DEVELOPMENT OF PLANTS 157 



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

 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 central 

 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 growth, 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 fall- 

 ing to the earth produces red streaks in the snow. Hazen has 

 been able to show that a most suggestive change in the mode of 

 life of the plants may also be induced by low temperature or by 

 a reduction of the volume of water in which they live. Under 

 such conditions, bodies are formed that resemble zoospores, but 

 that are not capable of motion owing to the lack of cilia. This 

 helps us to understand how the motionless or stationary forms 

 of plant life came about and how the motile stage became less 

 and less conspicuous. Owing to such causes as these and many 



