FILAMENTOUS GREEN ALG& 



221 



which lash about and cause the plant to swim rapidly around in 

 the water. In the swimming stage the plant is green, the chloro- 

 phyll usually not being obscured by the red pigment. As the small 

 pools dry out the swimming plant passes again into the resting 

 stage and acquires the red pigment. 



362. Single-celled green algae in colonies. The single- 

 celled algae just described lead an independent or individual 

 existence. There are others in 

 which the individuals are asso- 

 ciated in definite colonies. Some 

 of these are motile during their 

 vegetative existence as in Pan- 

 dorina morum. This is a colony 

 of sixteen like individuals ar- 

 ranged in the form of a sphere 

 enclosed in a thin gelatinous 

 sheath, each oval individual 

 with a pair of cilia projecting 

 beyond the sheath. The vibra- 

 tion of the cilia causes a rapid 

 rotary motion of the sphere. The 

 colony multiplies by each in- 



Fig. 175- 



Pandorina morum (Mull.) Bory. I, motile 

 colony; II, colony divided into 16 daughter 

 dividual dividing into Sixteen colonies; III, sexual colony, gametes escap- 

 ....,. ,_. ing; IV, V, conjugating gametes; VI, VII, 



Small individuals. The Small young and old zygospore; VIII, zygospore 

 . . . forming a large swarm spore, which is free in 



COlomeS Separate from the parent IX; X, same large swarm spore divided to 

 * . . form young colony. (After Pringsheim.) 



and grow to the normal size. 



Sexual reproduction takes place when these small individuals 

 separate and conjugate in pairs, usually a small one with a large 

 one, producing a zygospore. Some of the colony algae are non- 

 motile as in the water-net (Hydrodictyon). 



FILAMENTOUS GREEN 



The Confervas (Confervoidea). 



363. General characters. The larger number of these algae 

 are thread-like, either simple or branched. Some of the marine 

 forms are leaf -like, as the " sea lettuce." The chlorophyll is 



