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This Project Gutenbeg Etext was prepared by Sue Asscher asschers@dingoblue.net.au
THE DIFFERENT FORMS OF FLOWERS
ON PLANTS OF THE SAME SPECIES
BY CHARLES DARWIN, M.A., F.R.S.
INTRODUCTION.
CHAPTER I.
HETEROSTYLED DIMORPHIC PLANTS: PRIMULACEAE.
Primula veris or the cowslip.--Differences in structure between the two forms.-- Their degrees of fertility when legitimately and illegitimately united.--P. elatior, vulgaris, Sinensis, auricula, etc.--Summary on the fertility of the heterostyled species of Primula.--Homostyled species of Primula.--Hottonia palustris.--Androsace vitalliana.
CHAPTER II.
HYBRID PRIMULAS.
The oxlip a hybrid naturally produced between Primula veris and vulgaris.--The differences in structure and function between the two parent-species.--Effects of crossing long-styled and short-styled oxlips with one another and with the two forms of both parent-species.--Character of the offspring from oxlips artificially self-fertilised and cross-fertilised in a state of nature.--Primula elatior shown to be a distinct species.--Hybrids between other heterostyled species of Primula.--Supplementary note on spontaneously produced hybrids in the genus Verbascum.
CHAPTER III.
HETEROSTYLED DIMORPHIC PLANTS--continued.
Linum grandiflorum, long-styled form utterly sterile with own-form pollen.-- Linum perenne, torsion of the pistils in the long-styled form alone.--Homostyled species of Linum.--Pulmonaria officinalis, singular difference in self-fertility between the English and German long-styled plants.--Pulmonaria angustifolia shown to be a distinct species, long-styled form completely self-sterile.-- Polygonum fagopyrum.--Various other heterostyled genera.--Rubiaceae.--Mitchella repens, fertility of the flowers in pairs.--Houstonia.--Faramea, remarkable difference in the pollen-grains of the two forms; torsion of the stamens in the short-styled form alone; development not as yet perfect.--The heterostyled structure in the several Rubiaceous genera not due to descent in common.
CHAPTER IV.
HETEROSTYLED TRIMORPHIC PLANTS.
Lythrum salicaria.--Description of the three forms.--Their power and complex manner of fertilising one another.--Eighteen different unions possible.--Mid- styled form eminently feminine in nature.--Lythrum Graefferi likewise trimorphic.--L. hymifolia dimorphic.--L. hyssopifolia homostyled.--Nesaea verticillata trimorphic.--Lagerstroemia, nature doubtful.--Oxalis, trimorphic species of.--O. Valdiviana.--O. Regnelli, the illegitimate unions quite barren.- -O. speciosa.--O. sensitiva.--Homostyled species of Oxalis.--Pontederia, the one monocotyledonous genus known to include heterostyled species.
CHAPTER V.
ILLEGITIMATE OFFSPRING OF HETEROSTYLED PLANTS.
Illegitimate offspring from all three forms of Lythrum salicaria.--Their dwarfed stature and sterility, some utterly barren, some fertile.--Oxalis, transmission of form to the legitimate and illegitimate seedlings.--Primula Sinensis, illegitimate offspring in some degree dwarfed and infertile.--Equal-styled varieties of P. Sinensis, auricula, farinosa, and elatior.--P. vulgaris, red- flowered variety, illegitimate seedlings sterile.--P. veris, illegitimate plants raised during several successive generations, their dwarfed stature and sterility.--Equal-styled varieties of P. veris.--Transmission of form by Pulmonaria and Polygonum.--Concluding remarks.--Close parallelism between illegitimate fertilisation and hybridism.
CHAPTER VI.
CONCLUDING REMARKS ON HETEROSTYLED PLANTS.
The essential character of heterostyled plants.--Summary of the differences in fertility between legitimately and illegitimately fertilised plants.--Diameter of the pollen-grains, size of anthers and structure of stigma in the different forms.--Affinities of the genera which include heterostyled species.--Nature of the advantages derived from heterostylism.--The means by which plants became heterostyled.--Transmission of form.--Equal-styled varieties of heterostyled plants.--Final remarks.
CHAPTER VII.
POLYGAMOUS, DIOECIOUS, AND GYNO-DIOECIOUS PLANTS.
The conversion in various ways of hermaphrodite into dioecious plants.-- Heterostyled plants rendered dioecious.--Rubiaceae.--Verbenaceae.--Polygamous and sub-dioecious plants.--Euonymus.--Fragaria.--The two sub-forms of both sexes of Rhamnus and Epigaea.--Ilex.--Gyno-dioecious plants.--Thymus, difference in fertility of the hermaphrodite and female individuals.--Satureia.--Manner in which the two forms probably originated.--Scabiosa and other gyno-dioecious plants.--Difference in the size of the corolla in the forms of polygamous, dioecious, and gyno-dioecious plants.
CHAPTER VIII.
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.
INTRODUCTION.
THE subject of the present volume, namely the differently formed flowers normally produced by certain kinds of plants, either on the same stock or on distinct stocks, ought to have been treated by a professed botanist, to which distinction I can lay no claim. As far as the sexual relations of flowers are concerned, Linnæus long ago divided them into hermaphrodite, monœcious, diœcious, and polygamous species. This fundamental distinction, with the aid of several subdivisions in each of the four classes, will serve my purpose; but the classification is artificial, and the groups often pass into one another.
The hermaphrodite class contains two interesting sub-groups, namely, heterostyled and cleistogamic plants; but there are several other less important subdivisions, presently to be given, in which flowers differing in various ways from one another are produced by the same species.
Some plants were described by me several years ago, in a series of papers read before the Linnean Society,* the individuals of which exist under two or three forms, differing in the length of their pistils and stamens and in other respects. They were called by me dimorphic and trimorphic, but have since been better named by Hildebrand, heterostyled.**
* "On the Two Forms or Dimorphic Condition in the
Species of Primula, and on their remarkable Sexual Relations."
'Journal of the Proceedings of the Linnean Society,' vol. vi.
1862, p. 77.
"On the Existence of Two Forms, and on their Reciprocal Sexual
Relation, in several Species of the Genus Linum." Ibid. vol. vii.
1863, p. 69.
"On the Sexual Relations of the Three Forms of Lythrum
salicaria.' Ibid. vol. viii. 1864, p. 169.
"On the Character and Hybrid-like Nature of the Offspring from
the Illegitimate Unions of Dimorphic and Trimorphic Plants."
Ibid. vol. x. 1868, p. 393.
"On the Specific Differences between Primula veris, Brit. Fl.
(var. officinalis, Linn.), P. vulgaris, Brit. Fl. (var. acaulis,
Linn.), and P. elatior, Jacq.; and on the Hybrid Nature of the
Common Oxlip. With Supplementary Remarks on Naturally Produced
Hybrids in the Genus Verbascum." Ibid. vol. x. 1868, p. 437.
** The term "heterostyled" does not express all the differences between the forms; but this is a failure common in many cases. As the term has been adopted by writers in various countries, I am unwilling to change it for that of heterogone or heterogonous, though this has been proposed by so high an authority as Prof. Asa Gray: see the 'American Naturalist,' Jan. 1877, p. 42.
As I have many still unpublished observations with respect to these plants, it has seemed to me advisable to republish my former papers in a connected and corrected form, together with the new matter. It will be shown that these heterostyled plants are adapted for reciprocal fertilisation; so that the two or three forms, though all are hermaphrodites, are related to one another almost like the males and females of ordinary unisexual animals. I will also give a full abstract of such observations as have been published since the appearance of my papers; but only those cases will be noticed, with respect to which the evidence seems fairly satisfactory. Some plants have been supposed to be heterostyled merely from their pistils and stamens varying greatly in length, and I have been myself more than once thus deceived. With some species the pistil continues growing for a long time, so that if old and young flowers are compared they might be thought to be heterostyled. Again, a species tending to become diœcious, with the stamens reduced in some individuals and with the pistils in others, often presents a deceptive appearance. Unless it be proved that one form is fully fertile only when it is fertilised with pollen from another form, we have not complete evidence that the species is heterostyled. But when the pistils and stamens differ in length in two or three sets of individuals, and this is accompanied by a difference in the size of the pollen-grains or in the state of the stigma, we may infer with much safety that the species is heterostyled. I have, however, occasionally trusted to a difference between the two forms in the length of the pistil alone, or in the length of the stigma together with its more or less papillose condition; and in one instance differences of this kind have been proved by trials made on the fertility of the two forms, to be sufficient evidence.
The second sub-group above referred to consists of hermaphrodite plants, which bear two kinds of flowers—the one perfect and fully expanded—the other minute, completely closed, with the petals rudimentary, often with some of the anthers aborted, and the remaining ones together with the stigmas much reduced in size; yet these flowers are perfectly fertile. They have been called by Dr. Kuhn* cleistogamic, and they will be described in the last chapter of this volume. They are manifestly adapted for self-fertilisation, which is effected at the cost of a wonderfully small expenditure of pollen; whilst the perfect flowers produced by the same plant are capable of cross-fertilisation. Certain aquatic species, when they flower beneath the water, keep their corollas closed, apparently to protect their pollen; they might therefore be called cleistogamic, but for reasons assigned in the proper place are not included in the present sub-group. Several cleistogamic species, as we shall hereafter see, bury their ovaries or young capsules in the ground; but some few other plants behave in the same manner; and, as they do not bury all their flowers, they might have formed a small separate subdivision.
* 'Botanische Zeitung,' 1867, p. 65. Several plants are known occasionally to produce flowers destitute of a corolla; but they belong to a different class of cases from cleistogamic flowers. This deficiency seems to result from the conditions to which the plants have been subjected, and partakes of the nature of a monstrosity. All the flowers on the same plant are commonly affected in the same manner. Such cases, though they have sometimes been ranked as cleistogamic, do not come within our present scope: see Dr. Maxwell Masters, 'Vegetable Teratology,' 1869, p. 403.
Another interesting subdivision consists of certain plants, discovered by H. Müller, some individuals of which bear conspicuous flowers adapted for cross-fertilisation by the aid of insects, and others much smaller and less conspicuous flowers, which have often been slightly modified so as to ensure self-fertilisation. Lysimachia vulgaris, Euphrasia officinalis, Rhinanthus crista-galli, and Viola tricolor come under this head.*
* H. Müller, 'Nature,' Sept. 25, 1873 (vol. viii.), p. 433, and Nov. 20, 1873 (vol. ix.), p. 44. Also 'Die Befruchtung der Blumen,' etc., 1873, p. 294.
The smaller and less conspicuous flowers are not closed, but as far as the purpose which they serve is concerned, namely, the assured propagation of the species, they approach in nature cleistogamic flowers; but they differ from them by the two kinds being produced on distinct plants.
With many plants, the flowers towards the outside of the inflorescence are much larger and more conspicuous than the central ones. As I shall not have occasion to refer to plants of this kind in the following chapters, I will here give a few details respecting them. It is familiar to every one that the ray-florets of the Compositæ often differ remarkably from the others; and so it is with the outer flowers of many Umbelliferæ, some Cruciferæ and a few other families. Several species of Hydrangea and Viburnum offer striking instances of the same fact. The Rubiaceous genus Mussænda presents a very curious appearance from some of the flowers having the tip of one of the sepals developed into a large petal-like expansion, coloured either white or purple. The outer flowers in several Acanthaceous genera are large and conspicuous but sterile; the next in order are smaller, open, moderately fertile and capable of cross-fertilisation; whilst the central ones are cleistogamic, being still smaller, closed and highly fertile; so that here the inflorescence consists of three kinds of flowers.*
* J. Scott, 'Journal of Botany,' London, new series, vol. i. 1872, pp. 161-164.
From what we know in other cases of the use of the corolla, coloured bracteæ, etc., and from what H. Müller has observed* on the frequency of the visits of insects to the flower-heads of the Umbelliferæ and Compositæ being largely determined by their conspicuousness, there can be no doubt that the increased size of the corolla of the outer flowers, the inner ones being in all the above cases small, serves to attract insects. The result is that cross-fertilisation is thus favoured. Most flowers wither soon after being fertilised, but Hildebrand states** that the ray-florets of the Compositæ last for a long time, until all those on the disc are impregnated; and this clearly shows the use of the former.
* 'Die Befruchtung der Blumen,' pp. 108, 412.
** See his interesting memoir, 'Ueber die Geschlechtsverhältnisse bei den Compositen,' 1869, p. 92.
The ray-florets, however, are of service in another and very different manner, namely, by folding inwards at night and during cold rainy weather, so as to protect the florets of the disc.* Moreover they often contain matter which is excessively poisonous to insects, as may be seen in the use of flea-powder, and in the case of Pyrethrum, M. Belhomme has shown that the ray-florets are more poisonous than the disc-florets in the ratio of about three to two. We may therefore believe that the ray-florets are useful in protecting the flowers from being gnawed by insects.*
* 'Gardener's Chronicle,' 1861, p. 1067. Lindley, 'Vegetable Kingdom,' on Chrysanthemum, 1853, p. 706. Kerner in his interesting essay ('Die Schutzmittel der Blüthen gegen unberufene Gäste,' 1875, p. 19) insists that the petals of most plants contain matter which is offensive to insects, so that they are seldom gnawed, and thus the organs of fructification are protected. My grandfather in 1790 ('Loves of the Plants,' canto iii. note to lines 184, 188) remarks that "The flowers or petals of plants are perhaps in general more acrid than their leaves; hence they are much seldomer eaten by insects."
It is a well-known yet remarkable fact that the circumferential flowers of many of the foregoing plants have both their male and female reproductive organs aborted, as with the Hydrangea, Viburnum and certain Compositæ or the male organs alone are aborted, as in many Compositæ. Between the sexless, female and hermaphrodite states of these latter flowers, the finest gradations may be traced, as Hildebrand has shown.**
* Kerner clearly shows that this is the case: 'Die Schutzmittel des Pollens,' 1873, p. 28.
** 'Ueber die Geschlechtsverhältnisse bei den Compositen,' 1869, pp. 78-91.
He also shows that there is a close relation between the size of the corolla in the ray-florets and the degree of abortion in their reproductive organs. As we have good reason to believe that these florets are highly serviceable to the plants which possess them, more especially by rendering the flower-heads conspicuous to insects, it is a natural inference that their corollas have been increased in size for this special purpose; and that their development has subsequently led, through the principle of compensation or balancement, to the more or less complete reduction of the reproductive organs. But an opposite view may be maintained, namely, that the reproductive organs first began to fail, as often happens under cultivation,* and, as a consequence, the corolla became, through compensation, more highly developed. This view, however, is not probable, for when hermaphrodite plants become diœcious or gyno-diœcious—that is, are converted into hermaphrodites and females—the corolla of the female seems to be almost invariably reduced in size in consequence of the abortion of the male organs. The difference in the result in these two classes of cases, may perhaps be accounted for by the matter saved through the abortion of the male organs in the females of gyno-diœcious and diœcious plants being directed (as we shall see in a future chapter) to the formation of an increased supply of seeds; whilst in the case of the exterior florets and flowers of the plants which we are here considering, such matter is expended in the development of a conspicuous corolla. Whether in the present class of cases the corolla was first affected, as seems to me the more probable view, or the reproductive organs first failed, their states of development are now firmly correlated. We see this well illustrated in Hydrangea and Viburnum; for when these plants are cultivated, the corollas of both the interior and exterior flowers become largely developed, and their reproductive organs are aborted.
* I have discussed this subject in my 'Variation of Animals and Plants under Domestication,' chap. xviii. 2nd edit. vol. ii. pp. 152, 156.
There is a closely analogous subdivision of plants, including the genus Muscari (or Feather Hyacinth) and the allied Bellevalia, which bear both perfect flowers and closed bud-like bodies that never expand. The latter resemble in this respect cleistogamic flowers, but differ widely from them in being sterile and conspicuous. Not only the aborted flower-buds and their peduncles (which are elongated apparently through the principle of compensation) are brightly coloured, but so is the upper part of the spike—all, no doubt, for the sake of guiding insects to the inconspicuous perfect flowers. From such cases as these we may pass on to certain Labiatæ, for instance, Salvia Horminum, in which (as I hear from Mr. Thiselton Dyer) the upper bracts are enlarged and brightly coloured, no doubt for the same purpose as before, with the flowers suppressed.
In the Carrot and some allied Umbelliferæ, the central flower has its petals somewhat enlarged, and these are of a dark purplish-red tint; but it cannot be supposed that this one small flower makes the large white umbel at all more conspicuous to insects. The central flowers are said* to be neuter or sterile, but I obtained by artificial fertilisation a seed (fruit) apparently perfect from one such flower.
* 'The English Flora,' by Sir J. E. Smith, 1824, vol. ii. p. 39.
Occasionally two or three of the flowers next to the central one are similarly characterised; and according to Vaucher* "cette singulière dégénération s'étend quelquefois à l'ombelle entière." That the modified central flower is of no functional importance to the plant is almost certain.
* 'Hist. Phys. des Plantes d'Europe,' 1841, tom. ii. p. 614. On the Echinophora, p. 627.
It may perhaps be a remnant of a former and ancient condition of the species, when one flower alone, the central one, was female and yielded seeds, as in the umbelliferous genus Echinophora. There is nothing surprising in the central flower tending to retain its former condition longer than the others; for when irregular flowers become regular or peloric, they are apt to be central; and such peloric flowers apparently owe their origin either to arrested development—that is, to the preservation of an early stage of development—or to reversion. Central and perfectly developed flowers in not a few plants in their normal condition (for instance, the common Rue and Adoxa) differ slightly in structure, as in the number of the parts, from the other flowers on the same plant. All such cases seem connected with the fact of the bud which stands at the end of the shoot being better nourished than the others, as it receives the most sap.*
* This whole subject, including pelorism, has been discussed, and references given, in my 'Variation of Animals and Plants under Domestication,' chap. xxvi. 2nd edit. vol. ii. p. 338.
The cases hitherto mentioned relate to hermaphrodite species which bear differently constructed flowers; but there are some plants that produce differently formed seeds, of which Dr. Kuhn has given a list.*
* 'Bot. Zeitung,' 1867, p. 67.
With the Umbelliferæ and Compositæ, the flowers that produce these seeds likewise differ, and the differences in the structure of the seeds are of a very important nature. The causes which have led to differences in the seeds on the same plant are not known; and it is very doubtful whether they subserve any special end.
We now come to our second Class, that of monœcious species, or those which have their sexes separated but borne on the same plant. The flowers necessarily differ, but when those of one sex include rudiments of the other sex, the difference between the two kinds is usually not great. When the difference is great, as we see in catkin-bearing plants, this depends largely on many of the species in this, as well as in the next or diœcious class, being fertilised by the aid of the wind;* for the male flowers have in this case to produce a surprising amount of incoherent pollen. Some few monœcious plants consist of two bodies of individuals, with their flowers differing in function, though not in structure; for certain individuals mature their pollen before the female flowers on the same plant are ready for fertilisation, and are called proterandrous; whilst conversely other individuals, called proterogynous, have their stigmas mature before their pollen is ready. The purpose of this curious functional difference obviously is to favour the cross-fertilisation of distinct plants. A case of this kind was first observed by Delpino in the Walnut (Juglans regia), and has since been observed with the common Nut (Corylus avellana). I may add that according to H. Müller the individuals of some few hermaphrodite plants differ in a like manner; some being proterandrous and others proterogynous.** On cultivated trees of the Walnut and Mulberry, the male flowers have been observed to abort on certain individuals***, which have thus been converted into females; but whether there are any species in a state of nature which co-exist as monœcious and female individuals, I do not know.
* Delpino, 'Studi sopra uno Lignaggio Anemofilo.' Firenze, 1871.
† Delpino, 'Ult. Osservazioni sulla Dicogamia.' part ii. fasc. ii. p. 337. Mr. Wetterhan and H. Müller on Corylus, 'Nature,' vol. xi. p. 507, and 1875, p. 26. On proterandrous and proterogynous hermaphrodite individuals of the same species, see H. Müller, 'Die Befruchtung,' etc., pp. 285, 339.
*** 'Gardener's Chron.' 1847, pp. 541, 558.
The third Class consists of diœcious species, and the remarks made under the last class with respect to the amount of difference between the male and female flowers are here applicable. It is at present an inexplicable fact that with some diœcious plants, of which the Restiaceæ of Australia and the Cape of Good Hope offer the most striking instance, the differentiation of the sexes has affected the whole plant to such an extent (as I hear from Mr. Thiselton Dyer) that Mr. Bentham and Professor Oliver have often found it impossible to match the male and female specimens of the same species. In my seventh chapter some observations will be given on the gradual conversion of heterostyled and of ordinary hermaphrodite plants into diœcious or sub-diœcious species.
The fourth and last Class consists of the plants which were called polygamous by Linnæus; but it appears to me that it would be convenient to confine this term to the species which co-exist as hermaphrodites, males and females; and to give new names to several other combinations of the sexes, a plan which I shall here follow. Polygamous plants, in this confined sense of the term, may be divided into two sub-groups, according as the three sexual forms are found on the same individual or on distinct individuals. Of this latter or trioicous sub-group, the common Ash (Fraxinus excelsior) offers a good instance: thus, I examined during the spring and autumn fifteen trees growing in the same field; and of these, eight produced male flowers alone, and in the autumn not a single seed; four produced only female flowers, which set an abundance of seeds; three were hermaphrodites, which had a different aspect from the other trees whilst in flower, and two of them produced nearly as many seeds as the female trees, whilst the third produced none, so that it was in function a male. The separation of the sexes, however, is not complete in the Ash; for the female flowers include stamens, which drop off at an early period, and their anthers, which never open or dehisce, generally contain pulpy matter instead of pollen. On some female trees, however, I found a few anthers containing pollen-grains apparently sound. On the male trees most of the flowers include pistils, but these likewise drop off at an early period; and the ovules, which ultimately abort, are very small compared with those in female flowers of the same age.
Of the other or monoicous sub-group of polygamous plants, or those which bear hermaphrodite, male and female flowers on the same individual, the common Maple (Acer campestre) offers a good instance; but Lecoq states* that some trees are truly diœcious, and this shows how easily one state passes into another.
A considerable number of plants generally ranked as polygamous exist under only two forms, namely, as hermaphrodites and females; and these may be called gyno-diœcious, of which the common Thyme offers a good example. In my seventh chapter I shall give some observations on plants of this nature. Other species, for instance several kinds of Atriplex, bear on the same plant hermaphrodite and female flowers; and these might be called gyno-monœcious, if a name were desirable for them.
* 'Géographie Botanique,' tom. v. p. 367.
Again there are plants which produce hermaphrodite and male flowers on the same individual, for instance, some species of Galium, Veratrum, etc.; and these might be called andro-monœcious. If there exist plants, the individuals of which consist of hermaphrodites and males, these might be distinguished as andro-diœcious. But, after making inquiries from several botanists, I can hear of no such cases. Lecoq, however, states,* but without entering into full details, that some plants of Caltha palustris produce only male flowers, and that these live mingled with the hermaphrodites. The rarity of such cases as this last one is remarkable, as the presence of hermaphrodite and male flowers on the same individual is not an unusual occurrence; it would appear as if nature did not think it worth while to devote a distinct individual to the production of pollen, excepting when this was indispensably necessary, as in the case of diœcious species.
* 'Géographie Botanique,' tom. iv. p. 488.
I have now finished my brief sketch of the several cases, as far as known to me, in which flowers differing in structure or in function are produced by the same species of plant. Full details will be given in the following chapters with respect to many of these plants. I will begin with the heterostyled, then pass on to certain diœcious, sub-diœcious, and polygamous species, and end with the cleistogamic. For the convenience of the reader, and to save space, the less important cases and details have been printed in smaller type.
I cannot close this Introduction without expressing my warm thanks to Dr. Hooker for supplying me with specimens and for other aid; and to Mr. Thiselton Dyer and Professor Oliver for giving me much information and other assistance. Professor Asa Gray, also, has uniformly aided me in many ways. To Fritz Müller of St. Catharina, in Brazil, I am indebted for many dried flowers of heterostyled plants, often accompanied with valuable notes.
