DEVELOPMENT OF PLANTS 



175 



cells (Fig. 100, B) which escape by the rupturing of the old 

 mother wall and swim actively about by means of two cilia. 

 These cells, or zoospores, remain motile for varying periods, 

 during which time they increase in size, but finally the cilia are 



Fig. 100. Stages in the life history of Sphaerella: A, resting state of the 

 plant. B, first division. C, second division, the four cells, are about to 

 escape from the mother plant. D, one of the cells of C after escaping. This 

 is a zoospore of the first generation. E, zoospore at rest. F, forming four 

 new zoospores. G, one of these zoospores of the second generation. Note 

 that the red material, represented by the shaded area in the center of the 

 cell, has become greatly reduced and that the wall is becoming distended and 

 separated from the granular cytoplasm. H, third resting stage. 7, cell 

 dividing. K, zoospore of third generation with greatly distended cell wall 

 and small red area. Delicate' strands of cytoplasm connect the cell wall 

 and the central protoplasmic body. L, a resting cell dividing into a large 

 number of zoospores which are consequently smaller. — ^After Hazen. 



retracted and the plants remain in a quiescent state for a short 

 period (Fig. 100, E). A division of the nucleus now results 

 again in the formation of two or more zoospores (Fig. 100, F) 

 which repeat the life history noted above. This method of 

 reproduction may occur again and again and thus rapidly bring 

 about. a great increase in the number of plants. During the 



