334 Plant Life and Evolution 



as an inducement for variation, and for the preser- 

 vation of hereditary characters; but even among 

 animals, as Kellogg has shown from his studies on 

 both bees and aphides, the individuals produced by 

 parthenogenesis are quite as variable as those devel- 

 oped from fertilized ova. 



In following the history of the lower plants, it 

 is very evident that the result of fertilization, what- 

 ever may have inaugurated it, is twofold : first, the 

 development of a resting stage for carrying the plant 

 over unfavorable conditions of drought or cold ; and 

 second the increased production of new plants, as 

 each zygote usually produces more than one plant 

 on germination. The second cause is probably the 

 more important factor in the evolution of sex in 

 plants, and it has been closely associated with the 

 production of the resting stage, or terrestrial phase, 

 in the green algae and their descendants, the arche- 

 goniates; but in the strictly aquatic seaweeds, the 

 rapid multiplication by zoospores is quite sufficient, 

 as no resting stage is necessary, and where an elab- 

 orate sexual system is present, as in the red algae, 

 the same object is attained by the development of 

 a sporophyte from which numerous spores are pro- 

 duced without any resting stage. 



The better development of the sexual cells in the 

 fresh-water green algae when compared with their 

 marine relations, as well as the usual low grade of 

 sexuality, or its complete absence, in the brown algae, 

 and certain green seaweeds, implies that there is 



