CLEISTOGAMIC FLOWERS.
General character of cleistogamic flowers.
List of the genera producing such flowers, and their distribution in the
vegetable series.
Viola, description of the cleistogamic flowers in the several species; their
fertility compared with that of the perfect flowers.
Oxalis acetosella.
O. sensitiva, three forms of cleistogamic flowers.
Vandellia.
Ononis.
Impatiens.
Drosera.
Miscellaneous observations on various other cleistogamic plants.
Anemophilous species producing cleistogamic flowers.
Leersia, perfect flowers rarely developed.
Summary and concluding remarks on the origin of cleistogamic flowers.
The chief conclusions which may be drawn from the observations in this volume.
It was known even before the time of Linnaeus that certain plants produced two
kinds of flowers, ordinary open, and minute closed ones; and this fact formerly
gave rise to warm controversies about the sexuality of plants. These closed
flowers have been appropriately named cleistogamic by Dr. Kuhn. (8/1.
‘Botanische Zeitung’ 1867 page 65.) They are remarkable from their small size
and from never opening, so that they resemble buds; their petals are rudimentary
or quite aborted; their stamens are often reduced in number, with the anthers of
very small size, containing few pollen-grains, which have remarkably thin
transparent coats, and generally emit their tubes whilst still enclosed within
the anther-cells; and, lastly, the pistil is much reduced in size, with the
stigma in some cases hardly at all developed. These flowers do not secrete
nectar or emit any odour; from their small size, as well as from the corolla
being rudimentary, they are singularly inconspicuous. Consequently insects do
not visit them; nor if they did, could they find an entrance. Such flowers are
therefore invariably self-fertilised; yet they produce an abundance of seed. In
several cases the young capsules bury themselves beneath the ground, and the
seeds are there matured. These flowers are developed before, or after, or
simultaneously with the perfect ones. Their development seems to be largely
governed by the conditions to which the plants are exposed, for during certain
seasons or in certain localities only cleistogamic or only perfect flowers are
produced.
Dr. Kuhn, in the article above referred to, gives a list of 44 genera including
species which bear flowers of this kind. To this list I have added some genera,
and the authorities are appended in a footnote. I have omitted three names, from
reasons likewise given in the footnote. But it is by no means easy to decide in
all cases whether certain flowers ought to be ranked as cleistogamic. For
instance, Mr. Bentham informs me that in the South of France some of the flowers
on the vine do not fully open and yet set fruit; and I hear from two experienced
gardeners that this is the case with the vine in our hothouses; but as the
flowers do not appear to be completely closed it would be imprudent to consider
them as cleistogamic. The flowers of some aquatic and marsh plants, for instance
of Ranunculus aquatalis, Alisma natans, Subularia, Illecebrum, Menyanthes, and
Euryale, remain closely shut as long as they are submerged, and in this
condition fertilise themselves. (8/2. Delpino ‘Sull’ Opera, la Distribuzione dei
Sessi nelle Piante’ etc. 1867 page 30. Subularia, however, sometimes has its
flowers fully expanded beneath the water, see Sir J.E. Smith ‘English Flora’
volume 3 1825 page 157. For the behaviour of Menyanthes in Russia see Gillibert
in ‘Act. Acad. St. Petersb.’ 1777 part 2 page 45.--On Euryale ‘Gardener’s
Chronicle’ 1877 page 280.) They behave in this manner, apparently as a
protection to their pollen, and produce open flowers when exposed to the air; so
that these cases seem rather different from those of true cleistogamic flowers,
and have not been included in the list. Again, the flowers of some plants which
are produced very early or very late in the season do not properly expand; and
these might perhaps be considered as incipiently cleistogamic; but as they do
not present any of the remarkable peculiarities proper to the class, and as I
have not found any full record of such cases, they are not entered in the list.
When, however, it is believed on fairly good evidence that the flowers on a
plant in its native country do not open at any hour of the day or night, and yet
set seeds capable of germination, these may fairly be considered as
cleistogamic, notwithstanding that they present no peculiarities of structure. I
will now give as complete a list of the genera containing cleistogamic species
as I have been able to collect.
TABLE 8.38. List of genera including cleistogamic species (chiefly after Kuhn).
(8/3. I have omitted Trifolium and Arachis from the list, because Von Mohl says
‘Botanische Zeitung’ 1863 page 312, that the flower-stems merely draw the
flowers beneath the ground, and that these do not appear to be properly
cleistogamic. Correa de Mello ‘Journal of the Linnean Society Botany’ volume 11
1870 page 254, observed plants of Arachis in Brazil, and could never find such
flowers. Plantago has been omitted because as far as I can discover it produces
hermaphrodite and female flower-heads, but not cleistogamic flowers.
Krascheninikowia (vel Stellaria) has been omitted because it seems very doubtful
from Maximowicz’ description whether the lower flowers which have no petals or
very small ones, and barren stamens or none, are cleistogamic; the upper
hermaphrodite flowers are said never to produce fruit, and therefore probably
act as males. Moreover in Stellaria graminea, as Babington remarks ‘British
Botany’ 1851 page 51, “shorter and longer petals accompany an imperfection of
the stamens or germen.”
I have added to the list the following cases:
Several Acanthaceae, for which see J. Scott in ‘Journal of Botany’ London new
series volume 1 1872 page 161.
With respect to salvia see Dr. Ascherson in ‘Botanische Zeitung’ 1871 page 555.
For Oxybaphus and Nyctaginia see Asa Gray in ‘American Naturalist’ November 1873
page 692.
From Dr. Torrey’s account of Hottonia inflata ‘Bulletin of the Torrey Botanical
Club’ volume 2 June 1871, it is manifest that this plant produces true
cleistogamic flowers.
For Pavonia see Bouche in ‘Sitzungsberichte d. Gesellsch. Natur. Freunde’
October 20, 1874 page 90.
I have added Thelymitra, as from the account given by Mr. Fitzgerald in his
magnificent work on ‘Australian Orchids’ it appears that the flowers of this
plant in its native home never open, but they do not appear to be reduced in
size. Nor is this the case with the flowers of certain species of Epidendron,
Cattleya, etc. see second edition of my ‘Fertilisation of Orchids’ page 147,
which without expanding produce capsules. It is therefore doubtful whether these
Orchideae ought to have been included in the list. From what Duval-Jouve says
about Cryptostachys in ‘Bulletin of the Soc. Bot. de France’ tome 10 1863 page
195, this plant appears to produce cleistogamic flowers.
the other additions to the list are noticed in my text.)
DICOTYLEDONS.
BORAGINEAE:
Eritrichium.
CONVOLVULACEAE:
Cuscuta.
SCROPHULARINEAE:
Scrophularia.
Linaria.
Vandellia.
ACANTHACEAE:
Cryphiacanthus.
Eranthemum.
Daedalacanthus.
Dipteracanthus.
Aechmanthera.
Ruellia.
LABIATAE:
Lamium.
Salvia.
NYCTAGINEAE:
Oxybaphus.
Nyctaginia.
ASCLEPIADAE:
Stapelia.
CAMPANULACEAE:
Specularia.
Campanula.
PRIMULACEAE:
Hottonia.
COMPOSITAE:
Anandria.
CRUCIFERAE:
Heterocarpaea.
VIOLACEAE:
Viola.
CISTINEAE:
Helianthemum.
Lechea.
MALVEACEAE:
Pavonia.
MALPIGHIACEAE:
Gaudichaudia.
Aspicarpa.
Camarea.
Janusia.
POLYGALEAE:
Polygala.
BALSAMINEAE:
Impatiens.
GERANIACAEA:
Oxalis.
LEGUMINOSAE:
Ononis.
Parochaetus.
Chapmannia.
Stylosanthus.
Lespedeza.
Vicia.
Lathyrus.
Martinsia vel Neurocarpum.
Amphicarpaea.
Glycine.
Galactia.
Voandzeia.
DROSERACEAE:
Drosera.
MONOCOTYLEDONS.
JUNCEAE:
Juncus.
GRAMINEAE:
Leersia.
Hordeum.
Cryptostachys.
COMMELINEAE:
Commelina.
PONTEDERACEAE:
Monochoria.
ORCHIDEAE:
Schomburgkia.
Cattleya.
Epidendron.
Thelymitra.
The first point that strikes us in considering this list of 55 genera, is that
they are very widely distributed in the vegetable series. They are more common
in the family of the Leguminosae than in any other, and next in order in that of
the Acanthaceae and Malpighiaceae. A large number, but not all the species, of
certain genera, as of Oxalis and Viola, bear cleistogamic as well as ordinary
flowers. A second point which deserves notice is that a considerable proportion
of the genera produce more or less irregular flowers; this is the case with
about 32 out of the 55 genera, but to this subject I shall recur.
I formerly made many observations on cleistogamic flowers, but only a few of
them are worth giving, since the appearance of an admirable paper by Hugo Von
Mohl, whose examination was in some respects much more complete than mine. (8/4.
‘Botanische Zeitung’ 1863 page 309-28.) His paper includes also an interesting
history of our knowledge on the subject.
Viola canina.
The calyx of the cleistogamic flowers differs in no respect from that of the
perfect ones. The petals are reduced to five minute scales; the lower one, which
represents the lower lip, is considerably larger than the others, but with no
trace of the spur-like nectary; its margins are smooth, whilst those of the
other four scale-like petals are papillose. D. Muller of Upsala says that in the
specimens which he observed the petals were completely aborted. (8/5. Ibid. 1857
page 730. This paper contains the first full and satisfactory account of any
cleistogamic flower.) The stamens are very small, and only the two lower ones
are provided with anthers, which do not cohere together as in the perfect
flowers. The anthers are minute, with the two cells or loculi remarkably
distinct; they contain very little pollen in comparison with those of the
perfect flowers. The connective expands into a membranous hood-like shield which
projects above the anther-cells. These two lower stamens have no vestige of the
curious appendages which secrete nectar in the perfect flowers. The three other
stamens are destitute of anthers and have broader filaments, with their terminal
membranous expansions flatter or not so hood-like as those of the two
antheriferous stamens. The pollen-grains have remarkably thin transparent coats;
when exposed to the air they shrivel up quickly; when placed in water they
swell, and are then 8-10/7000 of an inch in diameter, and therefore of smaller
size than the ordinary pollen-grains similarly treated, which have a diameter of
13-14/7000 of an inch. In the cleistogamic flowers, the pollen-grains, as far as
I could see, never naturally fall out of the anther-cells, but emit their tubes
through a pore at the upper end. I was able to trace the tubes from the grains
some way down the stigma. The pistil is very short, with the style hooked, so
that its extremity, which is a little enlarged or funnel-shaped and represents
the stigma, is directed downwards, being covered by the two membranous
expansions of the antheriferous stamens. It is remarkable that there is an open
passage from the enlarged funnel-shaped extremity to within the ovarium; this
was evident, as slight pressure caused a bubble of air, which had been drawn in
by some accident, to travel freely from one end to the other: a similar passage
was observed by Michalet in V. alba. The pistil therefore differs considerably
from that of the perfect flower; for in the latter it is much longer, and
straight with the exception of the rectangularly bent stigma; nor is it
perforated by an open passage.
The ordinary or perfect flowers have been said by some authors never to produce
capsules; but this is an error, though only a small proportion of them do so.
This appears to depend in some cases on their anthers not containing even a
trace of pollen, but more generally on bees not visiting the flowers. I twice
covered with a net a group of flowers, and marked with threads twelve of them
which had not as yet expanded. This precaution is necessary, for though as a
general rule the perfect flowers appear considerably before the cleistogamic
ones, yet occasionally some of the latter are produced early in the season, and
their capsules might readily be mistaken for those produced by the perfect
flowers. Not one of the twelve marked perfect flowers yielded a capsule, whilst
others under the net which had been artificially fertilised produced five
capsules; and these contained exactly the same average number of seeds as some
capsules from flowers outside the net which had been fertilised by bees. I have
repeatedly seen Bombus hortorum, lapidarius, and a third species, as well as
hive-bees, sucking the flowers of this violet: I marked six which were thus
visited, and four of them produced fine capsules; the two others were gnawed off
by some animal. I watched Bombus hortorum for some time, and whenever it came to
a flower which did not stand in a convenient position to be sucked, it bit a
hole through the spur-like nectary. Such ill-placed flowers would not yield any
seed or leave descendants; and the plants bearing them would thus tend to be
eliminated through natural selection.
The seeds produced by the cleistogamic and perfect flowers do not differ in
appearance or number. On two occasions I fertilised several perfect flowers with
pollen from other individuals, and afterwards marked some cleistogamic flowers
on the same plants; and the result was that 14 capsules produced by the perfect
flowers contained on an average 9.85 seeds; and 17 capsules from the
cleistogamic ones contained 9.64 seeds,--an amount of difference of no
significance. It is remarkable how much more quickly the capsules from the
cleistogamic flowers are developed than those from the perfect ones; for
instance, several perfect flowers were cross-fertilised on April 14th, 1863, and
a month afterwards (May 15th) eight young cleistogamic flowers were marked with
threads; and when the two sets of capsules thus produced were compared on June
3rd, there was scarcely any difference between them in size.
Viola odorata (WHITE-FLOWERED, SINGLE, CULTIVATED VARIETY).
The petals are represented by mere scales as in the last species; but
differently from in the last, all five stamens are provided with diminutive
anthers. Small bundles of pollen-tubes were traced from the five anthers into
the somewhat distant stigma. The capsules produced by these flowers bury
themselves in the soil, if it be loose enough, and there mature themselves.
(8/6. Vaucher says ‘Hist. Phys. des Plantes d’Europe’ tome 3 1844 page 309, that
V. hirta and collina likewise bury their capsules. See also Lecoq ‘Geograph.
Bot.’ tome 5 1856 page 180.) Lecoq says that it is only these latter capsules
which possess elastic valves; but I think this must be a misprint, as such
valves would obviously be of no use to the buried capsules, but would serve to
scatter the seeds of the sub-aerial ones, as in the other species of Viola. It
is remarkable that this plant, according to Delpino, does not produce
cleistogamic flowers in one part of Liguria, whilst the perfect flowers are
there abundantly fertile (8/7. ‘Sull’ Opera, la Distribuzione dei Sessi nelle
Piante’ etc. 1867 page 30.); on the other hand, cleistogamic flowers are
produced by it near Turin. Another fact is worth giving as an instance of
correlated development: I found on a purple variety, after it had produced its
perfect double flowers, and whilst the white single variety was bearing its
cleistogamic flowers, many bud-like bodies which from their position on the
plant were certainly of a cleistogamic nature. They consisted, as could be seen
on bisecting them, of a dense mass of minute scales closely folded over one
another, exactly like a cabbage-head in miniature. I could not detect any
stamens, and in the place of the ovarium there was a little central column. The
doubleness of the perfect flowers had thus spread to the cleistogamic ones,
which therefore were rendered quite sterile.
Viola hirta.
The five stamens of the cleistogamic flowers are provided, as in the last case,
with small anthers, from all of which pollen-tubes proceed to the stigma. The
petals are not quite so much reduced as in V. canina, and the short pistil
instead of being hooked is merely bent into a rectangle. Of several perfect
flowers which I saw visited by hive-and humble-bees, six were marked, but they
produced only two capsules, some of the others having been accidentally injured.
M. Monnier was therefore mistaken in this case as in that of V. odorata, in
supposing that the perfect flowers always withered away and aborted. He states
that the peduncles of the cleistogamic flowers curve downwards and bury the
ovaries beneath the soil. (8/8. These statements are taken from Professor
Oliver’s excellent article in the ‘Natural History Review’ July 1862 page 238.
With respect to the supposed sterility of the perfect flowers in this genus see
also Timbal-Lagrave ‘Botanische Zeitung’ 1854 page 772.) I may here add that
Fritz Muller, as I hear from his brother, has found in the highlands of Southern
Brazil a white-flowered species of violet which bears subterranean cleistogamic
flowers.
Viola nana.
Mr. Scott sent me seeds of this Indian species from the Sikkim Terai, from which
I raised many plants, and from these other seedlings during several successive
generations. They produced an abundance of cleistogamic flowers during the whole
of each summer, but never a perfect one. When Mr. Scott wrote to me his plants
in Calcutta were behaving similarly, though his collector saw the species in
flower in its native site. This case is valuable as showing that we ought not to
infer, as has sometimes been done, that a species does not bear perfect flowers
when growing naturally, because it produces only cleistogamic flowers under
culture. The calyx of these flowers is sometimes formed of only three sepals;
two being actually suppressed and not merely coherent with the others; this
occurred with five out of thirty flowers which were examined for this purpose.
The petals are represented by extremely minute scales. Of the stamens, two bear
anthers which are in the same state as in the previous species, but, as far as I
could judge, each of the two cells contained only from 20 to 25 delicate
transparent pollen-grains. These emitted their tubes in the usual manner. The
three other stamens bore very minute rudimentary anthers, one of which was
generally larger than the other two, but none of them contained any pollen. In
one instance, however, a single cell of the larger rudimentary anther included a
little pollen. The style consists of a short flattened tube, somewhat expanded
at its upper end, and this forms an open channel leading into the ovarium, as
described under V. canina. It is slightly bent towards the two fertile anthers.
Viola Roxburghiana.
This species bore in my hothouse during two years a multitude of cleistogamic
flowers, which resembled in all respects those of the last species; but no
perfect ones were produced. Mr. Scott informs me that in India it bears perfect
flowers only during the cold season, and that these are quite fertile. During
the hot, and more especially during the rainy season, it bears an abundance of
cleistogamic flowers.
Many other species, besides the five now described, produce cleistogamic
flowers; this is the case, according to D. Muller, Michalet, Von Mohl, and
Hermann Muller, with V. elatior, lancifolia, sylvatica, palustris, mirabilis,
bicolor, ionodium, and biflora. But V. tricolor does not produce them.
Michalet asserts that V. palustris produces near Paris only perfect flowers,
which are quite fertile; but that when the plant grows on mountains cleistogamic
flowers are produced; and so it is with V. biflora. The same author states that
he has seen in the case of V. alba flowers intermediate in structure between the
perfect and cleistogamic ones. According to M. Boisduval, an Italian species, V.
Ruppii, never bears in France “des fleurs bien apparentes, ce qui ne l’empeche
pas de fructifier.”
It is interesting to observe the gradation in the abortion of the parts in the
cleistogamic flowers of the several foregoing species. It appears from the
statements by D. Muller and Von Mohl that in V. mirabilis the calyx does not
remain quite closed; all five stamens are provided with anthers, and some
pollen-grains probably fall out of the cells on the stigma, instead of
protruding their tubes whilst still enclosed, as in the other species. In V.
hirta all five stamens are likewise antheriferous; the petals are not so much
reduced and the pistil not so much modified as in the following species. In V.
nana and elatior only two of the stamens properly bear anthers, but sometimes
one or even two of the others are thus provided. Lastly, in V. canina never more
than two of the stamens, as far as I have seen, bear anthers; the petals are
much more reduced than in V. hirta, and according to D. Muller are sometimes
quite absent.
Oxalis acetosella.
The existence of cleistogamic flowers on this plant was discovered by Michalet.
(8/9. ‘Bulletin Soc. Bot. de France’ tome 7 1860 page 465.) They have been fully
described by Von Mohl, and I can add hardly anything to his description. In my
specimens the anthers of the five longer stamens were nearly on a level with the
stigmas; whilst the smaller and less plainly bilobed anthers of the five shorter
stamens stood considerably below the stigmas, so that their tubes had to travel
some way upwards. According to Michalet these latter anthers are sometimes quite
aborted. In one case the tubes, which ended in excessively fine points, were
seen by me stretching upwards from the lower anthers towards the stigmas, which
they had not as yet reached. My plants grew in pots, and long after the perfect
flowers had withered they produced not only cleistogamic but a few minute open
flowers, which were in an intermediate condition between the two kinds. In one
of these the pollen-tubes from the lower anthers had reached the stigmas, though
the flower was open. The footstalks of the cleistogamic flowers are much shorter
than those of the perfect flowers, and are so much bowed downwards that they
tend, according to Von Mohl, to bury themselves in the moss and dead leaves on
the ground. Michalet also says that they are often hypogean. In order to
ascertain the number of seeds produced by these flowers, I marked eight of them;
two failed, one cast its seed abroad, and the remaining five contained on an
average 10.0 seeds per capsule. This is rather above the average 9.2, which
eleven capsules from perfect flowers fertilised with their own pollen yielded,
and considerably above the average 7.9, from the capsules of perfect flowers
fertilised with pollen from another plant; but this latter result must, I think,
have been accidental.
Hildebrand, whilst searching various Herbaria, observed that many other species
of Oxalis besides O. acetosella produce cleistogamic flowers (8/10.
‘Monatsbericht der Akad. der Wiss. zu Berlin’ 1866 page 369.); and I hear from
him that this is the case with the heterostyled trimorphic O. incarnata from the
Cape of Good Hope.
Oxalis (Biophytum) sensitiva.
This plant is ranked by many botanists as a distinct genus, but as a sub-genus
by Bentham and Hooker. Many of the early flowers on a mid-styled plant in my
hothouse did not open properly, and were in an intermediate condition between
cleistogamic and perfect. Their petals varied from a rudiment to about half
their proper size; nevertheless they produced capsules. I attributed their state
to unfavourable conditions, for later in the season fully expanded flowers of
the proper size appeared. But Mr. Thwaites afterwards sent me from Ceylon a
number of long-styled, mid-styled, and short-styled flower-stalks preserved in
spirits; and on the same stalks with the perfect flowers, some of which were
fully expanded and others still in bud, there were small bud-like bodies
containing mature pollen, but with their calyces closed. These cleistogamic
flowers do not differ much in structure from the perfect ones of the
corresponding form, with the exception that their petals are reduced to
extremely minute, barely visible scales, which adhere firmly to the rounded
bases of the shorter stamens. Their stigmas are much less papillose, and smaller
in about the ratio of 13 to 20 divisions of the micrometer, as measured
transversely from apex to apex, than the stigmas of the perfect flowers. The
styles are furrowed longitudinally, and are clothed with simple as well as
glandular hairs, but only in the cleistogamic flowers produced by the long-
styled and mid-styled forms. The anthers of the longer stamens are a little
smaller than the corresponding ones of the perfect flowers, in about the ratio
of 11 to 14. They dehisce properly, but do not appear to contain much pollen.
Many pollen-grains were attached by short tubes to the stigmas; but many others,
still adhering to the anthers, had emitted their tubes to a considerable length,
without having come in contact with the stigmas. Living plants ought to be
examined, as the stigmas, at least of the long-styled form, project beyond the
calyx, and if visited by insects (which, however, is very improbable) might be
fertilised with pollen from a perfect flower. The most singular fact about the
present species is that long-styled cleistogamic flowers are produced by the
long-styled plants, and mid-styled as well as short-styled cleistogamic flowers
by the other two forms; so that there are three kinds of cleistogamic and three
kinds of perfect flowers produced by this one species! Most of the heterostyled
species of Oxalis are more or less sterile, many absolutely so, if
illegitimately fertilised with their own-form pollen. It is therefore probable
that the pollen of the cleistogamic flowers has been modified in power, so as to
act on their own stigmas, for they yield an abundance of seeds. We may perhaps
account for the cleistogamic flowers consisting of the three forms, through the
principle of correlated growth, by which the cleistogamic flowers of the double
violet have been rendered double.
Vandellia nummularifolia.
Dr. Kuhn has collected all the notices with respect to cleistogamic flowers in
this genus, and has described from dried specimens those produced by an
Abyssinian species. (8/11. ‘Botanische Zeitung’ 1867 page 65.) Mr. Scott sent me
from Calcutta seeds of the above common Indian weed, from which many plants were
successively raised during several years. The cleistogamic flowers are very
small, being when fully mature under 1/20 of an inch (1.27 millimetres) in
length. The calyx does not open, and within it the delicate transparent corolla
remains closely folded over the ovarium. There are only two anthers instead of
the normal number of four, and their filaments adhere to the corolla. The cells
of the anthers diverge much at their lower ends and are only 5/700 of an inch
(.181 millimetres) in their longer diameter. They contain but few pollen-grains,
and these emit their tubes whilst still within the anther. The pistil is very
short, and is surmounted by a bilobed stigma. As the ovary grows the two anthers
together with the shrivelled corolla, all attached by the dried pollen-tubes to
the stigma, are torn off and carried upwards in the shape of a little cap. The
perfect flowers generally appear before the cleistogamic, but sometimes
simultaneously with them. During one season a large number of plants produced no
perfect flowers. It has been asserted that the latter never yield capsules; but
this is a mistake, as they do so even when insects are excluded. Fifteen
capsules from cleistogamic flowers on plants growing under favourable conditions
contained on an average 64.2 seeds, with a maximum of 87; whilst 20 capsules
from plants growing much crowded yielded an average of only 48. Sixteen capsules
from perfect flowers artificially crossed with pollen from another plant
contained on an average 93 seeds, with a maximum of 137. Thirteen capsules from
self-fertilised perfect flowers gave an average of 62 seeds, with a maximum of
135. Therefore the capsules from the cleistogamic flowers contained fewer seeds
than those from perfect flowers when cross-fertilised, and slightly more than
those from perfect flowers self-fertilised.
Dr. Kuhn believes that the Abyssinian V. sessiflora does not differ specifically
from the foregoing species. But its cleistogamic flowers apparently include four
anthers instead of two as above described. The plants, moreover, of V.
sessiflora produce subterranean runners which yield capsules; and I never saw a
trace of such runners in V. nummularifolia, although many plants were
cultivated.
Linaria spuria.
Michalet says that short, thin, twisted branches are developed from the buds in
the axils of the lower leaves, and that these bury themselves in the ground.
(8/12. ‘Bulletin Soc. Bot. de France’ tome 7 1860 page 468.) They there produce
flowers not offering any peculiarity in structure, excepting that their
corollas, though properly coloured, are deformed. These flowers may be ranked as
cleistogamic, as they are developed, and not merely drawn, beneath the ground.
Ononis columnae.
Plants were raised from seeds sent me from Northern Italy. The sepals of the
cleistogamic flowers are elongated and closely pressed together; the petals are
much reduced in size, colourless, and folded over the interior organs. The
filaments of the ten stamens are united into a tube, and this is not the case,
according to Von Mohl, with the cleistogamic flowers of other Leguminosae. Five
of the stamens are destitute of anthers, and alternate with the five thus
provided. The two cells of the anthers are minute, rounded and separated from
one another by connective tissue; they contain but few pollen-grains, and these
have extremely delicate coats. The pistil is hook-shaped, with a plainly
enlarged stigma, which is curled down, towards the anthers; it therefore differs
much from that of the perfect flower. During the year 1867 no perfect flowers
were produced, but in the following year there were both perfect and
cleistogamic ones.
Ononis minutissima.
My plants produced both perfect and cleistogamic flowers; but I did not examine
the latter. Some of the former were crossed with pollen from a distinct plant,
and six capsules thus obtained yielded on an average 3.66 seeds, with a maximum
of 5 in one. Twelve perfect flowers were marked and allowed to fertilise
themselves spontaneously under a net, and they yielded eight capsules,
containing on an average 2.38 seeds, with a maximum of 3 in one. Fifty-three
capsules produced by the cleistogamic flowers contained on an average 4.1 seeds,
so that these were the most productive of all; and the seeds themselves looked
finer even than those from the crossed perfect flowers. According to Mr. Bentham
O. parviflora likewise bears cleistogamic flowers; and he informs me that these
flowers are produced by all three species early in the spring; whilst the
perfect ones appear afterwards, and therefore in a reversed order compared with
those of Viola and Oxalis. Some of the species, for instance Ononis columnae,
bear a fresh crop of cleistogamic flowers in the autumn.
Lathyrus nissolia.
This plant apparently offers a case of the first stage in the production of
cleistogamic flowers, for on plants growing in a state of nature, many of the
flowers never expand and yet produce fine pods. Some of the buds are so large
that they seem on the point of expansion; others are much smaller, but none so
small as the true cleistogamic flowers of the foregoing species. As I marked
these buds with thread and examined them daily, there could be no mistake about
their producing fruit without having expanded.
Several other Leguminous genera produce cleistogamic flowers, as may be seen in
Table 8.38; but much does not appear to be known about them. Von Mohl says that
their petals are commonly rudimentary, that only a few of their anthers are
developed, their filaments are not united into a tube and their pistils are
hook-shaped. In three of the genera, namely Vicia, Amphicarpaea, and Voandzeia,
the cleistogamic flowers are produced on subterranean stems. The perfect flowers
of Voandzeia, which is a cultivated plant, are said never to produce fruit
(8/13. Correa de Mello ‘Journal of the Linnean Society Botany’ volume 11 1870
page 254, particularly attended to the flowering and fruiting of this African
plant, which is sometimes cultivated in Brazil.); but we should remember how
often fertility is affected by cultivation.
Impatiens fulva.
Mr. A.W. Bennett has published an excellent description, with figures, of this
plant. (8/14. ‘Journal of the Linnean Society Botany’ volume 13 1872 page 147.)
He shows that the cleistogamic and perfect flowers differ in structure at a very
early period of growth, so that the existence of the former cannot be due merely
to the arrested development of the latter,--a conclusion which indeed follows
from most of the previous descriptions. Mr. Bennett found on the banks of the
Wey that the plants which bore cleistogamic flowers alone were to those bearing
perfect flowers as 20 to 1; but we should remember that this is a naturalised
species. The perfect flowers are usually barren in England; but Professor Asa
Gray writes to me that after midsummer in the United States some or many of them
produce capsules.
Impatiens noli-me-tangere.
I can add nothing of importance to Von Mohl’s description, excepting that one of
the rudimentary petals shows a vestige of a nectary, as Mr. Bennett likewise
found to be the case with I. fulva. As in this latter species all five stamens
produce some pollen, though small in amount; a single anther contains, according
to Von Mohl, not more than 50 grains, and these emit their tubes while still
enclosed within it. The pollen-grains of the perfect flowers are tied together
by threads, but not, so far as I could see, those of the cleistogamic flowers;
and a provision of this kind would here have been useless, as the grains can
never be transported by insects. The flowers of I. balsamina are visited by
humble-bees (8/15. H. Muller ‘Die Befruchtung’ etc. page 170.), and I am almost
sure that this is the case with the perfect flowers of I. noli-me-tangere. From
the perfect flowers of this latter species covered with a net eleven
spontaneously self-fertilised capsules were produced, and these yielded on an
average 3.45 seeds. Some perfect flowers with their anthers still containing an
abundance of pollen were fertilised with pollen from a distinct plant; and the
three capsules thus produced contained, to my surprise, only 2, 2, and 1 seed.
As I. balsamina is proterandrous, so probably is the present species; and if so,
cross-fertilisation was effected by me at too early a period, and this may
account for the capsules yielding so few seeds.
Drosera rotundifolia.
The first flower-stems which were thrown up by some plants in my greenhouse bore
only cleistogamic flowers. The petals of small size remained permanently closed
over the reproductive organs, but their white tips could just be seen between
the almost completely closed sepals. The pollen, which was scanty in amount, but
not so scanty as in Viola or Oxalis, remained enclosed within the anthers,
whence the tubes proceeded and penetrated the stigma. As the ovarium swelled the
little withered corolla was carried upwards in the form of a cap. These
cleistogamic flowers produced an abundance of seed. Later in the season perfect
flowers appeared. With plants in a state of nature the flowers open only in the
early morning, as I have been informed by Mr. Wallis, who particularly attended
to the time of their flowering. In the case of D. Anglica, the still folded
petals on some plants in my greenhouse opened just sufficiently to leave a
minute aperture; the anthers dehisced properly, but the pollen-grains adhered in
a mass to them, and thence emitted their tubes, which penetrated the stigmas.
These flowers, therefore, were in an intermediate condition, and could not be
called either perfect or cleistogamic.
A few miscellaneous observations may be added with respect to some other
species, as throwing light on our subject. Mr. Scott states that Eranthemum
ambiguum bears three kinds of flowers,--large, conspicuous, open ones, which are
quite sterile,--others of intermediate size, which are open and moderately
fertile--and lastly small closed or cleistogamic ones, which are perfectly
fertile. (8/16. ‘Journal of Botany’ London new series volume 1 1872 pages 161-
4.) Ruellia tuberosa, likewise one of the Acanthaceae, produces both open and
cleistogamic flowers; the latter yield from 18 to 24, whilst the former only
from 8 to 10 seeds; these two kinds of flowers are produced simultaneously,
whereas in several other members of the family the cleistogamic ones appear only
during the hot season. According to Torrey and Gray, the North American species
of Helianthemum, when growing in poor soil, produce only cleistogamic flowers.
The cleistogamic flowers of Specularia perfoliata are highly remarkable, as they
are closed by a tympanum formed by the rudimentary corolla, and without any
trace of an opening. The stamens vary from 3 to 5 in number, as do the sepals.
(8/17. Von Mohl ‘Botanische Zeitung’ 1863 pages 314 and 323. Dr. Bromfield
‘Phytologist’ volume 3 page 530, also remarks that the calyx of the cleistogamic
flowers is usually only 3-cleft, while that of the perfect flower is mostly 5-
cleft.) The collecting hairs on the pistil, which play so important a part in
the fertilisation of the perfect flowers, are here quite absent. Drs. Hooker and
Thomson state that some of the Indian species of Campanula produce two kinds of
flowers; the smaller ones being borne on longer peduncles with differently
formed sepals, and producing a more globose ovary. (8/18. ‘Journal of the
Linnean Society’ volume 2 1857 page 7. See also Professor Oliver in ‘Natural
History Review’ 1862 page 240.) The flowers are closed by a tympanum like that
in Specularia. Some of the plants produce both kinds of flowers, others only one
kind; both yield an abundance of seeds. Professor Oliver adds that he has seen
flowers on Campanula colorata in an intermediate condition between cleistogamic
and perfect ones.
The solitary almost sessile cleistogamic flowers produced by Monochoria
vaginalis are differently protected from those in any of the previous cases,
namely, within “a short sack formed of the membranous spathe, without any
opening or fissure.” There is only a single fertile stamen; the style is almost
obsolete, with the three stigmatic surfaces directed to one side. Both the
perfect and cleistogamic flowers produce seeds. (8/19. Dr. Kirk ‘Journal of the
Linnean Society’ volume 8 1864 page 147.)
The cleistogamic flowers on some of the Malpighiaceae seem to be more profoundly
modified than those in any of the foregoing genera. According to A. de Jussieu
they are differently situated from the perfect flowers; they contain only a
single stamen, instead of 5 or 6; and it is a strange fact that this particular
stamen is not developed in the perfect flowers of the same species. (8/20.
‘Archives du Museum’ tome 3 1843 pages 35-38, 82-86, 589, 598.) The style is
absent or rudimentary; and there are only two ovaries instead of three. Thus
these degraded flowers, as Jussieu remarks, “laugh at our classifications, for
the greater number of the characters proper to the species, to the genus, to the
family, to the class disappear.” I may add that their calyces are not glandular,
and as, according to Kerner, the fluid secreted by such glands generally serves
to protect the flowers from crawling insects, which steal the nectar without
aiding in their cross-fertilisation (8/21. ‘Die Schutzmittel der Bluthen gegen
unberufene Gaste’ 1876 page 25.), the deficiency of the glands in the
cleistogamic flowers of these plants may perhaps be accounted for by their not
requiring any such protection.
As the Asclepiadous genus Stapelia is said to produce cleistogamic flowers, the
following case may be worth giving. I have never heard of the perfect flowers of
Hoya carnosa setting seeds in this country, but some capsules were produced in
Mr. Farrer’s hothouse; and the gardener detected that they were the product of
minute bud-like bodies, three or four of which could sometimes be found on the
same umbel with the perfect flowers. They were quite closed and hardly thicker
than their peduncles. The sepals presented nothing particular, but internally
and alternating with them, there were five small flattened heart-shaped
papillae, like rudiments of petals; but the homological nature of which appeared
doubtful to Mr. Bentham and Dr. Hooker. No trace of anthers or of stamens could
be detected; and I knew from having examined many cleistogamic flowers what to
look for. There were two ovaries, full of ovules, quite open at their upper
ends, with their edges festooned, but with no trace of a proper stigma. In all
these flowers one of the two ovaries withered and blackened long before the
other. The one perfect capsule, 3 1/2 inches in length, which was sent me, had
likewise been developed from a single carpel. This capsule contained an
abundance of plumose seeds, many of which appeared quite sound, but they did not
germinate when sown at Kew. Therefore the little bud-like flower which produced
this capsule probably was as destitute of pollen as were those which I examined.
Juncus bufonius and Hordeum.
All the species hitherto mentioned which produce cleistogamic flowers are
entomophilous; but four genera, Juncus, Hordeum, Cryptostachys, and Leersia are
anemophilous. Juncus bufonius is remarkable by bearing in parts of Russia only
cleistogamic flowers, which contain three instead of the six anthers found in
the perfect flowers. (8/22. See Dr. Ascherson’s interesting paper in ‘Botanische
Zeitung’ 1871 page 551.) In the genus Hordeum it has been shown by Delpino that
the majority of the flowers are cleistogamic, some of the others expanding and
apparently allowing of cross-fertilisation. (8/23. ‘Bollettini del Comizio
agrario Parmense.’ Marzo e Aprile 1871. An abstract of this valuable paper is
given in ‘Botanische Zeitung’ 1871 page 537. See also Hildebrand on Hordeum in
‘Monatsbericht d. K. Akad Berlin’ October 1872 page 760.) I hear from Fritz
Muller that there is a grass in Southern Brazil, in which the sheath of the
uppermost leaf, half a metre in length, envelopes the whole panicle; and this
sheath never opens until the self-fertilised seeds are ripe. On the roadside
some plants had been cut down, whilst the cleistogamic panicles were developing,
and these plants afterwards produced free or unenclosed panicles of small size,
bearing perfect flowers.
Leersia oryzoides.
It has long been known that this plant produces cleistogamic flowers, but these
were first described with care by M. Duval-Jouve. (8/24. ‘Bulletin Bot. Soc. de
France’ tome 10 1863 page 194.) I procured plants from a stream near Reigate,
and cultivated them for several years in my greenhouse. The cleistogamic flowers
are very small, and usually mature their seeds within the sheaths of the leaves.
These flowers are said by Duval-Jouve to be filled by slightly viscid fluid; but
this was not the case with several that I opened; but there was a thin film of
fluid between the coats of the glumes, and when these were pressed the fluid
moved about, giving a similarly deceptive appearance of the whole inside of the
flower being thus filled. The stigma is very small and the filaments extremely
short; the anthers are less than 1/50 of an inch in length or about one-third of
the length of those in the perfect flowers. One of the three anthers dehisces
before the two others. Can this have any relation with the fact that in some
other species of Leersia only two stamens are fully developed? (8/25. Asa Gray
‘Manual of Botany of the United States’ 1856 page 540.) The anthers shed their
pollen on the stigma; at least in one instance this was clearly the case, and by
tearing open the anthers under water the grains were easily detached. Towards
the apex of the anther the grains are arranged in a single row and lower down in
two or three rows, so that they could be counted; and there were about 35 in
each cell, or 70 in the whole anther; and this is an astonishingly small number
for an anemophilous plant. The grains have very delicate coats, are spherical
and about 5/7000 of an inch (.0181 millimetres), whilst those of the perfect
flowers are about 7/7000 of an inch (.0254 millimetres) in diameter.
M. Duval-Jouve states that the panicles very rarely protrude from their sheaths,
but that when this does happen the flowers expand and exhibit well-developed
ovaries and stigmas, together with full-sized anthers containing apparently
sound pollen; nevertheless such flowers are invariably quite sterile. Schreiber
had previously observed that if a panicle is only half protruded, this half is
sterile, whilst the still included half is fertile. Some plants which grew in a
large tub of water in my greenhouse behaved on one occasion in a very different
manner. They protruded two very large much-branched panicles; but the florets
never opened, though these included fully developed stigmas, and stamens
supported on long filaments with large anthers that dehisced properly. If these
florets had opened for a short time unperceived by me and had then closed again,
the empty anthers would have been left dangling outside. Nevertheless they
yielded on August 17th an abundance of fine ripe seeds. Here then we have a near
approach to the single case as yet known of this grass producing in a state of
nature (in Germany) perfect flowers which yielded a copious supply of fruit.
(8/26. Dr. Ascherson ‘Botanische Zeitung’ 1864 page 350.) Seeds from the
cleistogamic flowers were sent by me to Mr. Scott in Calcutta, who there
cultivated the plants in various ways, but they never produced perfect flowers.
In Europe Leersia oryzoides is the sole representative of its genus, and Duval-
Jouve, after examining several exotic species, found that it apparently is the
sole one which bears cleistogamic flowers. It ranges from Persia to North
America, and specimens from Pennsylvania resembled the European ones in their
concealed manner of fructification. There can therefore be little doubt that
this plant generally propagates itself throughout an immense area by
cleistogamic seeds, and that it can hardly ever be invigorated by cross-
fertilisation. It resembles in this respect those plants which are now widely
spread, though they increase solely by asexual generation. (8/27. I have
collected several such cases in my ‘Variation under Domestication’ chapter 18
2nd edition volume 2 page 153.)
CONCLUDING REMARKS ON CLEISTOGAMIC FLOWERS.
That these flowers owe their structure primarily to the arrested development of
perfect ones, we may infer from such cases as that of the lower rudimentary
petal in Viola being larger than the others, like the lower lip of the perfect
flower,--from a vestige of a spur in the cleistogamic flowers of Impatiens,--
from the ten stamens of Ononis being united into a tube,--and other such
structures. The same inference may be drawn from the occurrence, in some
instances, on the same plant of a series of gradations between the cleistogamic
and perfect flowers. But that the former owe their origin wholly to arrested
development is by no means the case; for various parts have been specially
modified, so as to aid in the self-fertilisation of the flowers, and as a
protection to the pollen; for instance, the hook-shaped pistil in Viola and in
some other genera, by which the stigma is brought close to the fertile anthers,-
-the rudimentary corolla of Specularia modified into a perfectly closed
tympanum, and the sheath of Monochoria modified into a closed sack,--the
excessively thin coats of the pollen-grains,--the anthers not being all equally
aborted, and other such cases. Moreover Mr. Bennett has shown that the buds of
the cleistogamic and perfect flowers of Impatiens differ at a very early period
of growth.
The degree to which many of the most important organs in these degraded flowers
have been reduced or even wholly obliterated, is one of their most remarkable
peculiarities, reminding us of many parasitic animals. In some cases only a
single anther is left, and this contains but few pollen-grains of diminished
size; in other cases the stigma has disappeared, leaving a simple open passage
into the ovarium. It is also interesting to note the complete loss of trifling
points in the structure or functions of certain parts, which though of service
to the perfect flowers, are of none to the cleistogamic; for instance the
collecting hairs on the pistil of Specularia, the glands on the calyx of the
Malpighiaceae, the nectar-secreting appendages to the lower stamens of Viola,
the secretion of nectar by other parts, the emission of a sweet odour, and
apparently the elasticity of the valves in the buried capsules of Viola odorata.
We here see, as throughout nature, that as soon as any part or character becomes
superfluous it tends sooner or later to disappear.
Another peculiarity in these flowers is that the pollen-grains generally emit
their tubes whilst still enclosed within the anthers; but this is not so
remarkable a fact as was formerly thought, when the case of Asclepias was alone
known. (8/28. The case of Asclepias was described by R. Brown. Baillon asserts
‘Adansonia’ tome 2 1862 page 58, that with many plants the tubes are emitted
from pollen-grains which have not come into contact with the stigma; and that
they may be seen advancing horizontally through the air towards the stigma. I
have observed the emission of the tubes from the pollen-masses whilst still
within the anthers, in three widely distinct Orchidean genera namely Aceras,
Malaxis, and Neottia: see ‘The Various Contrivances by which Orchids are
Fertilised’ 2nd edition page 258.) It is, however, a wonderful sight to behold
the tubes directing themselves in a straight line to the stigma, when this is at
some little distance from the anthers. As soon as they reach the stigma or the
open passage leading into the ovarium, no doubt they penetrate it, guided by the
same means, whatever these may be, as in the case of ordinary flowers. I thought
that they might be guided by the avoidance of light: some pollen-grains of a
willow were therefore immersed in an extremely weak solution of honey, and the
vessel was placed so that the light entered only in one direction, laterally or
from below or from above, but the long tubes were in each case protruded in
every possible direction.
As cleistogamic flowers are completely closed they are necessarily self-
fertilised, not to mention the absence of any attraction to insects; and they
thus differ widely from the great majority of ordinary flowers. Delpino believes
that cleistogamic flowers have been developed in order to ensure the production
of seeds under climatic or other conditions which tend to prevent the
fertilisation of the perfect flowers. (8/29. ‘Sull’ Opera la Distribuzione dei
Sessi nelle Piante’ 1867 page 30.) I do not doubt that this holds good to a
certain limited extent, but the production of a large supply of seeds with
little consumption of nutrient matter or expenditure of vital force is probably
a far more efficient motive power. The whole flower is much reduced in size; but
what is much more important, an extremely small quantity of pollen has to be
formed, as none is lost through the action of insects or the weather; and pollen
contains much nitrogen and phosphorus. Von Mohl estimated that a single
cleistogamic anther-cell of Oxalis acetosella contained from one to two dozen
pollen-grains; we will say 20, and if so the whole flower can have produced at
most 400 grains; with Impatiens the whole number may be estimated in the same
manner at 250; with Leersia at 210; and with Viola nana at only 100. These
figures are wonderfully low compared with the 243,600 pollen-grains produced by
a flower of Leontodon, the 4,863 by an Hibiscus, or the 3,654,000 by a Paeony.
(8/30. The authorities for these statements are given in my ‘Effects of Cross
and Self-Fertilisation’ page 376.) We thus see that cleistogamic flowers produce
seeds with a wonderfully small expenditure of pollen; and they produce as a
general rule quite as many seeds as the perfect flowers.
That the production of a large number of seeds is necessary or beneficial to
many plants needs no evidence. So of course is their preservation before they
are ready for germination; and it is one of the many remarkable peculiarities of
the plants which bear cleistogamic flowers, that an incomparably larger
proportion of them than of ordinary plants bury their young ovaries in the
ground;--an action which it may be presumed serves to protect them from being
devoured by birds or other enemies. But this advantage is accompanied by the
loss of the power of wide dissemination. No less than eight of the genera in the
list at the beginning of this chapter include species which act in this manner,
namely, several kinds of Viola, Oxalis, Vandellia, Linaria, Commelina, and at
least three genera of Leguminosae. The seeds also of Leersia, though not buried,
are concealed in the most perfect manner within the sheaths of the leaves.
Cleistogamic flowers possess great facilities for burying their young ovaries or
capsules, owing to their small size, pointed shape, closed condition and the
absence of a corolla; and we can thus understand how it is that so many of them
have acquired this curious habit.
It has already been shown that in about 32 out of the 55 genera in the list just
referred to, the perfect flowers are irregular; and this implies that they have
been specially adapted for fertilisation by insects. Moreover three of the
genera with regular flowers are adapted by other means for the same end. Flowers
thus constructed are liable during certain seasons to be imperfectly fertilised,
namely, when the proper insects are scarce; and it is difficult to avoid the
belief that the production of cleistogamic flowers, which ensures under all
circumstances a full supply of seed, has been in part determined by the perfect
flowers being liable to fail in their fertilisation. But if this determining
cause be a real one, it must be of subordinate importance, as four of the genera
in the list are fertilised by the wind; and there seems no reason why their
perfect flowers should fail to be fertilised more frequently than those in any
other anemophilous genus. In contrast with what we here see with respect to the
large proportion of the perfect flowers being irregular, one genus alone out of
the 38 heterostyled genera described in the previous chapters bears such
flowers; yet all these genera are absolutely dependent on insects for their
legitimate fertilisation. I know not how to account for this difference in the
proportion of the plants bearing regular and irregular flowers in the two
classes, unless it be that the heterostyled flowers are already so well adapted
for cross-fertilisation, through the position of their stamens and pistils and
the difference in power of their two or three kinds of pollen, that any
additional adaptation, namely, through the flowers being made irregular, has
been rendered superfluous.
Although cleistogamic flowers never fail to yield a large number of seeds, yet
the plants bearing them usually produce perfect flowers, either simultaneously
or more commonly at a different period; and these are adapted for or admit of
cross-fertilisation. From the cases given of the two Indian species of Viola,
which produced in this country during several years only cleistogamic flowers,
and of the numerous plants of Vandellia and of some plants of Ononis which
behaved during one whole season in the same manner, it appears rash to infer
from such cases as that of Salvia cleistogama not having produced perfect
flowers during five years in Germany (8/31. Dr. Ascherson ‘Botanische Zeitung’
1871 page 555.), and of an Aspicarpa not having done so during several years in
Paris, that these plants would not bear perfect flowers in their native homes.
Von Mohl and several other botanists have repeatedly insisted that as a general
rule the perfect flowers produced by cleistogamic plants are sterile; but it has
been shown under the head of the several species that this is not the case. The
perfect flowers Viola are indeed sterile unless they are visited by bees; but
when thus visited they yield the full number of seeds. As far as I have been
able to discover there is only one absolute exception to the rule that the
perfect flowers are fertile, namely, that of Voandzeia; and in this case we
should remember that cultivation often affects injuriously the reproductive
organs. Although the perfect flowers of Leersia sometimes yield seeds, yet this
occurs so rarely, as far as hitherto observed, that it practically forms a
second exception to the rule.
As cleistogamic flowers are invariably fertilised, and as they are produced in
large numbers, they yield altogether a much larger supply of seeds than do the
perfect flowers on the same plant. But the latter flowers will occasionally be
cross-fertilised, and their offspring will thus be invigorated, as we may infer
from a wide-spread analogy. But of such invigoration I have only a small amount
of direct evidence: two crossed seedlings of Ononis minutissima were put into
competition with two seedlings raised from cleistogamic flowers; they were at
first all of equal height; the crossed were then slightly beaten; but on the
following year they showed the usual superiority of their class, and were to the
self-fertilised plants of cleistogamic origin as 100 to 88 in mean height. With
Vandellia twenty crossed plants exceeded in height twenty plants raised from
cleistogamic seeds only by a little, namely, in the ratio of 100 to 94.
It is a natural inquiry how so many plants belonging to various very distinct
families first came to have the development of their flowers arrested, so as
ultimately to become cleistogamic. That a passage from the one state to the
other is far from difficult is shown by the many recorded cases of gradations
between the two states on the same plant, in Viola, Oxalis, Biophytum,
Campanula, etc. In the several species of Viola the various parts of the flowers
have also been modified in very different degrees. Those plants which in their
own country produce flowers of full or nearly full size, but never expand (as
with Thelymitra), and yet set fruit, might easily be rendered cleistogamic.
Lathyrus nissolia seems to be in an incipient transitional state, as does
Drosera Anglica, the flowers of which are not perfectly closed. There is good
evidence that flowers sometimes fail to expand and are somewhat reduced in size,
owing to exposure to unfavourable conditions, but still retain their fertility
unimpaired. Linnaeus observed in 1753 that the flowers on several plants brought
from Spain and grown at Upsala did not show any corolla and yet produced seeds.
Asa Gray has seen flowers on exotic plants in the Northern United States which
never expanded and yet fruited. With certain English plants, which bear flowers
during nearly the whole year, Mr. Bennett found that those produced during the
winter season were fertilised in the bud; whilst with other species having fixed
times for flowering, but “which had been tempted by a mild January to put forth
a few wretched flowers,” no pollen was discharged from the anthers, and no seed
was formed. The flowers of Lysimachia vulgaris if fully exposed to the sun
expand properly, while those growing in shady ditches have smaller corollas
which open only slightly; and these two forms graduate into one another in
intermediate stations. Herr Bouche’s observations are of especial interest, for
he shows that both temperature and the amount of light affect the size of the
corolla; and he gives measurements proving that with some plants the corolla is
diminished by the increasing cold and darkness of the changing season, whilst
with others it is diminished by the increasing heat and light. (8/32. For the
statement by Linnaeus see Mohl in ‘Botanische Zeitung’ 1863 page 327. Asa Gray
‘American Journal of Science’ 2nd series volume 39 1865 page 105. Bennett in
‘Nature’ November 1869 page 11. The Reverend G. Henslow also says ‘Gardener’s
Chronicle’ 1877 page 271, also ‘Nature’ October 19, 1876 page 543, “that when
the autumn draws on, and habitually in winter for such of our wild flowers as
blossom at that season” the flowers are self-fertilised. On Lysimachia H. Muller
‘Nature’ September 1873 page 433. Bouche ‘Sitzungsbericht der Gesell.
Naturforsch. Freunde’ October 1874 page 90.)
The belief that the first step towards flowers being rendered cleistogamic was
due to the conditions to which they were exposed, is supported by the fact of
various plants belonging to this class either not producing their cleistogamic
flowers under certain conditions, or, on the other hand, producing them to the
complete exclusion of the perfect ones. Thus some species of Viola do not bear
cleistogamic flowers when growing on the lowlands or in certain districts. Other
plants when cultivated have failed to produce perfect flowers during several
successive years; and this is the case with Juncus bufonius in its native land
of Russia. Cleistogamic flowers are produced by some species late and by others
early in the season; and this agrees with the view that the first step towards
their development was due to climate; though the periods at which the two sorts
of flowers now appear must since have become much more distinctly defined. We do
not know whether too low are too high a temperature or the amount of light acts
in a direct manner on the size of the corolla, or indirectly through the male
organs being first affected. However this may be, if a plant were prevented
either early or late in the season from fully expanding its corolla, with some
reduction in its size, but with no loss of the power of self-fertilisation, then
natural selection might well complete the work and render it strictly
cleistogamic. The various organs would also, it is probable, be modified by the
peculiar conditions to which they are subjected within a completely closed
flower; also by the principle of correlated growth, and by the tendency in all
reduced organs finally to disappear. The result would be the production of
cleistogamic flowers such as we now see them; and these are admirably fitted to
yield a copious supply of seed at a wonderfully small cost to the plant.
I will now sum up very briefly the chief conclusions which seem to follow from
the observations given in this volume. Cleistogamic flowers afford, as just
stated, an abundant supply of seeds with little expenditure; and we can hardly
doubt that they have had their structure modified and degraded for this special
purpose; perfect flowers being still almost always produced so as to allow of
occasional cross-fertilisation. Hermaphrodite plants have often been rendered
monoecious, dioecious or polygamous; but as the separation of the sexes would
have been injurious, had not pollen been already transported habitually by
insects or by the wind from flower to flower, we may assume that the process of
separation did not commence and was not completed for the sake of the advantages
to be gained from cross-fertilisation. The sole motive for the separation of the
sexes which occurs to me, is that the production of a great number of seeds
might become superfluous to a plant under changed conditions of life; and it
might then be highly beneficial to it that the same flower or the same
individual should not have its vital powers taxed, under the struggle for life
to which all organisms are subjected, by producing both pollen and seeds. With
respect to the plants belonging to the gyno-dioecious sub-class, or those which
co-exist as hermaphrodites and females, it has been proved that they yield a
much larger supply of seed than they would have done if they had all remained
hermaphrodites; and we may feel sure from the large number of seeds produced by
many plants that such production is often necessary or advantageous. It is
therefore probable that the two forms in this sub-class have been separated or
developed for this special end.
Various hermaphrodite plants have become heterostyled, and now exist under two
or three forms; and we may confidently believe that this has been effected in
order that cross-fertilisation should be assured. For the full and legitimate
fertilisation of these plants pollen from the one form must be applied to the
stigma of another. If the sexual elements belonging to the same form are united
the union is an illegitimate one and more or less sterile. With dimorphic
species two illegitimate unions, and with trimorphic species twelve are
possible. There is reason to believe that the sterility of these unions has not
been specially acquired, but follows as an incidental result from the sexual
elements of the two or three forms having been adapted to act on one another in
a particular manner, so that any other kind of union is inefficient, like that
between distinct species. Another and still more remarkable incidental result is
that the seedlings from an illegitimate union are often dwarfed and more or less
or completely barren, like hybrids from the union of two widely distinct
species.
INDEX.
Acanthaceae.
Acer campestre.
Adoxa.
Aegiphila elata.
--mollis.
--obdurata.
Alefeld, Dr., on Linum.
Alisma natans.
Amphicarpaea.
Amsinckia spectabilis.
Variability in length of stamens and pistil.
Anchusa arvensis.
Androsace vitalliana.
Anthers, size of, in different forms.
contabescent.
Arachis.
Arnebia hispidissima.
Ascherson, Dr., on Salvia cleistogama.
Juncus bufonius.
Leersia oryzoides.
Asclepias.
Ash, the common.
Asperula scoparia.
Axell on Primula stricta.
Babington, Professor, on Primula elatior.
Stellaria graminea.
Baillon, emission of the tubes from pollen-grains.
Belhomme, M., on ray-florets.
Bennett, A.W., on Impatiens fulva.
flowers fertilised whilst in the bud state.
Bentham, Mr., on the differentiation of the sexes.
on the cleistogamic flowers of Ononis.
Boragineae.
Boreau on cowslip and primrose.
Borreria.
Bouche on Pavonia.
effect of temperature and light on corolla.
Bouvardia leiantha.
Braun on Dracocephalum.
Breitenbach, W., on Primula elatior.
Bromfield, Dr., on primrose and cowslip.
Primula elatior.
Specularia perfoliata.
Brown, Robert, on sexual changes.
Buckwheat, the common.
Caltha palustris.
Campanula colorata.
Cardamine amara.
Caspary, Professor, on Rhamnus catharticus.
Cattleya.
Chamissoa.
Cinchona micrantha.
Cleistogamic flowers.
list of genera.
on their origin.
Cnicus acaulis.
-- palustris.
Coccocypselum.
pollen-grains of.
Coprosma.
Cordia.
pistil of.
Corolla, difference in size in the sexes of the same species.
Corydalis.
Corylus avellana.
Cowslip, the common.
short- and long-styled.
Cratoxylon formosum.
Crocker, C.W., on Plantago lanceolata.
Cryptostachys.
Cuphea purpurea.
Darwin, Charles, on reproductive organs under cultivation.
intercrossed plants.
prepotency of pollen.
insects fertilising flowers.
Cephalanthera grandiflora.
Epidendron and Cattleya.
number of pollen-grains.
Darwin, W., on Pulmonaria angustifolia.
Datura arborea.
Delpino, plants fertilised by the wind.
on the walnut.
Polygonaceae.
pollen-grains.
Thymus serpyllum.
closed or cleistogamic flowers.
Viola odorata.
Dianthus barbatus.
Dickie, Dr., on Eriophorum angustifolium.
Dictamnus fraxinella.
Diodia.
Dioecious and sub-dioecious plants.
Discospermum.
Doubleday, H., on Primula elatior.
Dracocephalum Moldavicum.
Drosera Anglica.
-- rotundifolia.
Duval-Jouve, M., on Cryptostachys.
Leersia oryzoides.
Dyer, Thiselton, on Salvia Horminum.
Cratoxylon formosum.
Echium vulgare.
Epidendron.
Epigaea repens.
Eranthemum ambiguum.
Eriophorum angustifolium.
Erythroxylum.
pollen-grains of.
Euonymus Europaeus.
Euphrasia officinalis.
Euryale.
Faramea.
pollen-grains of.
Fitzgerald, Mr., on Thelymitra.
Forsythia suspensa.
stamens.
-- viridissima.
Fragaria Chiloensis.
-- elatior.
-- vesca.
-- Virginiana.
Fraxinus excelsior.
Galium cruciatum.
Gartner on the sterility of unions between distinct species.
Primula vulgaris and veris.
hybrid Verbascums.
prepotency of pollen.
variation in the sexual powers of plants.
contabescent anthers.
Gentianeae.
Geraniaceae.
Geranium sylvaticum.
Gesneria pendulina.
Gilia aggregata.
-- coronopifolia.
-- micrantha.
-- nudicaulis.
-- pulchella.
Gillibert on Menyanthes.
Gloriosa Lily, the.
Godron on hybrid Primulas.
Gray, Professor Asa, proposes the term heterogone or heterogonous.
on Linum.
Leucosmia Burnettiana and acuminata.
Forsythia suspensa.
Gilia pulchella.
G. coronopifolia.
Phlox subulata.
Mitchella repens.
heterostyled plants.
Coprosma.
Euonymus.
Rhamnus lanceolatus.
Epigaea repens.
Ilex opaca.
Plantago media.
Oxybaphus and Nyctaginia.
Impatiens fulva.
Leersia.
cleistogamic flowers.
Gyno-dioecious plants.
Hart, Mr., on Nepeta glechoma.
Hautbois Strawberry, the.
Hedyotis.
Henslow, Reverend Professor, on hybrid Primulae.
Henslow, Reverend G., on flowers self-fertilised during the winter.
Herbert, Dr., on hybrid Primulae.
Heterostyled plants, illegitimate offspring of.
essential character of.
summary of the differences of fertility between legitimately and illegitimately
fertilised plants.
diameter of pollen-grains.
size of anthers, structure of stigma.
list of genera.
advantages derived from Heterostylism.
means by which plants became heterostyled.
transmission of form.
equal-styled varieties.
final remarks.
-- dimorphic plants.
-- trimorphic plants.
Hibiscus, pollen-grains.
Hildebrand, Professor, introduces the word “heterostyled.”
on the ray-florets of the Compositae.
Primula Sinensis.
Linum grandiflorum.
L. perenne.
Pulmonaria officinalis.
P. azurea.
Polygonum fagopyrum.
Oxalis.
hermaphrodite plants becoming uni-sexual.
Hordeum.
Homostyled species of Primula.
Hooker Dr., on Campanula.
Hordeum.
Hottonia inflata.
-- palustris.
relative fertility.
anthers of.
papillae on stigma.
Houstonia coerulea.
Hoya carnosa.
Hybrid Primulas.
Hydrangea.
Hypericineae.
Hyssopus officinalis.
Ilex aquifolium
-- opaca.
Illegitimate offspring of heterostyled plants.
Lythrum salicaria, dwarfed stature and sterility.
Oxalis, transmission of form to seedlings.
Primula Sinensis, in some degree dwarfed.
equal-styled varieties.
Primula vulgaris.
transmission of form and colour.
seedlings.
P. veris.
dwarfed stature and sterility.
equal-styled varieties.
parallelism between illegitimate fertilisation and hybridism.
Illecebrum.
Impatiens, pollen-grains of.
-- balsamina.
-- fulva.
-- noli-me-tangere.
Juglans regia.
Juncus bufonius.
Jussieu, A. de, on Malpighiaceae.
Kerner, Professor, on ray-florets.
Auricula.
hybrid forms of Primula.
on use of hairs within the corolla.
size of corolla in male flowers.
use of glands as a protection to flowers.
Kirk, Dr., on Monochoria vaginalis.
Knoxia.
Koch on Primula longiflora.
Krascheninikowia.
Kuhn, Dr., on cleistogamic flowers.
list of plants producing differently formed seeds.
heterostyled plants.
Vandellia nummularifolia.
V. sessiflora.
Lagerstroemia Indica.
-- parviflora.
-- reginae.
Lathyrus nissolia.
Lecoq, H., on the common maple.
cowslips and primroses.
Primula elatior.
Linum Austriacum.
Lythrum hyssopifolia.
Rhamnus.
gyno-dioecious plants.
Scabiosa succisa.
Viola odorata.
Leersia oryzoides.
pollen-grains of.
Leggett, Mr., Pontederia cordata.
Legitimate unions, summary on the fertility of the two, compared with that of
the two illegitimate in Primula.
fertility of, compared with illegitimate.
Leighton, Reverend W.A., on the cowslip and primrose.
Verbascum virgatum.
Leontodon, pollen-grains.
Leptosiphon.
Leucosmia acuminata.
-- Burnettiana.
stigma.
Lily, the Gloriosa.
Limnanthemum Indicum.
pollen-grains.
anthers.
Linaria spuria.
Lindley on Fragaria elatior.
Linnaeus on Primula veris, vulgaris, and elatior.
Linum angustifolium.
-- Austriacum.
-- catharticum
-- corymbiferum.
-- flavum.
stamens.
-- grandiflorum.
various experiments.
pistils and stamens.
sterile with its own-form pollen.
-- Lewisii.
-- perenne.
torsion of the styles.
long-styled form.
stigma.
-- salsoloides.
-- trigynum.
-- usitatissimum.
Lipostoma.
Lysimachia vulgaris.
Lythrum Graefferi.
-- hyssopifolia.
-- salicaria.
power of mutual fertilisation between the three forms.
summary of results.
illegitimate offspring from the three forms.
concluding remarks on.
mid-styled form.
seeds.
-- thymifolia.
Malpighiaceae.
Manettia bicolor.
Maple, the common.
Marshall, W., on Primula elatior.
Plantago lanceolata.
Masters, Dr. Maxwell, on cleistogamic flowers.
Maximowicz on Krascheninikowia.
Meehan, Mr., on Mitchella.
Epigaea repens.
Melissa clinopodium
-- officinalis.
Mello, Correa de, on Arachis.
Voandzeia.
Mentha aquatica.
-- hirsuta.
-- vulgaris.
Menyanthes.
-- trifoliata.
Michalet on Oxalis acetosella.
Linaria spuria.
Mitchella.
-- repens.
Mohl, H. Von, on the common cowslip.
size of corolla in the sexes of the same species.
Trifolium and Arachis.
cleistogamic flowers.
Oxalis acetosella.
Impatiens noli-me-tangere.
Specularia perfoliata.
Mollia lepidota.
-- speciosa.
Monnier, M., on Viola.
Monochoria vaginalis.
Mulberry, the.
Muller, D., on Viola canina.
Muller, Fritz, on pollen of the Villarsia.
Faramea.
Posoqueria fragrans.
Nesaea.
Oxalis.
Pontederia.
Oxalis Regnelli.
Chamissoa.
Muller, H., on the frequency of visits by insects to the Umbelliferae and
Compositae.
on dichogamy.
on Anthophora and Bombylius sucking the cowslip.
Primula elatior.
-- villosa.
Hottonia palustris.
table of relative fertility of.
Linum catharticum.
Polygonum fagopyrum.
Lythrum salicaria.
on the origin of heterostylism.
on the Labiatae.
Thymus serpyllum.
Scabiosa arvensis.
Plantago lanceolata.
size of corolla in the two sexes of the same species.
Impatiens balsamina.
Lysimachia.
Myosotis.
Nepeta glechoma.
Nertera.
Nesaea verticillata.
Nolana prostrata, variability in length of stamens and pistil.
Nyctaginia.
Oldenlandia.
Oleaceae.
Oliver, Professor, on ovules of Primula veris.
Viola.
Campanula colorata.
Ononis columnae.
-- minutissima.
-- parviflora.
Origanum vulgare.
Oxalis acetosella.
pisil of.
cleistogamic flowers.
pollen-grains.
-- Bowii.
-- compressa.
-- corniculata.
-- Deppei.
-- hedysaroides.
-- homostyled species.
-- incarnata.
-- Regnelli.
-- rosea.
-- (Biophytum) sensitiva.
stigma.
-- speciosa.
-- stricta.
-- tropaeoloides.
-- Valdiviana.
Oxlip, the Bardfield.
--, the common.
differences in structure and function between the two parent-species.
effects of crossing.
a hybrid between the cowslip and primrose.
Oxybaphus.
Paeony, pollen-grains of.
Parallelism between illegitimate and hybrid fertilisation.
Pavonia.
Phlox Hentzii.
-- nivalis.
-- subulata.
Planchon on Linum salsoloides.
L. Lewisii.
on Hugonia.
Plantago lanceolata.
-- media.
Polemoniaceae.
Pollen-grains, relative diameter of.
Polyanthus.
Polygonaceae.
Polygonum bistorta.
-- fagopyrum.
pollen-grains.
Pontederia.
pollen-grains.
size of anthers.
-- cordata.
Posoqueria fragrans.
Primrose, the common.
Primula, the, heterostyled species of.
summary on.
homostyled species.
-- auricula.
-- equal-styled varieties.
-- cortusoides.
-- elata.
-- elatior, Jacq.
relative fertility of the two forms.
not a hybrid.
equal-styled var. of.
-- farinosa.
equal-styled var.
-- hirsuta.
-- involucrata.
-- longiflora.
-- mollis.
-- Scotica.
-- Sibirica.
-- Sikkimensis.
-- Sinensis.
relative fertility.
long-styled.
short-styled.
transmission of form, constitution and fertility.
equal-styled variety.
-- stricta.
-- veris.
difference in structure between the two forms.
degrees of fertility when legitimately or illegitimately united.
fertility possessed by illegitimate plants.
equal-styled red variety.
long-styled.
length of pistil.
-- verticillata.
-- villosa.
-- vulgaris (var. acaulis Linn.).
pollen-grains.
relative fertility of the two forms.
length of pistil
Primula vulgaris, var. rubra.
Prunella vulgaris.
Psychotria.
Pulmonaria angustifolia.
anthers.
-- azurea.
-- officinalis.
number of flowers.
pistil.
Ranunculus aquatilis.
Ray-florets, their use.
Rhamnus catharticus.
size of corolla.
-- frangula.
-- lanceolatus.
Rhinanthus crista-galli.
Rubiaceae.
size of anthers.
stigmas.
number of heterostyled genera.
Rudgea eriantha.
Rue, the common.
Ruellia tuberosa.
Salvia.
-- cleistogama.
-- Horminum.
Satureia hortensis.
Scabiosa arvensis.
-- atro-purpurea.
-- succisa.
Scott, J., on Primula auricula.
on Primula vulgaris.
on Primula var. rubra.
on Primula Sikkimensis.
on Primula farinosa.
homostyled Primulae.
hybrids.
length of pistil.
Hottonia palustris.
Androsace vitalliana.
Polyanthus.
Mitchella repens.
Acanthaceae.
Eranthemum ambiguum bearing three kinds of flowers.
Scrophularia aquatica.
Serratula tinctoria.
Sethia acuminata.
-- obtusifolia.
Smith, Sir J.E., on the carrot.
hybrid Verbascums.
Serratula tinctoria.
Cnicus.
Subularia.
Soldanella alpina.
Specularia perfoliata.
Spence, Mr., on Mollia.
Spermacoce.
Sprengel on Hottonia palustris.
Stellaria graminea.
Strawberry, the Hautbois.
Subularia.
Suteria.
Thelymitra.
Thomson, Dr., on Campanula.
Thrum-eyed, origin of term.
Thwaites, Mr., on ovules of Limnanthemum Indicum.
Sethia acuminata.
Discospermum.
Thymelia.
Thymus citriodorus.
-- serpyllum
-- vulgaris.
Timbal-Lagrave, M., on hybrids in genus Cistus.
Torrey, Dr., on Hottonia inflata.
Transmission of the two forms of heterostyled plants.
Treviranus on Androsace vitalliana.
Linum.
Vandellia nummularifolia.
-- sessifloria.
Vaucher on the carrot.
Soldanella alpina.
Lythrum salicaria.
-- thymifolia.
Ilex aquifolium.
on Labiatae.
Viola hirta and collina.
Verbascum, wild hybrids of.
-- lychnitis.
-- phoeniceum.
-- thapsus.
-- virgatum.
Viburnum.
Vicia.
Villarsia.
anthers.
Viola alba.
-- bicolor.
-- biflora.
-- canina.
-- collina.
-- elatior.
-- hirta.
-- ionodium.
-- lancifolia.
-- mirabilis.
-- nana.
pollen-grains of.
-- odorata.
-- palustris.
-- Roxburghiana.
-- Ruppii.
-- sylvatica.
-- tricolor.
Voandzeia.
Walnut, the.
Watson, H.C., on cowslips, primroses, and Oxlips.
Primula elatior.
Weddell, Dr., on hybrids between Aceras and Orchis.
Wetterhan, Mr., on Corylus.
Wichura, Max, on hybrid willows.
sterile hybrids.
Wirtgen on Lythrum salicaria.
Wooler, W., on Polyanthus.
Wray, Leonard, on Fragaria.