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Davidsonia Mar 1, 1972

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Spring 1972
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Jv Cover
Flowering branches of 'Mock orange1
Philadelphus lewisii
Delicate foliage and pink flowers of
'Bleeding heart", Dicentra formosa
Spring 1972
Davidsonia is published quarterly by The Botanical Garden of The University of British
Columbia. Vancouver 8, British Columbia, Canada. Annual subscription, four dollars.
Single numbers, one dollar. All editorial matters or information concerning subscriptions
should be addressed to the Director of the Botanical Garden.
A cknowledgements
The scanning electron microscope photographs found on pages one to three were
prepared by Laszlo L. Veto of the Electron Microscope Laboratories, Biological Sciences,
The University of British Columbia. The photograph of Rhododendron ciliatum on page
one was contributed by Dr. C.J. Marchant.
The article on Philadelphus was researched and prepared by Mrs. Sylvia Taylor. Pen
and ink illustrations throughout this issue are by Mrs. Lesley Bohm. Rhododendron Pollen
Spring is the time of year we associate with the flowering of the many fine rhododendrons in the Pacific
Northwest. The special development of flowers signifies that important period in the life of a plant when
pollination may take place leading in most instances to successful fertilization and development of seeds —
the promise of a new generation.
Most plants shed their pollen from the anthers of the stamens as individual units. The pollen grains are
often highly ornamented. Rhododendrons are somewhat different than most flowering plants as they shed
their pollen in groups of four or tetrads. The tetrads are loosely associated with one another by the
presence of special thread-like structures called viscin. The name viscin is applied to these thread-like
structures because it is so similar to the birdlime prepared from the berries of the mistletoe, Viscum
(Meeuse 1961). Viscin is apparently derived from substances lining the inside of the pollen sacs of the
anthers (Erdtman 1952). Viscin is also found in the anthers of the evening primrose, fuchsia and fireweed.
The pollen of rhododendrons is not elaborately sculptured as can be seen in the microphotographs. Each
pollen grain has a modified tetrahedral outline and possesses three germinal furrows. Each germinal furrow
is contiguous with a germinal furrow in a neighboring pollen grain of the tetrad (Wodehouse 1959). A single
pore is found in each germinal furrow or colpus, hence the pollen is classified as 3-colporate.
The pollen grain of a rhododendron is extremely small. Each individual grain is approximately 50
microns in diameter - you would need 40 pollen grains laid end to end to stretch across the head of an
ordinary straight pin! It is estimated that each anther (each anther has two pollen sacs) contains about
250,000 pollen grains. Each rhododendron flower would produce about two and one-half million pollen
grains, each with the potential of producing two male gametes to effect fertilization of the eggs in the many
ovules in the pistil. The detailed microphotographs of the anther and pollen were taken with a Mark IIA
Cambridge Stereoscan Scanning Electron Microscope.
To effect the transfer of pollen to the stigma of the flower, a process known as pollination takes place.
The prominent exsertion and position of the anthers, plus the open nature of the corolla, indicates the
rhododendron flower may be wind pollinated. However the anthers are not versatile (swinging back and
forth like a seesaw on the filament or anther stalk) and the pollen, dispersed in tetrads is held together
loosely by viscin. Rhododendrons are best adapted to pollination by hawkmoths (Meeuse 1961, p. 188),
bumblebees, hummingbirds and butterflies. Darwin (1888) records the visitation of the bumblebee to a
Continued on Page 3
The flower of Rhododendron
ciliatum Hook. f. showing the
open corolla, stigma and
style, and the ten stamens
with prominent anthers. Left: Details of the external wall structure of the anther. X800. Right: The anther of a rhododendron
showing the two pollen sacs and filament or stalk. Pollen tetrads are dispersing from the terminal pores of
each pollen sac. X21
Right: Detail of the terminal pore of
the anther. X94
Left: The upper portion of the anther showing the two
sacs with their terminal pores. X64 The arrangement of the
pollen grains in a tetrad. The
attachment of three of the
four grains can be clearly seen
in the lower left portion of
the tetrad. X434
A single pollen tetrad.
Germinal furrow or colpus
can be seen in the central
portion of the tetrad where
two of the grains are attached. The thread-like
material called viscin can be
seen in both photographs on
this page. X865
hybrid rhododendron effecting pollination by entering the flower by the corolla throat; but, also acting as a
'burglar', cutting through the base of the corolla to collect nectar and thus not effecting pollination. In
addition to natural pollinators man has carried out extensive pollination of the species. Such pollinations
have produced the many new hybrids and cultivars that are so widely used and admired in our parks and
Darwin, C. 1888. The Effects of Cross and Self Fertilisation in the Vegetable Kingdom. John Murray, Albemarle Street,
Erdtman, G. 1952. Pollen Morphology and Plant Taxonomy. Angiosperms. Almqvist & Wiksell, Stockholm.
Meeuse, J.D. 1961. The Story of Pollination. The Ronald Press Company, New York.
Wodehouse, R.P. 1959. Pollen Grains. Hafner Publishing Company, New York. Philadelphus  lewisii Pursh
Member of the Family Hydrangeaceae
Natural Distribution
Philadelphus lewisii occurs from British Columbia southwards to northern California, and eastward
through southern B.C., Washington and most of Oregon to the continental divide in Montana, and from the
Snake River north to Idaho. In British Columbia there is sporadic occurrence of the species through the
Fraser Valley and into the Fraser Canyon. There is local occurrence in the Okaganan and Similkameen
Valleys and at Shuswap Lake in the Kootenays.
The habitat is characterised as the Arid Transition Zone. Philadelphus lewisii tolerates greatly varied
ecological conditions ranging from gullies, watercourses, rocky cliffs, talus slopes and rocky hillsides of
sagebrush desert and ponderosa or lodgepole pine to coastal Douglas fir and redwood forests, from sea level
along the coast to 7000' on the east side of the Cascades. In B.C. it is more common at low elevations on
dry hillsides east of the Cascades, whereas to the west of them it occurs sporadically at the borders of
woods near streams and lakes.
Philadelphus lewisii is extremely variable in vegetative and floral characteristics and appears to be
particularly responsive to local ecological conditions.
Rounded to erect deciduous shrub, 5 - 8 (10) feet tall, loosely branched.
The root system is fibrous.
The bark is red brown or yellowish brown on new twigs becoming gray and transversely cracked with age
and eventually exfoliating.
Twigs are erect to pendulous, glabrous to markedly hairy. Pith is solid, white and of moderate texture.
Buds are less than 0.7 cm in diameter, solitary, sessile with 2 nearly valvate, mostly hairy scales. End bud
mostly absent.
Leaf scars are opposite, half round with a thin membrane more or less covering the bud.
Leaves opposite, light green, thick, (1) 2.5 - 7 (9) cm long, (0.5) 1 - 4 (6) cm broad on floriferous
branches, sometimes slightly larger on new growth, ovate to narrowly ovate to narrowly elliptic, acute or
rarely obtuse at apex, obtuse or acute at base, entire to remotely denticulate on the older branches, often
strongly but remotely serrate dentate on vigorous new shoots, from rather strongly strigose or strigose
villous on both surfaces to glabrous above and sometimes also below except along the veins and vein angles,
3 (5) veined with the strongest pair not arising from the base giving a very characteristic appearance, petiole
3 - 8 mm long.
Flowers are usually produced over a two week period, in June in B.C., from May — July in other parts of
the range. The inflorescence is a determinate terminal raceme on lateral branches with 3, or very rarely 1,
terminal flowers and several pairs of axillary ones, the lower 2, 4 or 6 arising from the axils of gradually
reduced leaves, the upper ones inconspicuously bracteate or even bractless, 3-11 flowers in each. Flowers
are 2 - 2.5 cm across, white, slightly fragrant to very fragrant, perfect and regular. Calyx is turbinate and
campanulate, glabrous to sparsely strigose, the lower 6 mm adnate to the ovary up to the level of the lobes
which are 5 - 6 mm long, narrowly ovate to ovate, more or less acuminate and 4 (5) in number. Corolla
cruciform, petals 4 (5), white, ovate to nearly obovate, (5) 10 - 20 (25) mm long, obtuse or rounded and
often emarginate. Stamens 25 - 40 (50), unequal, the longest about half the length of the petals, filaments Mt     r
6      h
s     h
i      h
4 ft
FIGURE I.    Philadelphus lewisii.   A. floral raceme and detail of leaf venation,   B. individual flower bud,   C. vertical section of
individual flower,   D. mature fruits,   E. cut-away view of mature capsule,   F. typical habit of shrub. glabrous or occasionally with 1 or more hairs above, anthers ovate-oblong, glabrous or with 1 to several
straight to crisped hairs. Styles 4 (5), about as long as the longest stamens, from nearly fully connate to
distinct for half their length, glabrous, branches often stigmatic on the inner surface for the full length.
Ovary nearly completely inferior at anthesis, the top glabrous to hairy, at first rounded, becoming enlarged
and more pointed in fruit, 4 locular with numerous ovules. Pedicels glabrous to pubescent, 2 - 7 mm long,
lower ones always in the axils of normal leaves.
Fruit a rather woody capsule, ovoid-elliptic, pointed at the ends, (5) 6 - 10 mm long, 4 - 5 mm diam.,
loculicidal, 3 - 5 valved, calyx persistent. Seeds brown, numerous, 1 - 3 mm long, linear-fusiform, tip finely
pointed and with an unequal erose-coronate caruncle about twice as long as the cylindrical embryo at the
funicular end, testa membraneous.
Seed can be sown as soon as ripe or stored dry in airtight containers in a cool place for periods up to a
year and then sown.
The usual method is to root either softwood or hardwood cuttings 3" - 4" long in shady soil in a closed
cold frame outdoors. Softwood cuttings should be rooted in early summer or September, hardwood
cuttings in early spring or in November.
Removal of rooted suckers from the base of old plants is an easy way of obtaining new plants.
Old plants may be divided by using a sharp spade or even chopping through the centre with an axe in
early spring before growth starts.
Fibrous rooted and therefore easily transplanted in spring at any size, bare rooted.
Conditions for cultivation
The shrub is very slow growing. The recommended hardiness zone in Canada is 4 (K. Wilson personal
communication) although the cultivar P. lewisii 'Waterton' is hardy to zone 2b (Sherk and Buckley 1968).
The shrub will grow well in any ordinary well drained soil with a minimum amount of attention and is
equally well at home in full sun or partial shade in the border. If the plant is healthy with sufficient water a
large number of flowers per branch will develop. If pruning is necessary it should be done immediately after
flowering since the flowers appear on wood formed the previous year. The wood should be cut back to
where new shoots are showing. Usually a small amount of renewal pruning every few years is sufficient,
especially when flowering branches are removed for house decoration, to maintain a balance of old and new
Landscape Value
Philadelphus lewisii is a member of a group of shrubs which are chiefly valued for the short period
during which they are in flower, because the foliage and fruit are not very interesting. It is well adapted for
a shrubbery where other species will hide the foliage and branches during the non-flowering season. The
shrubs are dense in habit and may be used as a screen or hedge. Some of the hydrids of other species in the
group may be used as specimen shrubs because of their more attractive growth habit.
Philadelphus lewisii is not available from B.C. suppliers because of lack of demand./1, lewisii 'Waterton'
may be obtained from some nurseries in B.C. but is more readily available from sources outside the
Varieties and Ornamental Cultivars
Plants west of the Cascades are often cited as Philadelphus gordonianus or P. lewisii var./ssp. gordonianus
because the leaves are much more hairy and/or the styles are more completely connate than in forms from east of the Cascades. There may be statistical analysis to support this but it is doubtful whether separate
specific rank is justified in view of the extreme variability of P. lewisii. It is also possible that plants of the
dry interior lowlands may constitute an ecological race with smaller more elliptic leaves. One cultivar is
known - P. lewisii 'Waterton' was selected from plants in the Waterton Lakes area of Alberta where it is
native. It has a neat bushy habit with flowers well spaced over the shrub and is considered one of the
hardiest Mock oranges, growing to a maximum of 6' in height. The hybrid P. X splendens may have been
produced by hybridization between P. grandiflorus andP. gordonianus =P. lewisii (see Hilliers' Manual).
Other Uses
The slender shoots were used as arrow shafts for war and hunting by the native tribes in the area of its
distribution. Cut flowering shoots are much used for home decoration. Flowers of P. coronarius have been
used in perfumery and to flavour tea; the leaves are said to impart a cucumber flavour to summer drinks.
Diseases and Problems of Cultivation
Mock oranges as a group are mostly free from injurious insect and disease pests.
Nectria cinnabarina — 'canker' may occur on the stems followed by dieback of the distal portions. Prune
and burn affected branches. Two species of aphids and an undetermined species of leaf miner also attack
Mock oranges but usually result in little injury.
H.N.W. Toms (1964) lists only two diseases which have been found on wild P. lewisii in B.C. -
Phyllactinia guttata — 'powdery mildew' and Septoria philadelphi - 'leaf spot', which is occasionally
serious in very rainy seasons. Pick off and burn all infected leaves as soon as they appear.
The generic name is derived from the Greek word philos for love and delphos for brother, indicative of
the closely approximated opposite shoots, and is said to commemorate Ptolemy Philadelphus, King of
Egypt 285 — 247 B.C. The specific name lewisii recognises the collection of the first specimen by Captain
Meriwether Lewis on July 4, 1804 'on the waters of Clark's River, Montana' i.e. Clark's Fork of the
Columbia River, Montana.
The common name Syringa is derived from syrinx a panpipe and is identical with the generic name for
the lilac — this arises from the use of Syringa by the old herbalists for 3 completely separate plants, a Lilac,
a Philadelphus and a Jasminum. The confusion arose because Lilac and P. coronarius (the commonly
cultivated Mock orange of the Pacific Northwest) were introduced together in 1562 into Europe from
Turkey, where the wood of both had been used to make pipes.
In the Language of Flowers the Mock orange signifies Memory because 'when we inhale this penetrating
odour, it seems to follow us everywhere for a considerable time'.
The Mock orange is the State Flower of Idaho.
Coats, Alice M. 1963. Garden Shrubs and their Histories. Vista Books, London.
Garman, E.H.  1963.  A  Pocket Guide to   Trees and Shrubs in British Columbia. British Columbia Forest Service,
Department of Lands, Forests, and Water Resources.
Hillier & Sons. 1971. Hilliers' Manual of Trees and Shrubs, Winchester, England.
Hitchcock, C.L. et al. 1961. Vascular Plants of the Pacific Northwest. Part 3. Saxifragaceae to Ericaceae. University of
Washington Press, Seattle.
Hu, Shiu-Ying. 1955. A Monograph of the genus Philadelphus. J. Arnold Arboretum 36:52-109.
Jepson, Willis Linn. 1957. A Manual of the Flowering Plants of California. University of California Press, Berkeley and Los
Sherk, Lawrence C. & Arthur R. Buckley. 1968. Ornamental Shrubs for Canada. Research Branch, Canada Department of
Agriculture. Publication 1286. Queen's Printer, Ottawa.
Toms, H.N.W. 1964. Plant Diseases of Southern British Columbia. A Host Index, pp. 143-224. Reprinted from: Canadian
Plant Diseases Survey. Volume 44. Canada Department of Agriculture Publication, Ottawa.
Trelease, William. 1931. Winter Botany. Dover Publications Inc., New York. (1967 republication). 8
Botanical Garden  News  and  Notes
Rhododendron Species Collection - A grant of monies from the Vancouver Chapter of the American
Rhododendron Society was recently made to the Botanical Garden. The grant will provide for additional
species to be added to the collection now established in the nursery.
Local Initiatives Program - This spring program is operated by the Botanical Garden under the Federal
assistance program. Work has nearly been completed on the installation of the drainage and irrigation
system in the nursery. Work has been initiated on the selective clearing and preparation of a portion of the
Marine Drive Garden for establishment of Asian tree and shrub species. The principal display garden for
rhododendrons will be developed in this garden area.
Acquisition of Special Collections — The Botanical Garden has been fortunate to obtain the complete
nursery stock of two well-known Vancouver Island nurseries during the past six months. Bulbs of Crocus,
Narcissus and Tulips were obtained from the Beaumaris Bulb Farm on the Saanich Peninsula, owned and
operated by Mr. and Mrs. G.M. Owen. The collection has been planted and is now growing in the new
nursery of the Botanical Garden. The other collection purchased represents a nursery of international
reputation owned and operated by Mr. and Mrs. E.L. Lohbrunner. The Lohbrunners have accumulated a
worldwide collection of alpine plants, in particular plants of the Cordilleran region of North America. In
addition an outstanding collection of conifers including many exotic dwarf types are in the material to be
transferred to the Botanical Garden at The University of British Columbia. These collections will provide an
important foundation for the new rock and alpine garden development.
The Alpine and Rock Garden Development - The first major garden component of the new Botanical
Garden at The University of British Columbia will be started late this spring. The landscape architects for
this project are the firm of Justice and Webb of Vancouver. At the March meeting of the Board of
Governors of The University of British Columbia formal approval was given to Stage I development of the
Botanical Garden. It is anticipated that the working drawings for this stage will be completed in the fall of
Staff News - Dr. John W. Neill was recently elected president of the British Columbia Society of
Landscape Architects. Miss Evelyn Jack, associated for many years with the Botanical Garden program on
the campus, has transferred to a full time position with the Department of Physical Plant. Her activities
with the Botanical Garden, particularly those associated with the Index Seminum and the propagation of
rhododendrons, will be missed by the department. The staff of the Botanical Garden wish her well in her
new position. Mr. Ken Asano, gateman at the Nitobe Memorial Garden, has retired. Mr. Patrick Griffiths of
the University of Bath horticultural program will be spending the summer with the Garden.
Data                                           1972
Mean temperature
Highest temperature
48° F
54° F
Lowest temperature
20° F
Grass minimum temperature
22° F
Total rainfall/No. days with rainfall
Total snowfall/No. days with snowfall
Total hours bright sunshine/possible
Max. wind speed for 1 hour/direction
13/SE & NW
Mean mileage of wind at 3'
Mean mileage of wind at 40'
* Site: The University of British Columbia, Vancouver, B.C., Canada
Position: lat. 49° 15'29"N; long. 123° 14'58"W. Elevation: 342.6' Botanical Garden Staff
Dr. Roy L. Taylor
Supervisor of Operations
Mr. Kenneth Wilson
Research Scientist (Cytogenetics)
Dr. Christopher J. Marchant
Research Scientist (Horticulture)
Dr. John W. Neill
Research Assistants
Mrs. Marilyn G. Hirsekorn
Mrs. Sylvia Taylor
Secretary to the Office
Mrs. Morag L. Brown
Plant Accession System
Mrs. Annie Y.M. Cheng
Senior Gardener
Mr. James O'Friel
Mr. Harold Duffill
Mr. Leonard Gibbs
Mr. Sam Oyama
Mr. Tomomichi Sumi
Mr. David Tarrant
Mr. Isao Watanabe
Trientalis latifolia, the 'Star flower'
of moist coastal woodlands Spring male catkins and old female
cones of 'Red alder', Alnus rubra
Volume 3     Number 1 Spring 1972
Rhododendron Pollen 1
Philadelphus lewisii or 'Mock orange' 4
Botanical Garden News and Notes 8
Climatological Summary 8


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