CHAP. IV. LYTHRUM SALICAEIA. 161 



are much less sterile than the other two unions 

 which are much less likely to be effected. The 

 same relation holds good even in a more striking 

 manner with the mid-styled form, and with the short- 

 styled form as far as the extreme sterility of all its 

 illegitimate unions allows of any comparison. We 

 are led, therefore, to conclude that the rule of in- 

 creased sterility, in accordance with increased in- 

 equality in length between the pistils and stamens, 

 is a purposeless result, incidental on those changes 

 through which the species has passed in acquiring cer- 

 tain characters fitted to ensure the legitimate fertilisa- 

 tion of the three forms. 



Another conclusion which may be drawn from 

 Tables 23, 24, and 25, even from a glance at them, 

 is that the mid-styled form differs from both the 

 others in its much higher capacity for fertilisation 

 in various ways. Not only did the twenty-four flowers 

 legitimately fertilised by the stamens of corresponding 

 lengths, all, or all but one, yield capsules rich in 

 seed; but of the other four illegitimate unions, that 

 by the longest stamens of the short-styled form was 

 highly fertile, though less so than the two legitimate 

 unions, and that by the mid-length stamens of the 

 long-styled form was fertile to a considerable degree; 

 the remaining two illegitimate unions, namely, with 

 this form's own pollen, were sterile, but in different 

 degrees. So that the mid-styled form, when fertilised 

 in the six different possible methods, evinces five 

 grades of fertility. By comparing compartments III. 

 and VI. in Table 24 we may see that the action of 

 the pollen from the shortest stamens of the long-styled 

 and mid-styled forms is widely different ; in the one 

 case above half the fertilised flowers yielded capsules 

 containing a fair number of seeds; in the other case 



