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 ma.s.s 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 v.a.g.i.n.alis 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 cla.s.sifications, for the greater number of the characters proper to the species, to the genus, to the family, to the cla.s.s 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 h.o.m.ological 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 dest.i.tute of pollen as were those which I examined.
Juncus bufonius and Hordeum.
All the species. .h.i.therto 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 gra.s.s 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 gra.s.s 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 as.e.xual 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 pa.s.sage 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 a.s.serts "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-ma.s.ses 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 pa.s.sage 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 quant.i.ty 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 circ.u.mstances 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 cla.s.ses, 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. .h.i.therto 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 a.n.a.logy. But of such invigoration I have only a small amount of direct evidence: two crossed seedlings of Ononis minutissima were put into compet.i.tion 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 cla.s.s, 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 pa.s.sage 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 cla.s.s 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 s.e.xes would have been injurious, had not pollen been already transported habitually by insects or by the wind from flower to flower, we may a.s.sume 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 s.e.xes 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-cla.s.s, 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-cla.s.s 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 a.s.sured. For the full and legitimate fertilisation of these plants pollen from the one form must be applied to the stigma of another. If the s.e.xual 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 s.e.xual 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.