156. Plant Biology 



formation of reefs in the ocean. About 5,700 species are classified in 360 

 genera. The following typical examples will be considered : Chlamydo- 

 monas, Ulothrix, Protococcus, Spirogyra, and desmids. 



Chlaniydonionas (klam id o -mo' nas) (Gr. chlamydos, cloak; monas, 

 one). — This simple, unicellular green alga is common in fresh water and 

 soils. Each cell is spherical or ovoid and contains a central nucleus, a 

 single, large, cup-shaped chloroplast with a pyrenoid, an eyespot, two 

 anteriorly located flagella of equal length, a cell wall of cellulose, and two 

 excretory contractile vacuoles near the anterior end (Fig. 61). Some in- 

 vestigators classify this organism as a single-celled protozoan (animal). 



FlaaelluTn 



Contractije Vacuo/e 

 Viamenh opoz 



Cell Wall 



Nucleus 

 Dense Cutoplas 



m 



Plasma Membrane 



rurenofd 

 Chloroplast 



Fig. 61. — A green alga (Chlamydonionas) of the phylum Chlorophyta. The 

 chloroplast is cup shaped ; the pigment spot is also known as the eye spot. Be- 

 cause of certain characteristics, this unicellular organism is considered by some to 

 be a protozoan (single-celled animal). 



At certain times there may be formed within the cell two, four, or eight 

 motile swarm spores (zoospores) which resemble the parent cell except 

 in size and which swim out to form new Chlamydomonas. In some in- 

 stances the contents of the parent cell may divide into eight, sixteen, or 

 thirty-two small gametes (sex cells) which resemble miniature Chlamydo- 

 monas plants. When released into water, two gametes of equal size, but 

 coming from different parent cells, fuse by the process of fertilization 

 known as isogamy (i-sog'amy) (Gr. isos, equal; gamos, marriage). In 

 the fusion of isogamous (alike) gametes, there is no differentiation into 



I 



