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 

 forms 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 lives 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 algae 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 



