DEVELOPMENT OF PLANTS 



171 



definite stimuli as the heat, moisture, light, food and other 

 conditions that occur in the environment. These stimuli pro- 

 duce enfeebled zoospores that are incapable ordinarily of growth 

 unless a fusion of two of them is effected. So sexuality arose 

 from the asexual state of the plant as a result of the influence 

 of the environment and differentiation of sex appeared owing 

 to the variation in the amount and kind of material in the gamete 

 producing cells. 



(c) Motionless Unicellular Algae. — In contrast to these mo- 

 tile plants, there is a large group of the Volvocales that have 

 lost their motility and spend a large part of their life as station- 

 ary plants. We have already noticed how this condition may 

 have come about, owing to the temperature and moisture con- 

 ditions that were unfavorable for the motile existence. Pleuro- 

 coccus (Fig. 104, A) is one of the most common of the green 



Fig. 104. Common forms of stationary unicellular green algae: A, Pleu- 

 rococcus showing stages in the division of the plants, the nucleus and the 

 irregular chloroplast. B, Scenedesmus. Below three of the plants are shown 

 dividing. C, Ankistrodesmus. At right three plants in process of division. 



