The Ohio Journal of Science 



Vol. XXII February, 1922 No. 4 



PROGRESSION OF SEXUAL EVOLUTION IN THE 

 PLANT KINGDOM* 



JOHN H. SCHAFFNER 



Department of Botany, Ohio State University 



Sexuality may manifest itself in at least three diifferent 

 ways: First, by the property of attraction and fusion of cells; 

 second, by the dimorphism of characters expressed through the 

 influence of the sexual state at the time of cell differentiation; 

 and third, by a difference in the nature of the chemical bodies 

 produced in cells in different sexual states. In the evolution 

 of sex there are, apparently, degrees of intensity of the sexual 

 state and degrees of persistency of any state established for the 

 time, whether male, female, or neutral. The main factor in the 

 evolution of sexual dimorphism has been the shifting of the time 

 in the ontogeny when sexual states arise from the neutral state 

 or when one sexual state gives place to another. The time of 

 sex determination ranges all the way from a late stage of 

 gametogenesis in the lowest forms backward through the onto- 

 genetic history until, as in diecious flowering plants of the 

 extreme type, the sexual state of both the sporophyte and 

 gametophyte is established at the time of fertilization of the 

 egg or even before. Of course, it must be remembered that 

 new hereditary factors are continually arising or old ones being 

 modified, in the upward progression of evolution, which come 

 to expression either in the presence of one sexual state or the 

 other or in the presence of both sexual states but with different 

 morphological values. In either case the dimorphism of sec- 

 ondary sexual characters becomes more pronounced or more 

 complex. 



* Papers from the Department of Botany, The Ohio State University. No. 133. 



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