ALGM COKTL\'UED: CH SSTFICA TIOM. 



I6l 



338. Family Tetrasporacese. — This family is well represented by Tetra- 

 spora lubrica forming slimy green net-like sheets attached to objects in 

 slow-running water. It is really a single-celled plant. The rounded cells 

 divide by cross walls into four cells, and these again, and so on, large num- 

 bers being held in loose sheets by the slime in which they are imbedded. 



339. Family Pleurocoooaceae. — The members of this family are all non- 

 motile in the vegetative stage. They consist of single individuals, or of 

 colonies. Pleurococcus %ailgari3 (Protococcus ^'ulgaris) 

 is a single-celled alga, usually obtained with Uttle difficulty. 

 It is often found on the shaded, and cool, or moist side of 

 trees, rocks, walls, etc., in damp places. This plant is 

 not motile. It multiplies by fission (fig. 165) into two, 

 then four, etc. These trells remain united for a time, then 

 separate. Sometimes the cells are found growing out into 

 filaments, and it is thought by some that P. vulgaris may 

 be only a simple stage of a higher alga. Eremosphgera 

 viridis is another single-celled alga found in fresh water 

 among filamentous forms. The cells are large and globose. 



340. Family Hydrodictyaceae. — These plants form colonies of cells. 

 Hydrodictyon reticulatum, the water net, is made up of large numbers of 

 cyUndrical cells so joined at their ends as to form a large open mesh or net. 

 Pediastrum forms circular flat colonies, as shown in fig. 166. Both of these 



Fig. 

 Pleurocuccus 

 (protococcus) 

 vulgaris. 



Fig. 166. 

 Pediastrum boryanum. A, mature coloTiy. most of the young colonies have 

 escaped from their mother cells; at g, a young colon\- is escaping; sp, empty 

 mother cells; B, >-oung colony; C, same colony with spores arranged in order. 

 (After Braun.) 



plants are rather common in fresh-water pools, the latter one intermingled 

 with filamentous alg;E, while the former forms large sheets or nets. !Mul- 

 tiplication in Hydrodictyon takes place by the pio.i ] lasm in one of the cells 



