24 



PLANT MORPHOLOGY 



Reproduction occurs chiefly by fission, the cell always dividing in the 

 plane of the valves (Fig. 14). The two valves separate and each daugh- 

 ter protoplast forms a new wall on its naked side, the new wall fitting 

 inside the old one. One of the cells is always as large as the parent cell, 



B 



Fig. 14. Cell division in Surirella calcarata, X275. 

 Laiderborn.) 



A to D, successive stages. {After 



but the other is smaller. Thus, as cell divisions continue, some of the 

 individuals become constantly smaller. After a minimum size for the 

 species has been reached, the original size is regained through the forma- 

 tion of auxospores. 



In the Centrales an auxospore is formed by the escape from its cell 

 wall of a protoplast that soon grows to the original size and develops a 

 new cell wall. An auxospore may directly become a new individual or 

 may form two new individuals by dividing in half. In most of the 



