CHAP. VI. ON HETEROSTYLED PLANTS. 255 



and often thicker than those on that of the short- 

 styled. For instance, the papillae on the long-styled 

 stigma of Hottonia palustris are more than twice the 

 length of those in the other form. This holds good 

 even in the case of Houstonia ccerulea, in which the 

 stigmas are much shorter and stouter in the long- 

 styled than in the short-styled form, for the papillae 

 on the former compared with those on the latter are 

 as 100 to 58 in length. The length of the pistil 

 in the long-styled form of Linum grandiflorum varies 

 much, and the stigmatic papillae vary in a corre- 

 sponding manner. From this fact I inferred at first 

 that in all cases the difference in length between the 

 stigmatic papillaB in the two forms was one merely of 

 correlated growth; but this can hardly be the true or 

 general explanation, as the shorter stigmas of the 

 long-styled form of Houstonia have the longer papillae. 

 It is a more probable view that the papilla?, which ren- 

 der the stigma of the long-styled form of various species 

 rough, serve to entangle effectually the large-sized pol- 

 len-grains brought by insects from the short-styled form, 

 thus ensuring its legitimate fertilisation. This view is 

 supported by the fact that the pollen-grains from the 

 two forms of eight species in Table 34 hardly differ in 

 diameter, and the papillaa on their stigmas do not differ 

 in length. 



The species which are at present positively or almost 

 positively known to be heterostyled belong, as shown in 

 the following table, to 38 genera, widely distributed, 

 throughout the world. These genera are included in 

 fourteen Families, most of which are very distinct from 

 one another, for they belong to nine of the several great 

 Series, into which phanerogamic plants have been di- 

 vided by Bentham and Hooker. 



