SINGLE-CELLED GREEN ALG& 



241 



of lakes, ponds, and streams in rocky areas of the North Tem- 

 perate region. It is often found in shallow depressions of rock 

 along the shore, where in dry weather it resembles a coat of dull 

 red paint on the rocks. The plant is now in the resting stage. 

 When rains come the plants revive, come out of the resting state, 

 are provided with two long lashes or whips (cilia or flagella) 

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

 the water. In the swimming 

 stage the plant is green, the 

 chlorophyll usually not being 

 obscured by the red pigment. 

 As the small pools dry out the 

 swimming plant passes again 

 into the resting stage and ac- 

 quires the red pigment. 



375. 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 

 Pandorina morum. This is a 

 colony of sixteen like individuals 

 arranged in the form of a sphere enclosed in a thin gelatinous 

 sheath, each oval individual with a pair of cilia projecting be- 

 yond the sheath. The vibration of the cilia causes a rapid 

 rotary motion of the sphere. The colony multiplies by each 

 individual dividing into sixteen small individuals. The small 

 colonies separate from the parent 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 

 nonmotile as in the water-net (Hydrodictyori). 

 16 



Fig. 184. 



Pandorina morum (Mull.) Bory. I, motile 

 colony; II, colony divided into 16 daughter 

 colonies; III, sexual colony, gametes escap- 

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

 young and old zygospore; VIII, zygospore 

 forming a large swarm spore, which is free 

 in IX; X, same large swarm spore divided to 

 form young colony. (After Pringsheim.) 



