66 POPULAR SCIENCE MONTHLY. 



THE OEIGIN OF SEX IN PLANTS. 



By Dr. BRADLEY MOORE DAVIS, 



UNIVERSITY OF CHICAGO. 



'y'^OOLOGISTS have held various views as to the origin of sex in 

 ■^-^ animals, but the subject is confessedly speculative. They have 

 very little data bearing upon the problem — the gap between the Pro- 

 tozoa and the Metazoa is so immense and characterized by such a 

 paucity of intermediate types. We pass directly from relatively sim- 

 ple conjugation among unicellular forms to the complicated conditions 

 in higher animals, where the sexual elements have reached a very high 

 state of specialization. 



Botany is very much more fortunate in this respect. It is not 

 difficult to understand the evolution of multicellular plants from the 

 unicellular, and we have a great deal of evidence that bears on the 

 origin and differentiation of sex. Greater interest is added to this 

 subject because we have reason to believe that sex has arisen in a num- 

 ber of divergent groups by identical processes but without relation to 

 one another, so that similar complex results have been worked out 

 independently. 



We shall deal entirely with that large group of the lower plants 

 known as the algae which includes all the plants below the liverworts 

 and mosses with the exception of the fungi. One need study the algae 

 but slightly to realize that they are a very diverse assemblage of forms 

 comprising many lines of ascent, some of which are marked out clearly, 

 but many of them mere fragments and remnants of former series that 

 have been broken up by the extinction of ancestral types. 



There are certain groups of algae well known to all students of 

 botany that have no place in the present discussion. Such for example 

 are the Conjugales comprising types such as Spirogyra, Zygnema, the 

 desmids, and again, the diatoms. However valuable these forms may 

 be for certain laboratory studies, they should never be cited as typical 

 illustrations of sexual processes among the lower plants. They are 

 rather extraordinarily specialized groups and have developed peculiar- 

 ities of a high order. Again, there are numbers of groups complex in 

 their organization, whose relationship to other forms is so remote that 

 we must place them quite apart by themselves. Such for example are 

 the stoneworts (Charales), the red algae (Khodophyceae) and some 



