Proceedings of the Ohio State Academy of Science 331 



SEXUAF. DIMORPHISM. 



Jf we are right in assuming that tlie difference in gametes is 

 an expression of a difference in nutritive function and that the 

 advantage of heterogamy is one merel}- of speciahzation in the 

 two cells, we may next inquire as to the probable cause of a 

 difference in the size, shape, color, etc., of the ovaries and sper- 

 maries, and finally of the unisexual individuals in the higher 

 forms. 



In such plants as Vaucheria the dift'erence in shape and size 

 of the gametangia is remarkable, when the simplicity of the other 

 parts of the plant is taken into consideration. Whether this dif- 

 ference is merely an expression of the activity of different heredi- 

 . tary characters set free by the determination of the sex of the 

 part, we may not be able to discover. But it is certainly appar- 

 ent that the sexual differentiation is brought ^bout like any other 

 differentiation in the growth of the hermaphrodite individual. 

 The sexual dimorphism of the parts is of no special significance. 

 The twist in the antheridium does not appear to be of any special 

 advantage ; for it does not result in bringing about the discharge 

 of the sperms in any constant direction in respect to the oogon- 

 ium. In Chara, the oogonium and antheridium are exceedingly 

 complex and also remarkably diff'erentiated in shape, size, and 

 finally in color. The oogonium is green corresponding to its 

 further nutritive function in ripening the oospore which becomes 

 packed with food material, while the antlieridium is a bright red. 

 Whatever purpose the bright red color of the antheridium may 

 have, it is not the result of any sexual selection. If it has any 

 significance, that significance is purely physiological and is prob- 

 ably an expression of internal activities closely bound up with 

 the nutritive hereditary tendencies wdiich produce the male 

 gametes. 



In certain species of Oedogonium the plant produces eggs 

 and the so-called androspores. These androspores produce 

 dwarf males whose sperms fertilize the eggs of the original par- 

 ent plant. Now, these dwarf males are of peculiar shape and 

 size. There is thus a very striking sexual dimorphism produced 



