PLEUROCOCCUS 



307 



occurs within the hollow of the sphere. The spore formed from 

 the fusion, now known as an oospore (meaning eggspore), forms a 

 new colony upon germination. 



There is much advantage gained by differentiating gametes. 

 The egg, owing to its size and loss of motihty, can store much 

 food for the next generation. The smallness of sperms makes it 

 possible for large numbers of them to be produced, and promotes 

 their movements through water. 



In summarizing the Volvocales, the following features are the 

 notable ones. The plant body consists of a single motile cell 

 having a chloroplast and well-defined nucleus and cytoplasm. 

 Some swim about independently, but the formation of colonies 

 is a marked feature of the group, and the colonies range from 

 simple to complex ones. By the division of cells new individuals 

 and new colonies are formed. Sexual reproduction advances 

 from isogamy to heterogamy. As 

 in the Blue-green Algae, the forma- 

 tion of colonies is a step toward 

 the formation of multicellular in- 

 dividuals. 



Unicellular Non-motile Green 

 Algae (Protococcales). — In con- 

 trast to the Volvocales, the absence 

 of cilia, except on reproductive cells, 

 is a notable feature of this group. 

 Some plants of this order are very 

 common on damp soil, walls, and on 

 the bark of trees, where they are 

 often exposed to long periods of 

 drought. Most of the group are 

 aquatic and occur mainly in fresh 

 water. Some enter into the forma- 

 tion of Lichens. Others are endo- 

 phytic, living in the intercellular 

 spaces of other plants, and some give the green color to certain 

 animals, such as the hydra and fresh-water sponge, which eat 

 them. They show considerable variation in their habit of form- 

 ing colonies and in methods of reproduction. 



Pleurococcus. — Pleurococcus {Fig. 262), often called Protococ- 

 cus, is the simplest plant of the group, and may be regarded as 



Fig. 262. — Pleurococcus vul- 

 garis. Above, a single plant 

 consisting of a single cell with a 

 definite wall, well defined nucleus, 

 and large lobed chloroplast; be- 

 low, left, plants dividing; and 

 below, right, a group of four 

 separate plants. X 540. After 

 Strasburger. 



