158 NATURE OF SEXUAL REPRODUCTION 



others, plants become stationary. At first, perhaps, simply an 

 aggregate of motionless cells, but finally there resulted chains or 

 filament of cells owing to the repeated division of the plants in 

 one plane. By division in two planes, membranous expansions 

 resulted and the more complex types arose by the division of 

 the cells in three planes and by modification of the cells. 



One other feature appears in the life of the Sphaerella that 

 indicates clearly how sexual reproduction came about. When 

 the conditions for growth are unfavorable, as for example, 

 through lack of moisture or low temperatures, there result plants 

 that are not so well nourished, or, at least they appear to lack 

 some of the qualifications that characterize the plants growing 

 under favorable conditions. The same condition often arises 

 in these plants after several generations have been formed. 

 Such plants, which may be characterized as weaker, or, at least 

 lacking in certain material, do not give rise to the ordinarv zoo- 

 spores, but to much smaller though similar bodies that are lack- 

 ing in cell walls (Fig. ioo, L). These small zoospores usually 

 perish or produce small plants unless two of them meet, when 

 a fusion may occur and thus a plant is formed that is capable 

 of repeating the life history of Sphaerella. By suitable nourish- 

 ment, however, these small zoospores may sometimes be made 

 to develop into the normal plant. It is very evident that these 

 small zoospores are lacking in some substance that is essential 

 to their growth. This is rarely furnished to them in nature 

 except when two of the bodies fuse. From numerous examples 

 appearing among different groups of Algae, it appears that 

 sexuality arose in this way. Owing to certain conditions of 

 light, temperature, food, etc., zoospores were formed that were 

 incapable of further growth. But, by the union of two of these 

 zoospores, a cell was formed possessed of renewed vigor and 

 capabilities of growth. So we can think of the sexual cells or 

 gametes as zoospores that are lacking in the materials essential 

 to growth and of the sexual process as a union of the two bodies 

 for the purpose of bringing together the missing material and 

 supplying the necessary energy for growth. 



A closely allied genus, Chlamydomonas (Fig. 101) has a life 



