The Algae 239 



season the cells in the filament stop dividing and food accumu- 

 lates in the form of starch and protein granules. The proto- 

 plasm in each cell then contracts into a spherical form and 

 secretes a heavy cell wall about itself inside the original cell 

 wall. In this way the cells of a filament form a row of ovoid 

 or spherical heavy-walled resting spores. Usually the walls 

 of these spores become yellow or brownish. The resting spore 

 remains dormant until the late fall or early spring. Then, 

 like a seed, it germinates, or begins to grow. The wall that 

 incloses the spore breaks and the protoplasm pushes out and 

 foniis a cylindrical vegetative cell which continues to grow 

 and divide until a new filament is formed. 



Microspora, then, in addition to the vegetative multipli- 

 cation of the cells shown by Protococcus, has specialized 

 swimming spores that multiply and spread the plant, and 

 resting spores that undergo a dormant period, after which, 

 when favorable conditions for growth appear, they produce 

 a new plant. Its life cycle and that of other similar algae 

 includes (i) an active chlorophyll- working period, during 

 which the plant grows and enlarges its body and accumulates 

 food; (2) a reproductive phase, which closes with the pro- 

 duction of resting spores ; and (3) a period of dormancy, dur- 

 ing which only the resting spores are alive. The length of 

 the dormant period for a particular alga is practically the same, 

 whether it fives in a permanent pond or in a pool that dries 

 up in summer. 



The living conditions of the pond algae. Curiously, the 

 ponds in which the alg:e are most abundant are the ones that 

 dry up in the summer. Yet these plants are among the most 

 delicate of all living things ; you may readily discover, by 

 putting the green masses in a dish and letting the water 



