1 82 



MOTIONLESS UNICELLULAR ALGAE 



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 Unicelhdar Algae. — In contrast to these motile 

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

 their motility and spend a large part of their life as stationary 

 plants. We have already noticed how this condition may have 



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 chloroplnst. B, Scenedesmus. Below three of the plants are shown 

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



come about, owing to the temperature and moisture conditions 

 that were unfavorable for the motile existence. Pleurococcus 

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

 and illustrates many of the features in the life history of the 

 stationary forms. These plants often cause the green coating 

 that appears upon the shaded or moist side of tree trunks, 

 fences and buildings. If a bit of this material is examined under 



