PLEUROCOCCUS 307 
occurs within the hollow of the sphere. The spore formed from 
the fusion, now known as an odspore (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 motility, 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 Big. 262.—Preumocnciis: ule 
garis. Above, a single plant 
the bark of trees, where they are consisting of a single cell with a 
often exposed to long periods of definite wall, well definednucleus, 
drought. Most of the group are and large lobed chloroplast; be- 
aquatic and occur mainly in fresh low, left, plants dividing; and 
water. Some enter into the forma- Helow, ight, grep Gb Zour 
: : separate plants. X 540. After 
tion of Lichens. Others are endo-  gtrasburger. 
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 
