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 zoospores, 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 zoospores 

 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 oospore 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, Conjugates^ and 

 Siphonales. 



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. 



