleate, 7 c e l l e d embryo sac"; the other macrospores d i s i n t e -grate, and the antipodals p e r s i s t f o r some time a f t e r f e r t i l i z -a t i o n (Cooper). In some gametophytes an e a r l y 6 nucleate stage i s followed by the f u s i o n of two to form the f u s i o n nucleus, the formation of three c e l l s at the micropylar end, the egg and synergids, and the disappearance of the s i x t h which d i s i n t e -grates e a r l y , long before the female gametophyte i s mature. The occurrence i n hybrid 7-7 i s s i m i l a r to that found by Reeves i n - I I -s a t i v a . In other gametophytes of H 7-7 there were 5 or 7. Where 5 n u c l e i are formed, the egg, synergids and one c e l l of the -r f u s i o n nucleus form at the micropylar end. The evidence i n d i c -ates that the nucleus at the antipodal end does not d i v i d e , hut fuses d i r e c t l y w i t h the nucleus which migrates from the micro-p y l a r end. This delays the formation of. endosperm. Just p r i o r to f e r t i l i z a t i o n the micropylar ends of the syfiergids elongate, and'-extend''into t he .'•micropyle '(Cooper) . I n H 7-7 the synergids are beaked, the beaks extending towards the micropyle. In s a t i v a f e r t i l i z a t i o n takes place between 24 and 27 hours a f t e r p o l l i n a t i o n . The synergids are not broken down by the p o l l e n tube but p e r s i s t f o r some time a f t e r f e r t i l i z a t i o n (Cooper). I n H 7-7 f e r t i l i z a t i o n occurs at the same time but the synergids disappear r a p i d l y . I n s a t i v a 2 c e l l e d proembryos are to be found 31 hours a f t e r p o l l i n a t i o n . The endosperm at t h i s time contains 2 -4 n u c l e i (Cooper). I n H 7-7 at 48 hours, endo-sperm w i t h l e n t i c u l a r c e l l s had formed i n the upper 3 ovules. In s a t i v a the male n u c l e i are not rounded but more elongate (Cooper). I n H 7-7 they are a l s o more elongate, some being s p i r a l or vermiform. Circumstances i n d i c a t e d that the l a t t -er were supernumerary sperms which had a r r i v e d a f t e r f e r t i l -i z a t i o n had taken place. Although there i s an abundance of p o l l e n tubes i n the ovaries of s a t i v a at the time of f e r t i l i z a t i o n , only about h a l f or l e s s of the ovules show the presence of proembryos at 31 and 48 hours a f t e r p o l l i n a t i o n . The cytoplasm of an embryo sac i n -12-whioh f e r t i l i z a t i o n has not occurred remains apparently normal f o r a considerable time, the f i r s t evidences of d i s i n t e g r a t i o n being found i n the ovules c o l l e c t e d 72 hours a f t e r p o l l i n a t i o n . This d i s i n t e g r a t i o n continues and u l t i m a t e l y the whole ovule becomes involved, so that at 120 hours a f t e r p o l l i n a t i o n the u n f e r t i l i z e d ovules are small and very much shrunken. In heavy seed-setting l i n e s of a l f a l f a there i s an average of 3-4 seeds per pod, whereas 10-12 ovules are present i n each ovule. An examination of about 100 ovaries revealed a range i n , number of ovules per ovary from 8-14, and a.range i n number of f e r t i l i z e d ovules per ovary from 1-5, the average being 3-4 (Cooper), In H 7-7 there are numerous p o l l e n tubes, and exam-i n a t i o n of ovaries reveals a range i n number of 12-16 ovules per ovary. The average number of f e r t i l i z e d ovaries per ovary i n H7-7 i s three. Seed development occurs most fr e q u e n t l y i n the f i f t h , s i x t h and seventh ovules from the s t y l a r end of the ovary. D i s i n t e g r a t i o n i n ovules where f e r t i l i z a t i o n has f a i l e d proceeds r a p i d l y , some ovules d i s i n t e g r a t i n g before t r i p p i n g . At 96 hours the ovules have become completely shrunken. As i s c h a r a c t e r i s t i c of the Leguminosae, the ovules s a t i v a a r i s e i n two rows, one on each side of the v e n t r a l sutre The f u n i c u l u s of the ovule becomes geniculate i n shape and the ovule curved so that i t becomes campylotropous, with the micro-pyle against the f u n i c u l u s . The embryo sac i s long and somewhat curved at maturity (Reeves). The ovules of H 7-7 are paired campylotropous, and the embryo sec i s curved at maturity. - r o -sy way of. introducing the work to f o l l o w i t would be w e l l to study f u r t h e r the development of the female gametophyte, p a r t i c u l a r l y that of H 7-7. There are c e r t a i n c h a r a c t e r i s t i c s which are outstanding and i n d i c a t e to a large extent whether an i n d i v i d u a l plant i s high or low seed producing. In general those ovules which l i e i n the upper regions of the ovary have t h e i r female -garnet ophytes uinore advanced than those i n the lower regions (Chart I,, rows I & 3 ) . Whereas the nuclei, of the ovules i n the lower p o r t i o n of the ovary have not yet fused to form the f u s i o n nucleus, i n the upper regions of the ovary f u s i o n has occured and sperms have entered the embryo sac.(Chart I, row I ) . S i m i l a r l y (Chart I, row 3) the gametes i n the I4th. ovule are j u s t ready to fuse, and s e v e r a l c e l l s of the inner n u t r i t i v e l ayer s t i l l remain,, while i n the uppermost ovules not only equally dimensioned endosperm c e l l s , c h a r a c t e r i s t i c of the wandering and d i v i d i n g c o n d i t i o n , have formed, but l e n t i c -u l a r endosperm c e l l s are developing towards the periphery of the sac. In the I4th. ovule the presence of sperms, the absence of endosperm and the f a c t that f u s i o n of tiie egg and sperm has not occurred, i s good evidence that endosperm.does not develop u n t i l f e r t i l i z a t i o n has taken place, and there i s evidence that a f t e r a c e r t a i n stage of maturity f u s i o n does not occur although l a t e a r r i v i n g sperms, may be present„ The main supply -of food w i t h i n tne female gametophyte a f t e r the t a p e t a l layer has "begun to d i s i n t e g r a t e , i s i n the form of s t a r c h g r a i n s . They are very abundant around the female -14-gametophyte c e l l s of low seed producers, and i n nigh seed prod-ucers are more abundant i n the lower ovules than i n the upper ovules:. Comparing H 7-7 w i t h H 68-5 and Parent Grimm at 2. days a f t e r a r t i f i c i a l t r i p p i n g (Chart 1, rows. 3,4, & 5) i t i s ev i d -e n t l t h a t while E 7-7, a high seed producer has l i t t l e or no st a r c h i n the upper ovules, Parent Grimm has an intermediate number of s t a r c h grains and E 68-5, low seed producer, has numerous s t a r c h g r a i n s . I t would appear that the s t a r c h present i n low-seed producers i s i n an unavailable form and cannot be u t i l i z e d as food m a t e r i a l . In addit;io.n to the f a c t that the upper ovules are more advanced than the lower ovules, the ovules of d i f f e r e n t plants vary i n degree of development. Parent Grimm i s most advanced, showing young embryos- i n two of the oviiles at two. days, a f t e r t r i p p i n g (Chart I , row 5 & P l a t e I I , f i g . 16). The endo-sperm cells.; however are only i n the wandering and d i v i d i n g con-d i t i o n whereas i n H 7-7 endosperm w i t h l e n t i c u l a r c e l l s has developed (Chart I , row 3 ) . In Parent Grimm a few s t a r c h grains remain, and the inner n u t r i t i v e layer; i s almost complete i n the tenth or l a s t ovule. In the sac of the f i r s t ovule of H 7-7 endosperm w i t h l e n t i c u l a r c e l l s has formed and i s beginning to- develop towards the periphery of the sac. There are s t i l l a few d i s i n t e g r a t i n g c e l l s of the inner n u t r i t i v e l ayer of the megasporangium, but no trace of s t a r c h g r a i n s * In the second and t h i r d ovules.,, some of the endosperm c e l l s have become l e n t i c u l a r i n shape, but the remainder are rounded, c h a r a c t e r i s t i c of the wandering and d i v -i d i n g c o n d i t i o n (Plate I I , f i g . 18). Ovules 6 and 7 show best development of the male and female gametophytea and contain supernumerary s p i r a l sperms. In the 13th ovule there s t i l l remain a number of c e l l s of the inner n u t r i t i v e l a y e r . There i s a. large representation of s t a r c h g r a i n s , as before mentioned, and s e v e r a l giant food bodies which are, by a l l appearances p r o t e i n i n nature. In the l a s t ovule no endosperm has developed, st a r c h grains are p l e n t i f u l , and the inner n u t r i t i v e l a y e r s t i l l remains f a i r l y i n t a c t (Plate I I I , f i g . 21), although i n i t i a l d i s i n t e g r a t i o n i s evident. H 71—26 and H 7-33, low and intermediate seed prod-ucers, of the Don type r e s p e c t i v e l y , show no endoaperm develop-ment, and abundant food m a t e r i a l around the female gametophyte (Plate IV, f i g s . 24-26 & 27-30). Results i n my l a s t paper, "A study of the p a r t i a l s t e r i l i t y of c e r t a i n a l f a l f a s t r a i n s I " , i n d i c a t e d that i n 1935, H 7-33 would again produce a law seed y i e l d as i t had done i n 1923. Judging from the sections made t h i s year the y i e l d would again be low. The egg, f u s i o n nucleus and synergids of H 71-26 have a very collapsed appearance (Plate IV, f i g s . 27-30). The f u s i o n nucleus, never @©uid d i v i d e , a,nd the synergids, apparently d i s -i n t e g r a t i n g , do not promise normal development. The lower por-t i o n of the ovary i s d i s t o r t e d , and numerous long h a i r s are on the outside w a l l of the ovary - a Don c h a r a c t e r i s t i c . I t would appear that i n the m a j o r i t y of ovules some development took place, and then" d i s i n t e g r a t i o n began. Seed production as i n t e r p r e t e d from data represented i n charts I and 2. In the hybrid a l f a l f a s 68-5 and 7-7, and Parent Grimm best seed production occurs i n the f i f t h , s i x t h and seventh ovules from the s t y l a r region. Above these the ovules develop i n p a r t , forming endosperm only i n the m a j o r i t y of cases, whereas i n other instances the embryo forms, but no endosperm as i n H 7-7 at 6 days, or there i s a poor development of both as i n E 7-7 at 9 days. The f i r s t ovules mentioned above may be ref e r r e d to as the seed-producing ovules. Those above as the upper non-seed-producing ovules. Non seed production i n the upper ovules may be a t t r i b u t e d to e i t h e r of two p o s s i b l e causes. I t may be the r e s u l t of cross p o l l i n a t i o n - the p o l l e n tubes a r r i v i n g at the ovules before the maturity of the egg and f u s i o n nucleus r e s u l t -ing i n poor development of both embryo and endosperm as shown i n H 7-7 at 9 days, or i t may be that the p o l l e n tubes a r r i v e before the maturity of the egg but at maturity of the f u s i o n nucleus, r e s u l t i n g i n development of endosperm but not of embryo as i n H 7-7 at 2 days and H 7~7 at 9 days. This time r e -l a t i o n s h i p appears to l o s e s i g n i f i c a n c e i f the development of the f u s i o n nueleus i s delayed i n i t s e a r l y stages (see page I I ) . I t i s not l i k e l y t hat the egg would become retarded i n develop-ment i n l a t e r stages, thus a l l o w i n g the f u s i o n nucleus to super--r.7-cede-.ft inndevelopment. This i s a p h y s i o l o g i c a l explanation. A second theory to e x p l a i n non-seed-production i n the upper ovules i s s e l f p o l l i n a t i o n and a Low degree of c o m p a t i b i l i t y . The f i r s t evidence of s e l f f e r t i l i z a t i o n i s found i n H 7-7 at the time of nat u r a l t r i p p i n g . P o l l e n tubes were seen passing down the s t y l e (Plate. VI, f i g . . I ) , and. sperms were seen i n a p o l l e n tube enter-ing the-micropyle of an ovule In the upper p o r t i o n of the.ovary (Plate ,L, f i g . 6 ) . Ho. a c t u a l fusions .were seen. The low degree of c o m p a t i b i l i t y between tft.2 male and female gametes appears f o r the most part etween the egg and sperm, r e s u l t i n g i n ovules, containing endosperm only, as shown i n H. 7-7 at 2-days, and Parent Grimm at 2-days.; In other cases as i n H 7-7 at 6 days where the ovule contains an embryo but no endosperm, apparent i n c o m p a t i b i l i t y i s between the f u s i o n nucleus and sperm. Where i n f e r i o r embryas and. endosperm develop i n the same embryo i n c o m p a t i b i l i t y i s between both the egg and sperm, and f u s i o n nucleus and sperm. In the f i f t h , s i x t h and seventh ovules, or tne seed-producing ovules, seed production appears to be the r e s u l t of cross p o l l i n a t i o n - the p o l l e n tubes a r r i v i n g at the ovules at the m aturity of the egg and 244- hours a f t e r t r i p p i n g . This i s believed to bring about best seed development r e s u l t i n g i n the production of normal embryos, and a la r g e amount of endosperm. In the lower non-seed-producing ovules examined, complete abortion has occured i n a l l ovules except ovule 12 of H 7-7, 6 days. As p o l l e n tubes were seen to ovule 9 and sperms i n ovule 14 of H 7-7 at 2 days (see Gharts I & 2 ) , evidence -18-indioates that p o l l e n tubes penetrate as f a r as the fourteenth ovule. Yet at 6 and 9 days a f t e r a r t i f i c i a l t r i p p i n g complete abortion has occured below the seventh ovule i n p r a c t i c a l l y a l l cases (Chart I I , rows 10-13). - h i s i s i n t e r p r e t e d as the r e s u l t of cross p o l l i n a t i o n followed by the a r r i v a l of sperms l a t e r than maturity of the egg and f u s i o n nucleus. An i n d i c a t i o n of the f a c t that s e l f p o l l i n a t i o n occurs i n the upper non-seed-producing ovules, i s that there are d i f f -erent degrees and combination of development of embryo and endosperm i n the upper ovules, whereas i n the lower non-seed-producing ovules complete abortion occurs. East and Y a r n e l l (1929, Studies on s e l f - s t e r i l i t y VIII. S e l f - s t e r i l i t y allelomorphs. Genetics 14, 455-487) e x p l a i n the d i f f e r n c e between s e l f - f e r t i l e and s e l f - s t e r i l e plants by r a p i d i t y of p o l l e n tube growth. Own p o l l e n germinates on own stigmas j u s t as r e a d i l y as does f o r e i g n , and the f i r s t increment of growth of the two types takes place at the same r a t e . A f t e r that own p o l l e n tubes continue to grow s t e a d i l y and normally at the same rate they s t a r t e d , but the rate of growth of fo r e i g n tubes i s continuously accelerated as though they were r e c e i v i n g some stimulus which was i n e f f e c t i v e on own p o l l e n tubes. Own p o l l e n tubes f a i l to reach the ovary before the stigma and s t y l e have decayed while f o r e i g n tubes with t h e i r accelerated growth reach t h e i r goal before the way has become blocked. In a l f a l f a own p o l l e n tubes are i n t e r p r e t e d as slow i n growth, and as f e r t i l i z i n g the upper ovules thus i n c a p a c i t -a t i n g the l a t t e r to "become f e r t i l i z e d by f o r e i g n p o l l e n . A f t e r t r i p p i n g the accelerated f o r e i g n tubes f e r t i l i z e the f i f t h , s i x t h and seventh ovules hut below that region the egg has become too mature and f e r t i l i z a t i o n f a i l s . The t w e l f t h ovule of E 7-7 at 6 days may be regarded as an exception i n that development has occured where aborted ovules are u s u a l l y found. This probably i l l u s t r a t e s the f a c t that rapid p o l l e n tubes i n H 7-7 grow to that p o i n t , TEis i s again shown i n H 7-7 at 2-days where sperms were seen i n the fourteenth ovule. O r d i n a r i l y however the egg has passed matur-i t y before p o l l e n tubes penetrate below the tenth ovule, and o r d i n a r i l y f u s i o n does not take place, as i n the case of the fourteenth ovule ( P l a t e I I I , f i g s . 21-23). -20-Comparison of E 7-7, a high seed-producer, a'nd E 68-5, a low seed-producer, at 2-, 6-, and 9-days a f t e r t r i p p i n g . 1. f here i s a smaller number of ovules i n E 68-5 - a maximum of I I as compared w i t h 18. 2. P o l l e n tubes are seen to the f i r s t ovule only at 2-days i n E 68-5; and f i n a l l y , sperms were found i n the 8th ovule as compared with the 14 i n E 7-7. l o supernumerary sperms were seen i n E 68-5 whereas several were found i n E 7-7. 3. Embryos develop i n ovules from 2-6 i n E 68-5. In E 7r-7 they are found from 2-12. 4. Quite a large amount of st a r c h i s present i n the embryo sacs of E 68-5 at 6 and 9-days but none was observed i n E 7-7, and at two days star c h was present i n a l l ovules of E 68-5 but only i n the lower ovuJ.es of E 7-7. 5* THe best developed ovules of E 68-5 at 9-days,are not as w e l l developed as those of E 7-7 at 9-days, p a r t i c u l a r l y i n respect to endosperm. 6. In E 68-5 the d i s i n t e g r a t i o n and shrinkage of ovules i s more rapid and i s complete by the 6th or 9th day, while i n E 7-7 a l l stages of development from the many ce l l e d embryo and endosperm to the completely disorganized ovule are present at t h i s time. Comparison of H 7-7, H 68-5 and Parent Grimm at 2-days a f t e r t r i p p i n g . 1. H 68-5 i s d i s t i n c t l y retarded i n development as compared w i t h H 7-7 and Parent Grimm. 2. H 68-5 has fewer ovules than H 7-7 and Parent Grimm. 3. No embryos had formed i n H 68-5 whereas they had i n H 7-7 and Parent Grimm. 4. In ovule 14 of H 7-7 the gametes are ready t o fuse, and i n ovules 8 of H 68-5 they are a l i t t l e past maturity f o r e f f e c t i v e f u s i o n whereas ovules 8-10 of Parent Grimm appear to be d i s i n t e g r a t i n g . 5. P o l l e n tubes were seen only as f a r as the 1st ovule i n H 68-5 whereas they were seen down as f a r as the 9th i n H 7-7. 6. The upper ovules of K 68-5 seem to be at the same stage of development as the lower ovules whereas i n H 7-7 and Parent Grimm there i s a d i s t i n c t d i f f e r e n c e , the upper ovules developing e a r l i e r . 7. D i s i n t e g r a t i o n of eggs to some extent occurs i n a l l three. 8. Starch i s abundant i n a l l ovules of H 68-5 but only i n the lower ovules of H 7-7. Review of l i t e r a t u r e * S t e r i l i t y i s c l a s s i f i e d i n t o three fundamentally d i f f e r e n t kinds (Crane and Lawrence). Generational s t e r i l i t y , i s s t e r i l i t y due to the f a i l u r e of any of the processes con-cerned w i t h the normal a l t e r n a t i o n of generations, namely, development of p o l l e n , embryo sac, embryo and endosperm* and the r e l a t i o n s of these with'one another and w i t h t h e i r parents regardless of the cross made. Morphological s t e r i l i t y i s sup-pression or abortion o r the reproductive organs* The t h i r d kind i s i n c o m p a t i b i l i t y * I n c o m p a t i b i l i t y best explains the s t e r i l i t y found i n the hybrid a l f a l f a s studied i n t h i s work. Of the three theories of i n c o m p a t i b i l i t y - the g e n e t i c a l , the chromosome, and the p h y s i o l o g i c a l , the l a s t most nearly p a r a l l e l s t h e s s i t u a t i o n . P h y s i o l o g i c a l l y , s e l f - and c r o s s - i n c o m p a t i b i l i t y may be brought about by f a i l u r e of pollen.tubes to reach the egg, by f a i l u r e of zygote formation a f t e r the male nucleus has enter-ed the embryo sac, or abo r t i o n of embryos during the e a r l y stages. In a l f a l f a , i n c o m p a t i b i l i t y i s brought about by f a i l u r e of the p o l l e n tubes to reach the egg i n time f o r e f f e c t i v e f u s i o n of the gametes • Jo-at (19Q7) f i n d s that i n L i Hum the p o l l e n tubes of s e l f - i n c o m p a t i b l e p l a n t s grow so slowly that f e r t i l i z a t i o n i s not achieved* High seed production i s therefore r e l a t e d to rate of growth of p o l l e n tubes. -Martin (1913) found that p o l l e n tubes i n s e l f - p o l l i n a t e d red clover grew much more slowly than - 2 4 -p o l l e n tubes i n a cross p o l l i n a t e d p l a n t , Goe and. Martin (1920) and Williams say t h i s f a c t may be c o r r e l a t e d with the slow growth of i t s own p o l l e n tubes. Sansome studied the p o l l e n tubes i n Grane^s apple m a t e r i a l and found on s t y l e d i s s e c t i o n that there was a great d i f f e r e n c e i n growth r a t e i n the s t y l e , between compatible and incompatible p o l l e n . Other i n v e s t i g a t o r s among- whom are Correns (1913) f o r Gardamine pratense, Gbmpton (1913) f o r Roseda, Moore (1917) f o r Tradescantia, Grane and Lawrence f o r c h e r r i e s , plums and apples. East and Mangelsdorf f o r Hlcotiana* hold that the p o l l e n tube growth i n the s t y l e i s the cause of c o m p a t i b i l i t y or i n c o m p a t i b i l i t y of the matings i n these p l a n t s . "yfliether incompatible p o l l e n tubes are i n h i b i t e d i n growth, or whether both i n h i b i t i o n and a c c e l e r a t i o n may be present i n these plants, i s as yet undecided. The success of p o l l i n a t i o n In the bud, or incompatible matings i n H l c o t i a n a , red c l over and cabbage does not decide the matter since the development of i n h i b i t o r y subatances may be r e s t r i c t e d to a c e r t a i n period i n s t y l a r degeneration. End-season f e r t i l i t y , by which normally, s e l f - s t e r i l e plants may give some seeds l a t e i n the season of growth, haa been reported i n ITieotiana, East (1923) and Anderson (1924), and i n Lythrum, Stout (1922). I t has been used by Kaki&aki (1930) to support the view that there i s an i n h i b i t o r y a c t i o n which wanes at the end of the season. This hypothesis, however, i s open to the objection that the flower does not wither so q u i c k l y at that time, therefore the - 2 5 -alower growing tubes achieve f e r t i l i z a t i o n owing to the time f a c t o r alone* Discussion and conclusions The development of the female gametophyte, at the time the p o l l e n reaches the ovule, i s a very important f a c t o r i n seed, production, not only i n d i f f e r e n t flowers hut i n ovules, of the same flower. Consequently the v a r i e t i e s which show a high degree of s e l f t r i p p i n g at the time 24+ hours before ovule maturity are i n most cases high seed producing.. V a r i e t i e s which are not r e a d i l y s e l f t r i p p e d may be f a c i l i t a t e d i n seed prod-u c t i o n by the v i s i t s of insects.. That i s the i n s e c t s seem to have 2 b e n e f i c i a l e f f e c t s ; 1. A greater proportion of t r i p p i n g before the time of matur-i t y of the egg. 2. Transfer of p o l l e n since cross p o l l e n seems to show greater c o m p a t i b i l i t y (Carlson; P i p e r ; Frandsen; K i r k ; T o r s e l l ; Dwyer) i In other words there seem to be 2 important f a c t o r s i n a l f a l f a , seed production; ( i ) c o m p a t i b i l i t y of the p o l l e n (Carlson; Piper; Prandsen; K i r k ; T o r s e l l ; Dwyer) and (2) the time of a r r i v a l of the p o l l e n tube i n r e l a t i o n to the develop-ment of the ovule. This time f a c t o r may be s a t i s f i e d by w e l l regulated s e l f t r i p p i n g or by the frequent v i s i t s of s u i t a b l e i n s e c t s . I t would appear that i n extreme cases the greater weight of humble bees as compared w i t h honey bees i s a f a c t o r in. this, time period of t r i p p i n g . -27-The work recorded here gives some very d e f i n i t e evidence as to the importance of the time of p o l l i n a t i o n and f e r t i l i z a t i o n and the number and c o n d i t i o n of the ovules at t h a t p e r i o d . -28-Oonclusions 1. S e l f f e r t i l i z a t i o n occurs i n a l f a l f a , at l e a s t i n c e r t a i n forms. F e r t i l i z a t i o n sometimes occurs before t r i p p i n g . 2. Development of the male gametophyte f o l l o w s the usual type found i n Angiosperms. 3. Development of the female gametophyte i s unusual i n some respects. The usual 8 n u c l e i found i n the female gametophyte i s reduced i n number to 6 i n most cases: 5 and 7 were also obi 4. Where there i s low seed production there i s poor develop-ment of glandular h a i r s and p o l l e n tubes. 5. In the s t y l e vascular bundles are i n v a r i a b l y found i n the v i c i n i t y of p o l l e n tubes. 6. Development of glandular h a i r s and vascular bundles are r e l a t e d to rate of growth of p o l l e n tubes. 7. F e r t i l i z a t i o n occurs between 24 and 27 hours a f t e r p o l l i n -a t i o n and r a p i d disappearance of the synergids occurs. 8. A small number of ovules as i n Parent Grimm and H 68-5 gives a r e l a t i v e l y small margin of development time and o r d i n a r i l y a correspondingly small number of seeds. 9. Those ovules which l i e i n the upper regions of the ovary -29-have female gametophytes more advanced, than those of the ovules i n the lower regions. 10. The food store and general c o n d i t i o n of the female gam-etophyte, which does not seem to follow any r e g u l a r sequence w i t h i n the ovules of any given ovary, may determine seed development. -30-Bib l i a g r a p h y Anderson, E.M.., 1923, G e n e t i c a l aspects of s e l f - and cross-s t e r i l i t y . Amer.. Jour. Bat;, 10, 468-473. Armstrong, J.K., and White, W.J . . , 1935, Factors i n f l u e n c i n g seed s e t t i n g i n a l f a l f a . Amer. Jour. Bot. 17, 239-246. Brin k , R.A., and Cooper, D.C., 1936, The mechanism of p o l l i n a t -i o n i n a l f a l f a (Medicago s a t i v a ) . Amer. Jour. Bot. 23, 678-683. Coe, H.S,, and M a r t i n , J.N., 1920, Sweet clover seed I . P o l l i n -a t i o n studies of seed production. U.S. Dept. A g r i c . B u l l . , 344.. Crane and Lawrence, I93'I, S t e r i l i t y and I n c o m p a t i b i l i t y i n d i p l o i d and p o l y p l o i d f r u i t s . Jour. Genet., 24, 97-107. Crane and Lawrence, 1934, The genetics of garden p l a n t s . Macmillan & Co., L t d . , S t . Martins S t r e e t , London, 180-194. Cqo.per, D.C.,, I9J35, Macrosporogenesis and embryology of Medicago, Jour. A g r i c . Res. 51, 471-477. Dwyer, R.E.P., 1936, Lucerne breeding techneque. A new system of close breeding.. Herbage Reviews, V o l . 4, libs. 1-2, March - June. E a s t , E.M., 1923, Genetical aspects of s e l f - and c r o s s - s t e r i l i t y . Amer. Jour. Bot., 10, 468-473. -31-K a k i z a k i , 1 9 3 0 , Studies on the genetics and physiology of s e l f and cross i n c o m p a t i b i l i t y i n the common cabbage. Jap. Jour. Bot. 5 , 133. M a r t i n , 1913, '-Che physiology of the p o l l e n of T r i f o l i u m pratense. Bot. Gaz., 56, IJ-2-I26. Moore, C.W., 1917, S e l f - s t e r i l i t y . Jour. Hered., 8, 203-207. Reeves, R.G., 1930, Development of the ovule and embryo sac of a l f a l f a . Amer. Jour. Bot., 17, 239-246, i l l u s . . Sansome and P h i l i p , 1932, Recent Advances i n Plant Genetics. P. B l a k i s t o n ' s Son & Go. Inc., 1012 Walnut S t r e e t , P h i l a d e l p h i a . Sharp, 1934, I n t r o d u c t i o n to Cytology. Mc. Graw-Hill Book c o . , Inc. New York and London. Stout, A.B., 1922, S t e r i l i t i e s i n l i l i e s . J . Hered., 13, 369-373. D e s c r i p t i o n of p l a t e s . 3?3_3, *t s X© F i g . I H 7-7; Stage 3 Hounded up microsporocytes showing one of the 2 d i v i s i o n s i n the formation of the quartet of microspores. (X 1350) Fig„ 2- H 7-7: Stage 3 A t e t r a d of microspores.. (X 1350) F i g . 3 H 7-7 s Stage 4 A mature microspore containing tube and generative c e l l s . (X 1350) F i g . 4 H 7-7: Stage 4 A shrunken p o l l e n g r a i n whicn has not germinated. (X 1350) F i g * 5 H 7-7; 2 days a f t e r a r t i f i c i a l t r i p p i n g A p o l l e n tube making i t s - way down the s t y l e showing the tube, nucleus and 2 sperms. , (X 2400) F i g . 6 H 7-7: Within 24 hours a f t e r n a t u r a l t r i p p i n g . Two p o l l e n tubes, one of v/hich shows tae tube nucleus and two sperms., about to enter the micropyle. (X 778) F i g . 7 H 7-7: Stage 2. Result of the f i r s t d i v i s i o n of the megaspore mother c e l l . (X 1350) F i g . 9 H 7-7: Stage 3 Result of the second d i v i s i o n of the megaspore mother c e l l . (X 1350) -33-j?ig. 10 H 7-7: Stage 4. L a t e r stage showing one mega spore e n l a r g i n g and the others d i s i n t e g r a t i n g . (X 1350) f i g . I I H 7-7: Stage 4. Shows a s i m i l a r stage except that the megaspore has d i v i d e d to form the f i r s t 2 n u c l e i of the female gametophyte. (X 1350) F i g . 8 H 7-7: Stage 4. Megaspore has d i v i d e d to form 4 n u c l e i of the female gametophyte (X 1350) JTig, 12 H 7-7: Stage 4, D i v i s i o n s i n the megaspore mother •: c e l l r e s u l t i n g i n 6 n u c l e i . (X 1350) II - 3 . 5 -P l a t e I I E g . 13, 14 & 15 P i g . 13 H 7-7: 2 days a f t e r a r t i f i c i a l t r i p p i n g showing the egg and 2 synergids. (X 1350) P i g . 14 Same ovule showing the egg and 2 supernumerary sperms. (X 1350) P i g . 15 Same ovule showing the f u s i o n nucleus. (X 1350) P i g . 16 Parent Grimm: 2 days a f t e r a r t i f i c i a l t r i p p i n g show- ' ing a young embryo and several wandering endosperm c e l l s . (X 1350) P i g s . 17 and 18 H 7-7; 2. days a f t e r a r t i f i c i a l t r i p p i n g showing remnants of the inner n u t r i t i v e l a y e r and formation of endosperm but no development of an embryo. (X 1350) PLATE 11 -37-Plate I I I F i g s . 19 & SO F i g . 19 H 7-7: Time of a r t i f i c i a l t r i p p i n g showing the egg and a nucleolus which has become dislodged from the synergid - also a p o r t i o n of a sperm. (X 1350) F i g . 20 Same ovule showing the two synergids, the fu s i o n nucleus and the remainder of the sperm shown i n f i g . 19. (X 1350) F i g , 21, 22 & 23. F i g . 21 H 7-7: 2 days a f t e r a r t i f i c i a l t r i p p i n g show-in g the f u s i o n nucleus and a sperm - al s o s t a r c h g r a i n s . (X 778) F i g . 22 Same ovule showing the two synergids and the second sperm entering the egg - also starch grains and remnants of the inner n u t r i t i v e l a y e r . (X 778) F i g . 23 Same ovule showing a glandular h a i r , the micro-pyle of the ovule and the embryo sac contain-i n g the two synergids, and a mass of giant food bodies. There are no antipodal c e l l s . ( X 778) PLATE 111. - 3 9 -P l a t e IV. Pi g s . 24,25 & 26 P i g . 24 H 68-5: 2 days a f t e r a r t i f i c i a l t r i p p i n g show-ing i n the eighth ovule, the greater part of the egg, abundant starc h grains and remnants of the inner n u t r i t i v e l a y e r . (X 778) P i g . 25. Same ovule showing one beaked synergid and a sperm which has a r r i v e d too l a t e f o r e f f e c t i v e f u s i o n of the gametes and numerous st a r c h grains (X 778) P i g . 26 Same ovule showing the f u s i o n nucleus alongside of which i s a sperm which has a r r i v e d too l a t e f o r e f f e c t i v e f u s i o n of the gametes. (X 778) Pig s . 27, 28, 29 & 30. Pi g . 27 H 71-26: 2 days a f t e r a r t i f i c i a l t r i p p i n g show-ing a shrunken egg and a very large number of st a r c h g r a i n s . (X 778) Pi g s . 28-30 Other c e l l s of tne same sac showing the shrunken co n d i t i o n of the female gametophyte. (X 778) -41-Plate V. F i g . I H 7-7: Stage 4. Shows the paired campylotropous con d i t i o n of the ovules, the l e f t ovule of which shows a megaspore which has di v i d e d to form 4 n u c l e i of the female gametophyte (See plate I, f i g . 8). (X 260) F i g . 2 H 7-7: 2. days a f t e r a r t i f i c i a l t r i p p i n g showing pa i r e d campylotropous. (X 160) Plate VI. F i g . I H 7-7: Within 24 hours a f t e r n a t u r a l t r i p p i n g showing two pollen-tubes passing down the s t y l e . (X 726) F i g . 2 H 7-7: 2 days a f t e r a r t i f i c i a l t r i p p i n g showing a p o l l e n tube growing along a gland h a i r and a p o r t i o n of an ovule. (X 726) Pl a t e V I I . P i g . I H 7-7 : I day a f t e r a r t i f i c i a l t r i p p i n g . Shows an embryo sac containing one an t i p o d a l c e l l and the egg f u l l of food m a t e r i a l and surrounded by s t a r c h g r a i n s . X 270) F i g . 2 H 7-7: Time o f i . r r t i f i c i a l t r i p p i n g showing f u s i o n of the n u c l e i i n the formation of the f u s i o n nucleus, the egg and one synergid. (X 160) PLATE V F i g . 2 - 4 6 -C H A R T 11. 

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