50 DIMOEPHIC LEAVES IN RELATION TO HEREDITY. 



diffeioiil kinds of internodes may influence the germ cells and the 

 process of conjugation. The phenomena of sexuality are closely 

 connected with contrasted expressions of characters. Sexuality is 

 primarily a physiological fact, and only secondarily morphological. 

 The physiological value of sexual differentiation must be sought 

 finally in a greater efliciency of the process of conjugation. 



In the higher groups of plants and animals there is a double differ- 

 entiation of sexual characters. The male and female germ cells not 

 only become more and more unlike as the scale of organization is 

 ascended, but sexual inequalities also become more and more developed 

 in the organisms that produce the two kinds of germ cells. Not only 

 the inequalities of the germ cells but also the sexual differentiation 

 of the parent organisms must be supposed to relate in some unknown 

 manner to an increased efliciency of conjugation. Many of the 

 secondary sexual characters of plants and animals are like dimorphic 

 differences in having no direct or obvious use in relation to the 

 external environment, but the}" may have relation to the internal 

 functions of heredity. Even if considered as mere reflections or antici- 

 pations of divergent tendencies of expression embodied in the germ 

 cells, secondary sexual characters would still have physiological 

 significance as showing the fundamental tendency toward alternative 

 expression of characters. 



In view of these and other indications that diversity and alternative 

 expression of characters among the members of species have physiolog- 

 ical functions in increasing the efliciency of reproduction, it becomes 

 reasonable to consider the possibility that the series of sudden and 

 complete changes in the expression of characters involved in the 

 development of the successive types of internode individuals in plants 

 may also be a factor of heredity. If contrasted parental characters 

 and changes of external conditions aft'ect the vigor of organisms, wliy 

 may not frequent changes of characters during the process of develo})- 

 ment be supposed to have a similar advantage ? The speciahzation 

 of two or more different kinds of leaves, branches, or flowers on the 

 same plant may be compared with the alternative inheritance shown 

 in the sexes and castes of animals, and both classes of specialization 

 may have similar relations to the physiology of reproduction. Fre- 

 quent conjugations between germ cells representing difl'erent lines of 

 descent may be rendered less necessary in plants because of the 

 numerous changes of characters that take place during the ordinary 

 processes of growth. 



CONCLUSIONS. 



A definite dimorphism of the leaves exists in an Egyptian variety 

 of the Deccan hemp {Hibiscus cannahinus). The leaves of the upper 

 part of the stalk are deeply three lobed, while those of the lower part 



221 



