DEVELOPMENT OF PLANTS 167 



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

 aggregate of motionless cells, but finally there resulted chains or 

 filaments 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 ordinary zoospores, 

 but to much smaller though similar bodies that are lacking in 

 cell walls (Fig. 100, 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 nourishment, how- 

 ever, these small zoospores may sometimes be made to develop 

 into the normal plant. It is very evident that these small zoo- 

 spores 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 neces- 

 sary energy for growth. 



A closely allied genus, Chamlydomonas (Fig. loi) has a life 



