372 COOK 



of Aconitum purple and creamy or greenish white flowers are 

 described. Pink flowers also appear occasionally as definite 

 variants of white-flowered species of Achillcea. 



Semisexual Species. — A species consisting of bisexual organ- 

 isms divided into semisexes, that is, into groups differing in 

 characters which conduce to interbreeding between the groups. 



This is the condition reached by many species in which the 

 individuals are all bisexual, but differ among themselves in char- 

 acters which insure, or at least facilitate, cross-fertilization. In 

 the well known instance of Lythrum there are three castes of 

 plants with short, medium, and long styles and filaments, and 

 three different kinds of pollen grains and stigmatic papillae. A 

 long-styled plant produces only short and medium stamens, and 

 must be fertilized by pollen from long stamens, to be found 

 only on other plants. The semisexes of the primrose were 

 described by Darwin. Similar conditions are known in Oxalis, 

 Houstonia, and many other genera. 



Among plants, at least, it might appear that semisexual con- 

 ditions are more advantageous than the next stage of completely 

 differentiated sexes. Cross-fertilization is secured, but at the 

 same time all individuals may produce seed, and not merely 

 half of them. That complete sexual differentiation has been 

 attained notwithstanding, and in so many different groups, 

 affords an intimation of the importance of symbasic heterism in 

 the structural economy of organisms. The fact loses none of 

 its significance if we reflect that the complete separation of the 

 sexes in plants reduces by half the facilities of the species for 

 producing seeds. All individuals being stationary, the males 

 can contribute to the welfare of species by none of the accessory 

 habits which have been so richly developed among the animals. 

 Indeed, it is by no means unlikely that the tendency of selective 

 influence on many plants has been to keep them in the semi- 

 sexual condition, sexually differentiated only far enough to 

 secure cross-fertilization, but not far enough to preclude the 

 production of seeds by all individuals. 



Sexual Species. — A species consisting of unisexual organ- 

 isms, or divided into two sexes, male and female, so that inter- 

 breeding between the sexes is necessary to reproduction. 



