The Different Forms of Flowers on Plants of the Same Species

In the Carrot and some allied Umbelliferae, 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. (Introduction/12. 'The English Flora' by Sir J.E. Smith 1824 volume 2 page 39.) Occasionally two or three of the flowers next to the central one are similarly characterised; and according to Vaucher "cette singuliere degeneration s'etend quelquefois a l'ombelle entiere." (Introduction/13. 'Hist. Phys. des Plantes d'Europe' 1841 tome 2 page 614. On the Echinophora page 627.) That the modified central flower is of no functional importance to the plant is almost certain. 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. (Introduction/14. This whole subject, including pelorism, has been discussed, and references given in my 'Variation of Animals and Plants under Domestication' chapter 26 2nd edition volume 2 page 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. (Introduction/15. 'Botanische Zeitung' 1867 page 67.) With the Umbelliferae and Compositae, 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 monoecious 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 dioecious 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. (Introduction/16. Delpino 'Studi sopra uno Lignaggio Anemofilo' Firenze 1871.) Some few monoecious 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. Muller the individuals of some few hermaphrodite plants differ in a like manner; some being proterandrous and others proterogynous. (Introduction/17. Delpino 'Ult. Osservazioni sulla Dicogamia' part 2 fasc 2 page 337. Mr. Wetterhan and H. Muller on Corylus 'Nature' volume 11 page 507 and 1875 page 26. On proterandrous and proterogynous hermaphrodite individuals of the same species, see H. Muller 'Die Befruchtung' etc. pages 285, 339.) 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 monoecious and female individuals, I do not know. (Introduction/18. 'Gardener's Chronicle' 1847 pages 541, 558.) The third Class consists of dioecious 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 dioecious plants, of which the Restiaceae 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 dioecious or sub-dioecious species. The fourth and last Class consists of the plants which were called polygamous by Linnaeus; but it appears to me that it would be convenient to confine this term to the species which coexist 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 dioecious, and this shows how easily one state passes into another. (Introduction/19. 'Geographie Botanique' tome 5 page 367.) 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- dioecious, 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-monoecious, if a name were desirable for them. 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-monoecious. If there exist plants, the individuals of which consist of hermaphrodites and males, these might be distinguished as andro-dioecious. 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. (Introduction/20. 'Geographie Botanique' tome 4 page 488.) 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 dioecious species. 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 dioecious, sub-dioecious, 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 Muller of St. Catharina, in Brazil, I am indebted for many dried flowers of heterostyled plants, often accompanied with valuable notes. 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. (FIGURE 1.1. Primula veris. Left: Long-styled form. Right: Short-styled form.) It has long been known to botanists that the common cowslip (Primula veris, Brit. Flora, var. officinalis, Lin.) exists under two forms, about equally numerous, which obviously differ from each other in the length of their pistils and stamens. (1/1. This fact, according to Von Mohl 'Botanische Zeitung' 1863 page 326, was first observed by Persoon in the year 1794.) This difference has hitherto been looked at as a case of mere variability, but this view, as we shall presently see, is far from the true one. Florists who cultivate the Polyanthus and Auricula have long been aware of the two kinds of flowers, and they call the plants which display the globular stigma at the mouth of the corolla, "pin-headed" or "pin-eyed," and those which display the anthers, "thrum-eyed." (1/2. In Johnson's Dictionary, "thrum" is said to be the ends of weavers' threads; and I suppose that some weaver who cultivated the Polyanthus invented this name, from being struck with some degree of resemblance between the cluster of anthers in the mouth of the corolla and the ends of his threads.) I will designate the two forms as the long-styled and short-styled. The pistil in the long-styled form is almost exactly twice as long as that of the short-styled. The stigma stands in the mouth of the corolla or projects just above it, and is thus externally visible. It stands high above the anthers, which are situated halfway down the tube and cannot be easily seen. In the short-styled form the anthers are attached near the mouth of the tube, and therefore stand above the stigma, which is seated in about the middle of the tubular corolla. The corolla itself is of a different shape in the two forms; the throat or expanded portion above the attachment of the anthers being much longer in the long-styled than in the short-styled form. Village children notice this difference, as they can best make necklaces by threading and slipping the corollas of the long-styled flowers into one another. But there are much more important differences. The stigma in the long-styled form is globular; in the short-styled it is depressed on the summit, so that the longitudinal axis of the former is sometimes nearly double that of the latter. Although somewhat variable in shape, one difference is persistent, namely, in roughness: in some specimens carefully compared, the papillae which render the stigma rough were in the long- styled form from twice to thrice as long as in the short-styled. The anthers do not differ in size in the two forms, which I mention because this is the case with some heterostyled plants. The most remarkable difference is in the pollen- grains. I measured with the micrometer many specimens, both dry and wet, taken from plants growing in different situations, and always found a palpable difference. The grains distended with water from the short-styled flowers were about .038 millimetres (10 to 11/7000 of an inch) in diameter, whilst those from the long-styled were about .0254 millimetres (7/7000 of an inch), which is in the ratio of 100 to 67. The pollen-grains therefore from the longer stamens of the short-styled form are plainly larger than those from the shorter stamens of the long-styled. When examined dry, the smaller grains are seen under a low power to be more transparent than the larger grains, and apparently in a greater degree than can be accounted for by their less diameter. There is also a difference in shape, the grains from the short-styled plants being nearly spherical, those from the long-styled being oblong with the angles rounded; this difference disappears when the grains are distended with water. The long-styled plants generally tend to flower a little before the short-styled: for instance, I had twelve plants of each form growing in separate pots and treated in every respect alike; and at the time when only a single short-styled plant was in flower, seven of the long-styled had expanded their flowers. We shall, also, presently see that the short-styled plants produce more seed than the long-styled. It is remarkable, according to Professor Oliver, that the ovules in the unexpanded and unimpregnated flowers of the latter are considerably larger than those of the short-styled flowers (1/3. 'Natural History Review' July 1862 page 237.); and this I suppose is connected with the long-styled flowers producing fewer seeds, so that the ovules have more space and nourishment for rapid development. To sum up the differences:--The long-styled plants have a much longer pistil, with a globular and much rougher stigma, standing high above the anthers. The stamens are short; the grains of pollen smaller and oblong in shape. The upper half of the tube of the corolla is more expanded. The number of seeds produced is smaller and the ovules larger. The plants tend to flower first. The short-styled plants have a short pistil, half the length of the tube of the corolla, with a smooth depressed stigma standing beneath the anthers. The stamens are long; the grains of pollen are spherical and larger. The tube of the corolla is of uniform diameter except close to the upper end. The number of seeds produced is larger. I have examined a large number of flowers; and though the shape of the stigma and the length of the pistil both vary, especially in the short-styled form, I have never met with any transitional states between the two forms in plants growing in a state of nature. There is never the slightest doubt under which form a plant ought to be classed. The two kinds of flowers are never found on the same individual plant. I marked many cowslips and primroses, and on the following year all retained the same character, as did some in my garden which flowered out of their proper season in the autumn. Mr. W. Wooler, of Darlington, however, informs us that he has seen early blossoms on the Polyanthus, which were not long-styled, but became so later in the season. (1/4. I have proved by numerous experiments, hereafter to be given, that the Polyanthus is a variety of Primula veris.) Possibly in this case the pistils may not have been fully developed during the early spring. An excellent proof of the permanence of the two forms may be seen in nursery-gardens, where choice varieties of the Polyanthus are propagated by division; and I found whole beds of several varieties, each consisting exclusively of the one or the other form. The two forms exist in the wild state in about equal numbers: I collected 522 umbels from plants growing in several stations, taking a single umbel from each plant; and 241 were long-styled, and 281 short-styled. No difference in tint or size could be perceived in the two great masses of flowers. We shall presently see that most of the species of Primula exist under two analogous forms; and it may be asked what is the meaning of the above-described important differences in their structure? The question seems well worthy of careful investigation, and I will give my observations on the cowslip in detail. The first idea which naturally occurred to me was, that this species was tending towards a dioecious condition; that the long-styled plants, with their longer pistils, rougher stigmas, and smaller pollen-grains, were more feminine in nature, and would produce more seed;--that the short-styled plants, with their shorter pistils, longer stamens and larger pollen-grains, were more masculine in nature. Accordingly, in 1860, I marked a few cowslips of both forms growing in my garden, and others growing in an open field, and others in a shady wood, and gathered and weighed the seed. In all the lots the short-styled plants yielded, contrary to my expectation, most seed. Taking the lots together, the following is the result:-- TABLE 1.1. Column 1: Plant. Column 2: Number of Plants. Column 3: Number of Umbels Produced. Column 4: Number of Capsules Produced. Column 5: Weight of Seed In Grains. Short-styled cowslips : 9 : 33 : 199 : 83. Long-styled cowslips : 13 : 51 : 261 : 91. If we compare the weight from an equal number of plants, and from an equal number of umbels, and from an equal number of capsules of the two forms, we get the following results:-- TABLE 1.2. Column 1: Plant. Column 2: Number of Plants. Column 3: Weight of Seed in grains. ... Column 4: Number of Umbels. Column 5: Weight of Seed. ... Column 6: Number of Capsules. Column 7: Weight of Seed in grains. Short-styled cowslips : 10 : 92 :: 100 : 251 :: 100 : 41. Long-styled cowslips : 10 : 70 :: 100 : 178 :: 100 : 34. So that, by all these standards of comparison, the short-styled form is the more fertile; if we take the number of umbels (which is the fairest standard, for large and small plants are thus equalised), the short-styled plants produce more seed than the long-styled, in the proportion of nearly four to three. In 1861 the trial was made in a fuller and fairer manner. A number of wild plants had been transplanted during the previous autumn into a large bed in my garden, and all were treated alike; the result was:-- TABLE 1.3. Column 1: Plant. Column 2: Number of Plants. Column 3: Number of Umbels. Column 4: Weight of Seed in grains. Short-styled cowslips : 47 : 173 : 745. Long-styled cowslips : 58 : 208 : 692. These figures give us the following proportions:-- TABLE 1.4. Column 1: Plant. Column 2: Number of Plants. Column 3: Weight of Seed in grains. ... Column 4: Number of Umbels. Column 5: Weight of Seed in grains. Short-styled cowslips : 100 : 1585 :: 100 : 430. Long-styled cowslips : 100 : 1093 :: 100 : 332. The season was much more favourable this year than the last; the plants also now grew in good soil, instead of in a shady wood or struggling with other plants in the open field; consequently the actual produce of seed was considerably larger. Nevertheless we have the same relative result; for the short-styled plants produced more seed than the long-styled in nearly the proportion of three to two; but if we take the fairest standard of comparison, namely, the product of seeds from an equal number of umbels, the excess is, as in the former case, nearly as four to three. Looking to these trials made during two successive years on a large number of plants, we may safely conclude that the short-styled form is more productive than the long-styled form, and the same result holds good with some other species of Primula. Consequently my anticipation that the plants with longer pistils, rougher stigmas, shorter stamens and smaller pollen-grains, would prove to be more feminine in nature, is exactly the reverse of the truth. In 1860 a few umbels on some plants of both the long-styled and short-styled form, which had been covered by a net, did not produce any seed, though other umbels on the same plants, artificially fertilised, produced an abundance of seed; and this fact shows that the mere covering in itself was not injurious. Accordingly, in 1861, several plants were similarly covered just before they expanded their flowers; these turned out as follows:--