THE ALG^E. 83 



served at the right time, these swarm spores may be seen 

 in active motion. The motile condition does not last long, 

 but the spores arrange themselves in a methodical way in 

 the cell. The cell wall now breaks down, and the colony 

 of little cells develops into a full grown Water Net, 

 which escapes through the gelatinous wall of the mother 

 colony. 



There is also a sexual method of reproduction. Enor- 

 mous numbers of microzoospores are formed in certain of 

 the tubular cells of the mother colony. By the breaking 

 down of the cell wall, these microzoospores escape. Each 

 has a red spot known as the eye spot. Each also has 

 four flagella. After a period of motility, they come to- 

 gether in pairs and form green zygospores enveloped with 

 a wall of cellulose. This is the resting spore. It closely 

 resembles the resting spore of Protococcus. Buried in the 

 mud at. the bottom of ponds and streams, these resting 

 spores retain their vitality against drought for a long 

 time, but will not endure exposure to strong light. 

 When the conditions are again favorable for growth, the 

 endochrome, or colored contents of the spore, separates 

 into either two or four parts, the cellulose wall breaks 

 away, and the newly formed zoospores work their way 

 out of the cell by means of the two flagella with which 

 each is provided. They are not very active. They soon 

 become angular in shape, in which condition they rest for 

 a time. They then divide and subdivide, forming numer- 

 ous microzoospores, which are active for a time. These 

 then arrange themselves in the net form, develop a new 

 gelatinous covering, and grow very rapidly into a full 

 sized Water Net. The alternation of generations is clearly 

 seen in Hydrodictyon. The plant itself is the gameto- 

 phy te ; the resting zygospores and the angular resting 

 spores represent the sporophyte. 



The other plants of this class are not so abundant as 



