[Chap. XLVII THE ALGAE 613 



We have discussed so far some of the general characteristics, distri- 

 bution, and economic aspects of algae as a whole. They constitute such 

 a large and diversified assemblage of plants, however, that a better 

 understanding of their growth and reproduction depends on a study 

 of some of the commoner ones in more detail. We shall discuss in the 

 next few pages some representatives of each of the important classes 

 of algae. 



The Green Algae: Chlorophyceae 



About 5000 species of green algae have been described and named. 

 As a group of plants they are quite varied in structure, in appearance, 

 and in several other ways. They are usually predominantly green in 

 color during the vegetative phase, containing chlorophvll, xanthophyll, 

 and carotene in about the proportion found in the seed plants. They 

 grow chiefly in water, both fresh and marine, but occur also on and in 

 the soil and on many other kinds of moist substrates. Biologically they 

 influence both the abundance and the distribution of certain other 

 plants and of certain animals. Green algae are by far the most common 

 ones in lichens. They may be parasitized by animals and occur inside 

 animals, such as hydras and rotifers. In size thev varv from the micro- 

 scopic forms to the filamentous, highly branched cladophoras, which 

 may be several feet in length and are attached to rocks and stones of 

 streams and lakes. A few common species are described below. 



Pleurococcus. On the bark of trees and shrubs throughout the world 

 there occur green-colored areas of Pleurococcus, sometimes several 

 square feet in extent. This alga grows most frequently on the shaded 

 parts of the tree. In drier regions it may be restricted to the lee side. 

 Although most abundant near the ground, it mav be found on the bark 

 all the way to the tops of trees in moist open forests. No spores are 

 formed and no sexual reproduction occurs in this alga. 



Under the microscope the green areas of Pleurococcus are seen to 

 consist of masses of rounded solitary cells or of groups of two to several 

 cells ( Fig. 278 ) . The chloroplast occupies most of the cell, and the cell 

 walls are comparatively thick. The plant may survive prolonged droughts 

 and remain dormant during long periods of low temperature, in spite 

 of the fact that there are no apparent structural modifications that might 

 prevent desiccation of the cells. When periods of rain and higher tem- 

 peratures return, the cells again become active. Propagation takes place 

 only through the division and subsequent separation of the cells. No 



