176 . LIFE HISTORY OF SPHAERELLA 



divisions, the red coloring matter is usually rapidly replaced by 

 chlorophyll until but a red speck remains and the zoospores soon 

 appear as rather ovate green cells surrounded by delicate walls 

 which become widely separated from the chlorophyll owing to 

 the accumulation of water (Fig. loo, G). Nearly all zoospores 

 that occur among the algae are characterized by a small red body, 

 known as the eye spot, which is located near the ciliated end of 

 the zoospore. It has been supposed that this body is sensitive 

 to light and gives the zo5spore a sense of direction. While this 

 is questionable, since the eye spot may be lacking (as in Sphae- 

 rella), it is certain that these beautiful bodies swim about, cili- 

 ated end foremost, with a rotary motion from right to left and 

 adjust themselves to a suitable illumination. It can easily be 

 demonstrated that the zoospores of Sphaerella are keenly sensi- 

 tive to different intensities of light by placing them in a glass 

 dish by a window when the zoospores will congregate on the 

 illuminated side unless the light is too intense, when a reverse 

 action takes place. 



Thus we see that the life of the plants is largely a motile one, 

 each generation being characterized by a short resting stage 

 during which division occurs and a longer motile zoospore stage. 

 In fact, the zoospore may be looked upon as the original state 

 of these plants, while the resting condition is a departure due 

 to changes in the environment or the condition of the organism. 

 This common occurrence of motility in the lower types of life 

 indicates that possibly such was the condition of the first life upon 

 the earth. 



(a) Conditions Affecting the Life of Sphaerella. — Changes in 

 the surroundings sometimes produce remarkable variations in 

 the life of the plant. This fact is well illustrated in Sphaerella. 

 If the water dries up, a thick wall is formed about the"cehtral 

 protoplasmic body which becomes dense and of a deep red color 

 while the delicate distended wall and cilia disappear. This varia- 

 tion adapts the plant to conditions unfavorable to gro.wth, such 

 as drought and severe temperature. The so-called red snow is 

 due to a certain species of these plants that are swept off from 

 the rocks in this resting condition by the winter winds and falling 



