The Different Forms of Flowers on Plants of the Same Species

by Charles Darwin

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.

In the foregoing chapters all the heterostyled plants known to me have been more or less fully described. Several other cases have been indicated, especially by Professor Asa Gray and Kuhn, in which the individuals of the same species differ in the length of their stamens and pistils (6/1. Asa Gray ‘American Journal of Science’ 1865 page 101 and elsewhere as already referred to. Kuhn ‘Botanische Zeitung’ 1867 page 67.); but as I have been often deceived by this character taken alone, it seems to me the more prudent course not to rank any species as heterostyled, unless we have evidence of more important differences between the forms, as in the diameter of the pollen-grains, or in the structure of the stigma. The individuals of many ordinary hermaphrodite plants habitually fertilise one another, owing to their male and female organs being mature at different periods, or to the structure of the parts, or to self-sterility, etc.; and so it is with many hermaphrodite animals, for instance, land-snails or earth-worms; but in all these cases any one individual can fully fertilise or be fertilised by any other individual of the same species. This is not so with heterostyled plants: a long-styled, mid-styled or short-styled plant cannot fully fertilise or be fertilised by any other individual, but only by one belonging to another form. Thus the essential character of plants belonging to the heterostyled class is that the individuals are divided into two or three bodies, like the males and females of dioecious plants or of the higher animals, which exist in approximately equal numbers and are adapted for reciprocal fertilisation. The existence, therefore, of two or three bodies of individuals, differing from one another in the above more important characteristics, offers by itself good evidence that the species is heterostyled. But absolutely conclusive evidence can be derived only from experiments, and by finding that pollen must be applied from the one form to the other in order to ensure complete fertility.

In order to show how much more fertile each form is when legitimately fertilised with pollen from the other form (or in the case of trimorphic species, with the proper pollen from one of the two other forms) than when illegitimately fertilised with its own-form pollen, I will append Table 6.33 giving a summary of the results in all the cases hitherto ascertained. The fertility of the unions may be judged by two standards, namely, by the proportion of flowers which, when fertilised in the two methods, yield capsules, and by the average number of seeds per capsule. When there is a dash in the left hand column opposite to the name of the species, the proportion of the flowers which yielded capsules was not recorded.

TABLE 6.33. Fertility of the legitimate unions taken together, compared with that of the illegitimate unions together. The fertility of the legitimate unions, as judged by both standards, is taken as 100.

Column 1: Name of species. Column 2: Illegitimate unions : proportional number of flowers which produced capsules. Column 3: Illegitimate unions : average number of seeds per capsule.

Primula veris : 69 : 65.

Primula elatior : 27 : 75.

Primula vulgaris : 60 : 54.

Primula Sinensis : 84 : 63.

Primula Sinensis (second trial) : 0 : 53.

Primula Sinensis (Hildebrand) : 100 : 42.

Primula auricula (Scott) : 80 : 15.

Primula Sikkimensis (Scott) : 95 : 31.

Primula cortusoides (Scott) : 74 : 66.

Primula involucrata (Scott) : 72 : 48.

Primula farinosa (Scott) : 71 : 44.

Average of the nine species of Primula : 88.4 : 69.

Hottonia palustris (H. Muller) : - : 61.

Linum grandiflorum (the difference probably is much greater) : - : 69.

Linum perenne : - : 20.

Linum perenne (Hildebrand) : 0 : 0.

Pulmonaria officinalis (German stock, Hildebrand) : 0 : 0.

Pulmonaria angustifolia : 35 : 32.

Mitchella repens : 20 : 47.

Borreria, Brazilian sp. : - : 0.

Polygonum fagopyrum : - : 46.

Lythrum salicaria : 33 : 46.

Oxalis Valdiviana (Hildebrand) : 2 : 34.

Oxalis Regnelli : 0 : 0.

Oxalis speciosa : 15 : 49.

The two or three forms of the same heterostyled species do not differ from one another in general habit or foliage, as sometimes, though rarely, happens with the two sexes of dioecious plants. Nor does the calyx differ, but the corolla sometimes differs slightly in shape, owing to the different position of the anthers. In Borreria the hairs within the tube of the corolla are differently situated in the long-styled and short-styled forms. In Pulmonaria there is a slight difference in the size of the corolla, and in Pontederia in its colour. In the reproductive organs the differences are much greater and more important. In the one form the stamens may be all of the same length, and in the other graduated in length, or alternately longer and shorter. The filaments may differ in colour and thickness, and are sometimes nearly thrice as long in the one form as in the other. They adhere also for very different proportional lengths to the corolla. The anthers sometimes differ much in size in the two forms. Owing to the rotation of the filaments, the anthers, when mature, dehisce towards the circumference of the flower in one form of Faramea, and towards the centre in the other form. The pollen-grains sometimes differ conspicuously in colour, and often to an extraordinary degree in diameter. They differ also somewhat in shape, and apparently in their contents, as they are unequally opaque. In the short-styled form of Faramea the pollen-grains are covered with sharp points, so as to cohere readily together or to an insect; whilst the smaller grains of the long-styled form are quite smooth.

With respect to the pistil, the style may be almost thrice as long in the one form as in the other. In Oxalis it sometimes differs in hairiness in the three forms. In Linum the pistils either diverge and pass out between the filaments, or stand nearly upright and parallel to them. The stigmas in the two forms often differ much in size and shape, and more especially in the length and thickness of their papillae; so that the surface may be rough or quite smooth. Owing to the rotation of the styles, the papillose surface of the stigma is turned outwards in one form of Linum perenne, and inwards in the other form. In flowers of the same age of Primula veris the ovules are larger in the long-styled than in the short-styled form. The seeds produced by the two or three forms often differ in number, and sometimes in size and weight; thus, five seeds from the long-styled form of Lythrum salicaria equal in weight six from the mid-styled and seven from the short-styled form. Lastly, short-styled plants of Pulmonaria officinalis bear a larger number of flowers, and these set a larger proportional number of fruit, which however yield a lower average number of seed, than the long-styled plants. With heterostyled plants we thus see in how many and in what important characters the forms of the same undoubted species often differ from one another—characters which with ordinary plants would be amply sufficient to distinguish species of the same genus.

As the pollen-grains of ordinary species belonging to the same genus generally resemble one another closely in all respects, it is worth while to show, in Table 6.34, the difference in diameter between the grains from the two or three forms of the same heterostyled species in the forty-three cases in which this was ascertained. But it should be observed that some of the following measurements are only approximately accurate, as only a few grains were measured. In several cases, also, the grains had been dried and were then soaked in water. Whenever they were of an elongated shape their longer diameters were measured. The grains from the short-styled plants are invariably larger than those from the long-styled, whenever there is any difference between them. The diameter of the former is represented in the table by the number 100.

TABLE 6.34. Relative diameter of the pollen-grains from the forms of the same heterostyled species; those from the short-styled form being represented by 100.