Heterostyled Flowers 409 
for a method unsuitable in itself, such as a modification standing in 
the way of self-pollination, and on the other hand as a means of in- 
creasing the chance of pollination in the case of flowers in which self- 
pollination was possible, but which might, in accidental circumstances, 
be prevented. It was, therefore, very important to obtain experimental 
proof of the conclusion to which Darwin was led by the belief of the 
majority of breeders and by the evidence of the widespread occurrence 
of cross-pollination and of the remarkable adaptations thereto. 
This was supplied by the researches which are described in the 
two other works named above. The researches on which the con- 
clusions rest had, in part at least, been previously published in 
separate papers: this is the case as regards the heterostyled plants. 
The discoveries which Darwin made in the course of his investigations 
of these plants belong to the most brilliant in biological science. 
The case of Primula is now well known. C. K. Sprengel and 
others were familiar with the remarkable fact that different individuals 
of the European species of Primula bear differently constructed 
flowers; some plants possess flowers in which the styles project 
beyond the stamens attached to the corolla-tube (long-styled form), 
while in others the stamens are inserted above the stigma which is 
borne on a short style (short-styled form). It has been shown by 
Breitenbach that both forms of flower may occur on the same plant, 
though this happens very rarely. An analogous case is occasionally 
met with in hybrids, which bear flowers of different colour on the 
same plant (e.g. Dianthus caryophyllus). Darwin showed that the 
external differences are correlated with others in the structure of 
the stigma and in the nature of the pollen. The long-styled flowers 
have a spherical stigma provided with large stigmatic papillae ; the 
pollen grains are oblong and smaller than those of the short-styled 
flowers. The number of the seeds produced is smaller and the ovules 
larger, probably also fewer in number. The short-styled flowers have 
a smooth compressed stigma and a corolla of somewhat different 
form; they produce a greater number of seeds. 
' These different forms of flowers were regarded as merely a case 
of variation, until Darwin showed “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.” We 
have here an example of hermaphrodite flowers which are sexually 
different. There are essential differences in the manner in which 
fertilisation occurs. This may be effected in four different ways ; 
there are two legitimate and two illegitimate types of fertilisation. 
The fertilisation is legitimate if pollen from the long-styled flowers 
1 Forme of Flowers (1st edit.), p. 2. 
