THE ORIGIN OF SEX IN PLANTS. 



67 



forms of the brown algae (Phaeophyceae). These groups give us no 

 data on the problems that we are to consider. 



There is left for us a numerous and varied array of algae, represent- 

 ing several lines of ascent, all tending to diverge from one another. 

 But these forms have some important points in common, particularly 

 as concerns certain events in their life histories. There is immense 

 variety in the form of the plant body which ranges from a single cell 

 to structures with stalks and leaf-like organs. There are likewise ex- 

 hibited many degrees of sexual development, from a few forms which 

 actually appear to illustrate -the dawning of sex through various inter- 

 mediate stages to many types in which the sexual elements have become 

 highly specialized. The story of the differentiation of sex, that is, the 

 evolution of the egg and sperm from the primitive sexual elements, 

 is most interesting, but would require extended treatment. It must be 

 left for some future paper. Our problem is to understand how the 

 primitive sexual elements arose. 



Almost all the algae in the groups referred to in the paragraph 

 above have one phase in their life histories in common. They usually 

 present a period, although sometimes very short, when the protoplasm 

 of the cells is in the form of free-swimming elements. These are 

 called zoospores or swarm-spores and they are commonly little pear- 

 shaped bodies, the pointed ends bearing 2 or perhaps 4 delicate hair- 

 like organs, called cilia, whose vibrations give the zoospores their rapid 

 movement. A glance at the illustrations will show the form of these 

 motile cells. 



Zoospores are likely to be produced in greatest quantity at certain 

 seasons or under particular conditions of light or temperature, and 

 their purpose is plainly the rapid 

 propagation of the species. But 

 there is a deep significance in 

 their general conformity to a cer- 

 tain type of structure and their 

 almost universal presence in the 

 groups that we are considering. 

 In a certain sense the zoospore 

 represents a return on the part of 

 these algae to primitive ancestral 

 conditions. 



There are many unicellular 

 algae that pass a large part, per- fig. 1. stages in the Life history of chla- 

 haps the greater part, of their mydomonas. «, vegetative cell. 6, small ga- 



_ ^ ° _ r ' METE, c, Conjugation of Gametes, d, Sexually 



lives as motile cells with a struc- formed spore, e, first division of spoee. 



ture essentiallv thp «nmp n^ /, Quirscent condition, cells Multiplying by 

 Ture esseniiany tne same as division. (After gokoschankin.) 



