MICHIGAN ACADEMY OF SCIENCE. 59 



THE SEXUAL CYCLE IN PLANTS.* 



BY ERNST A. BESSEY. 



There are many groups of plants in which sexuality is entirely 

 lacking, lu some of these the indications are that this lack of 

 sexuality is not due to its loss in the course of evolution from 

 sexual ancestors, but that it is primitive; in other words lias not 

 yet been evolved. The Myxophyceae are good representatives of 

 such plants. Possibly, also, some of the Protococcoideae which lack 

 sexuality are to be classed as primitively sexless, but this is rather 

 doubtful. 



On the other hand we have numerous cases where the absence 

 of sexuality is almost certainly due to its loss in the course of 

 evolution from ancestors that possessed it. Thus we have in the 

 Class Ascomyceteae a grou}) of plants that, Avhatever theory as 

 to their phylogeny may be accepted, have descended from forms 

 possessing sexuality. This is borne out by the fact that a form 

 of sexuality is present in most of the species of the class. How- 

 ever, the closely related genera Eremascus and Endomyces differ 

 in this that the former possesses- and the latter lacks sexuality. 

 The same is true of the closely related family Saccharomycetaceae, 

 the yeasts, in which ascus formation is preceded by conjugation in 

 some forms and conjugation does not occur in others. Among the 

 ferns, too, and the floAvering plants there are quite a number of 

 species in which the new generation is produced apogamously, i. 

 e., without the sexual union. Perhaps the commonest example is 

 the common dandelion {Lcontodon taraxacum L) in which the 

 pollen still continues to be formed in spite of the fact that the 

 embryo develops from an unfertilized egg cell. Such plants as 

 these are therefore secondarily sexless. 



What was the origin of sexual reproduction is not yet clear. 

 Dr. Coulter is probably correct in his belief that the original 

 gametes were modified zoospores. It is not so sure, however, that 

 sexuality originated -fZe novo at many different points in the Vege- 

 table and Animal Kingdoms. It is indeed hard to conceive how 

 the almost identical phenomena of sexuality in animals and the 

 various groups of plants can have had separate origin. Indeed, 



♦Address of the retiring president March 29, 1916. 



