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 1 ." 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 Forms of Flowers (1st edit.), p. 2. 



