302 THALLOPHYTES 
nucleus is well organized and quite distinct, from the cytoplasm. 
In some, the cells of the colony have their protoplasts joined 
by protoplasmic strands and have thus become so closely asso- 
ciated that they constitute a multicellular plant. Some repro- 
duce entirely by cell division, but many of them have more 
specialized methods of reproduction. Many Green Algae form 
swimming cells called zodéspores, each of which is able to pro- 
duce a new plant directly. Others also form gametes or sex 
cells which fuse and form a cell that develops a new plant either 
directly or indirectly. The simplest gametes occurring in the 
group are alike as to size, structure, and behavior and are called 
isogametes. When isogametes pair and fuse, a cell called a 
zygospore or zygote is formed, and this spore may form zoéspores 
or develop a new plant directly. The fusion of similar gametes 
is called conjugation. The more advanced Green Algae form 
morphologically unlike gametes called heterogametes, of which the 
large ones are called eggs and the small ones are called sperms. 
The spore formed by the fusion of an egg and a sperm, that is, by 
the fusion of unlike gametes, is called an odspore and the fusion is 
called fertilization. Often the gametes are produced in special 
organs called sex organs. It is evident that the Green Algae 
resemble the higher plants much more than do the Blue-green 
Algae and are, therefore, considered more advanced. It is sup- 
posed that from plants like the Green Algae the higher plants 
have come. 
Among the Green Algae there is much diversity in character 
of plant body and methods of reproduction. About 9000 
species are known and these are commonly grouped into five 
orders — Volvocales, Protococcales, Confervales, Conjugales, and 
Stphonales. 
Unicellular Motile Green Algae (Volvocales).— These Green 
Algae are regarded as one-celled plants, although some of them 
form colonies of considerable size and complexity. They live in 
the water and chiefly in fresh water. Their vegetative cells have 
cilia and swim about like the lower animals. It is this motile 
habit that distinguishes them from other Green Algae. On ac- 
count of their motility and some animal-like structural features, 
they are sometimes regarded as animals. They are microscopic 
plants, but some of them form colonies that are sometimes visible 
to the naked eye. 
