26 PLANT MORPHOLOGY 



some of these are widely distributed and often abundant. Some green 

 algae grow as scums on the surface of quiet water, while others are 

 attached to various objects beneath the surface. A few forms grow on 

 moist soil, rocks, or tree trunks. Most of the green algae are multicellular 

 but some are unicellular, these occurring either as isolated cells or as 

 colonies. The Chlorophyceae are generally regarded as the group of 

 algae from which the bryophytes and other higher groups of green plants 

 have been derived. Lime-secreting forms are known as fossils as far 

 back as the Ordovician. The Chlorophyceae number over 5,000 species, 

 nearly all of which are included in seven principal orders: Volvocales, 

 Chlorococcales, Ulotrichales, Oedogoniales, Conjugales, Siphonocla- 

 diales, and Siphonales. 



1. Volvocales 



The Volvocales are a distinct group of primitive green algae that are 

 widely distributed in fresh water. Only a few members are marine. 

 They appear to have been derived from green flagellates, which they 

 resemble in many ways, and to have given rise, in turn, to the other 

 groups of Chlorophyceae. The Volvocales include about 50 genera and 

 300 species. The main genera are Chlamydomonas, Sphaerella, Gonium, 

 Pandorina, Eudorina, and Volvox. 



Chlamydomonas. This is a unicellular alga that does not form per- 

 manent colonies. It is widely distributed in pools and ditches and on 

 damp ground. The vegetative cell, which is free-swimming, is generally 

 spherical or egg-shaped (Fig. 16A). A cell wall is always present. At 

 the anterior end are a pair of cilia, equal in length, a red eyespot, and 

 two (rarely more) small contractile vacuoles. Surrounding the nucleus 

 is a small m.ass of colorless cytoplasm lying in the depression of a large 

 cup-shaped chloroplast. Embedded in the chloroplast is a conspicuous 

 spherical pyrenoid. Pyrenoids are protein bodies that function as centers 

 of starch formation. Although occurring in some members of certain 

 other algal groups, they are especially characteristic of the Chlorophyceae. 



Chlamydomonas reproduces asexually by means of zoospores. The 

 vegetative cell becomes quiescent by retraction of the ciha and then its 

 protoplast divides internally to form two, four, or eight daughter proto- 

 plasts, each of which, after enlarging slightly, forms a new cell wall and a 

 pair of cilia while within the parent cell (Fig. 165, C). By the breaking 

 down of the original cell wall, the small cells are set free as zoospores, 

 each soon undergoing further enlargement to become an adult vegeta- 

 tive cell (Fig. 16-D). Under conditions unfavorable for vegetative activ- 

 ity, Chlamydomonas may pass into a "palmella" stage. The daughter 

 cells, produced by the internal division of a vegetative cell, increase in 

 number but, instead of escaping, become surrounded by abundant muci- 



