CHAP. VI. ON HETEROSTYLED PLANTS. 259 



number of seeds which it yields in whatever manner 

 it may be fertilised, and by its pollen (the grains of 

 which are of smaller size than those from the cor- 

 responding stamens in the other two forms) when 

 applied to the stigma of any form producing fewer 

 seeds than the normal number. If we suppose the 

 process of deterioration of the male organs in the mid- 

 styled form to continue, the final result would be the 

 production of a female plant; and Lythrum salicaria 

 would then consist of two heterostyled hermaphrodites 

 and a female. No such case is known to exist, but it 

 is a possible one, as hermaphrodite and female forms 

 of the same species are by no means rare. Although 

 there is no reason to believe that heterostyled plants 

 are regularly becoming dioecious, yet they offer sin- 

 gular facilities, as will hereafter be shown, for such 

 conversion; and this appears occasionally to have been 

 effected. 



We may feel sure that plants have been rendered 

 heterostyled to ensure cross-fertilisation, for we now 

 know that a cross between the distinct individuals of 

 the same species is highly important for the vigour and 

 fertility of the offspring. The same end is gained by 

 dichogamy or the maturation of the reproductive ele- 

 ments of the same flower at different periods, by 

 diceciousness self-sterility the prepotency of pollen 

 from another individual over a plant's own pollen, and 

 lastly, by the structure of the flower in relation to the 

 visits of insects. The wonderful diversity of the means 

 for gaining the same end in this case, and in many 

 others, depends on the nature of all the previous 

 changes through which the species had passed, and on 

 the more or less complete inheritance of the successive 

 adaptations of each part to the surrounding conditions. 

 Plants which are already well adapted by the structure 



