METABOLISM OF GREEN PLANTS 33 



passing through the cellulose wall, extend for some distance 

 into the surrounding water. These threads of cytoplasm, or 

 FLAGELLA, lash vigorously and pull the cell rapidly through 

 the water. The activity of the flagella is one expression of a 

 fundamental property of protoplasm, CONTRACTILITY, which 

 is exhibited in its most specialized form in the muscles of 

 the higher animals. 



Returning now to the life-history of Sphaerella. The four 

 free-swimming individuals, which have arisen from the parent 

 dormant phase, may each divide many times so that instead 

 of four there may be, before long, thousands of flagellated 

 cells, all direct lineal descendants of the original resting cell. 

 If this number seems high, one only has to determine how 

 many cells there would be at, say, the twenty-fifth generation 

 by raising 2 to the 25th power. Sooner or later, however, 

 these active cells withdraw their flagella and again become 

 dormant forms. 



But Sphaerella is still more versatile. Now and then, 

 probably influenced by environmental conditions, the proto- 

 plasm within the wall of a spherical dormant form divides 

 rapidly into 32 or 64 relatively small cells which, when set 

 free, are termed GAMETES. These differ structurally from 

 the active form already described chiefly by the absence of 

 the prominent cell wall and vacuole. But it is the behavior 

 rather than the structure of these small cells which is char- 

 acteristic. After swimming about for a time by means of 

 their flagella, they come together in pairs, the two cells of a 

 pair completely fusing nucleus with nucleus and cyto- 

 plasm with cytoplasm to form a single cell, or ZYGOTE, 

 with four flagella. Soon the individual absorbs its flagella 

 and, secreting about itself a heavy cell wall, enters upon a dor- 

 mant stage with the characteristics and potentialities de- 

 scribed above. 



