24 ELEMENTARY STUDIES IN BOTANY 



no spores are "sexual," the only sexual cells being the 

 gametes. 



Spores and oospores are not produced by Algae continu- 

 ously, for under certain conditions Algse may vegetate, with- 

 out producing any spores ; under other conditions they may 

 produce spores freely; and under still other conditions 

 gametes appear and oospores are formed. In the ordinary 

 course of the seasons, the spores are produced during the 

 growing season and multiply individuals, in fact they do 

 most of the reproduction. The gametes, on the other hand, 

 usually appear towards the end of the growing season for 

 the plant, and so the formation of the oospores is about the 

 last activity of the plant. The spores germinate at once, 

 but the oospores, appearing late in the season, develop 

 heavy walls, remain dormant through the winter, and ger- 

 minate at the beginning of the next growing season. In 

 such plants, therefore, the oospores are the only structures 

 that remain alive through the winter. It may be said, 

 therefore, that spores multiply the plant, while oospores 

 protect it through the winter and start it again. In those 

 Algse in which there is no sex, and therefore no oospores, 

 ordinary vegetative cells become heavy-walled and protect 

 the plant through the winter. 



17. Life-history formulae. The life-history of a plant 

 means the complete history of its life, beginning at any point, 

 for example, the spore, and continuing until spores appear 

 again. It is helpful to express the outlines of a life-history 

 by a formula, and the following formulae illustrate the life- 

 histories of the Algse we have been considering. Vegetative 

 multiplication may be indicated by P P P, etc., in which 

 " P " stands for " plant," and which indicates that one plant 

 produces another directly, without any special cells. Spore- 

 reproduction may be indicated by P a P o P o, etc., 

 which indicates that the plant produces a spore which pro- 

 duces another plant, and so on. Sex-reproduction may be 



