Heterostylism 



133 



pollen and the stigmas in the two cases. The pollen grains of 

 the short-styled flowers are larger than those of the long-styled 

 flowers, and the stigma in the latter case is globular, whilst in 

 the former it is depressed in the centre. A number of careful 

 experiments by Professor Darwin and others show that long- 

 styled flowers are fertilised by the pollen from short-styled, and 

 vice versa the short-styled are fertilised by the pollen from the 

 long-styled. Such a cross Mr. Darwin calls legitimate union, 

 whilst the term illegitimate union is applied to the union of 

 long-styled pollen with long-styled stigma. This illegitimate 

 union will not produce seeds to the extent that the legitimate 

 will, and in some dimorphic flowers none at all are formed. 



Fig. 254.— Trimorphic forms oiLythrum, 



In some species of Lythrum we have trimorphic flowers : 

 one with long style, six medium stamens, and six short stamens; 

 one with medium style, six long stamens, and six short stamens ; 

 and one with short style, six long stamens, and six medium 

 stamens. In such cases we always find that the long style is 

 fertilised by the long stamens, the medium style by the medium 

 stamens, and the short style by the short stamens, as shown by 

 the lines in the diagram. 



Whilst these provisions for cross-fertilisation are the rule 

 amongst flowers, in some few cases there is a special arrange- 

 ment for self-fertilisation. The Sweet Violet is a good example. 

 The well-known flowers that appear in the spring do not, 



