254 CONCLUDING EEMABKS Chap. VL 



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 coerulea, in which the 

 stigmas are much shorter and stouter in the long- 

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

 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 grandifiorum 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 papillae 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 papillae, which 

 render the stigma of the long-styled form of various 

 species rough, serve to entangle effectually the large- 

 sized pollen-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 papillae 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 dis- 

 tributed 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 divided by Bentham and Hooker. 



