450 General Botany 



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 it germinates; the outer 

 wall that incloses the spore breaks and the protoplasm and 

 delicate inner wall push out and form a cylindrical vegetative 

 cell, which continues to grow and divide, producing a new fila- 

 ment. 



Microspora, then, in addition to the vegetative multiplication 

 of the cells shown by Protococcus, has swimming spores that 

 multiply and spread the plant, a stage that recalls Chlamydomo- 

 nas. It also has 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 

 algas include (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 production 

 of resting spores; and (3) a period of dormancy, during 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. 



Ulothrix. Another green alga occurring on the margins of 

 lakes, in running streams, and in clear springs is Ulothrix. It 

 has a filamentous body similar in many respects to Microspora, 

 and like that form it is attached to rocks and other objects. 

 Its methods of reproduction, however, are more numerous and 

 more complex than those of Microspora, and they will serve to 

 exemplify the reproductive processes of many other forms of 

 algae. When the filaments are mature, the protoplasm within 

 some of the cells divides into two, four, or eight parts, each of 

 which contains nucleus, cytoplasm, chloroplast, and vacuole. 

 Each of these parts becomes oval in shape and develops into a 

 swimming spore with four cilia. An opening appears at one 



