160 HETEROSTYLED TRIMORPHIC PLANTS. CHAP. IV. 



long-styled form, from the use of the pollen of the 

 shortest stamens of the mid-styled form and of the 

 mid-length stamens of the short-styled form. The 

 same rule also holds good with the mid-styled and 

 short-styled forms, when illegitimately fertilised with 

 pollen from the stamens more or less unequal in 

 length to their pistils. Certainly the difference in 

 sterility in these several cases is slight; but, as far as 

 we are enabled to judge, it always increases with the 

 increasing inequality of length between the pistil and 

 the stamens which are used in each case. 



The correspondence in length between the pistil in 

 each form and a set of stamens in the other two forms, 

 is probably the direct result of adaptation, as it is of 

 high service to the species by leading to full and 

 legitimate fertilisation. But the rule of the increased 

 sterility of the illegitimate unions according to the 

 greater inequality in length between the pistils and 

 stamens employed for the union can be of no service. 

 With some heterostyled dimorphic plants the dif- 

 ference of fertility between the two illegitimate unions 

 appears at first sight to be related to the facility of 

 self- fertilisation ; so that when from the position of 

 the parts the liability in one form to self-fertilisation 

 is greater than in the other, a union of this kind 

 has been checked, by having been rendered the 

 more sterile of the two. But this explanation does 

 not apply to Lythrum; thus the stigma of the long- 

 styled form is more liable to be illegitimately fer- 

 tilised with pollen from its own mid-length stamens, 

 or with pollen from the mid-length stamens of the 

 short-styled form, than by its own shortest stamens 

 or those of the mid-styled form; yet the two former 

 unions, which it might have been expected would 

 have been guarded against by increased sterility, 



