THE TITALLOPIIYTA 2G7 



green color which the plant body thus displays has given the class 

 its name. It is well represented in both fresh and salt water and 

 a few species thrive in damp situations on land. The cell is 

 much more highly differentiated here than among the blue-green 

 algae, possessing a nucleus, one or more chloroplasts (often 

 called chromatophores) and usually a sap-cavity, thus resembling 

 in its essential details the cells of the higher plants. Pyrenoids, 

 or centers of starch formation, are prominent in the chloroplasts. 

 The plant body may consist of a single cell, a filament, or a plate 

 of cells. Most species (though not all) produce zoospores, motile 

 reproductive cells which swim about by the aid of one or more 

 lashes or cilia and which grow directly into new plants. These 

 are developed in modified cells or sporangia. Various types 

 of sexual reproduction are also found in this class, ranging 

 from instances where the gametes are entirely similar to those 

 where they have become markedly distinguishable as sperms 

 and eggs. Because of all this structural diversity and of the fact 

 that they are thought to be near the main line of ascent from 

 lower algae to bryophytes, the Chlorophyceae have received 

 intensive study, particularly with regard to the development 

 of the multicellular individual and the evolution of sex. 



To classify this great class thoroughly it is necessary to dis- 

 tinguish within it a large number of orders, but for the purposes 

 of our brief survey we can conveniently group these into five: 

 The Protococcales, the Confervales, the _ 



Conjugales, the Siphonales, and the 

 Charales. 



1. Protococcales or One-celled Green 

 Algae. — These are chiefly microscopic 

 plants. The individuals are single-celled 

 and they may be completely separate 

 or loosely joined into colonies, and are 

 either motile or non-motile. Fig. 142. — PUurococcus. 



Plei,rococcus (Fig. 142), which forms ^^Vol' c^enl '"x ISi'" 

 the green stain found on damp bark, 



rocks, and similar places, is perhaps the most common type. It 

 consists of a single cell containing one large chloroplastid and 

 reproduces only by cell division. The daughter cells may some- 

 times cohere for a time in small groups. This is one of the 

 algae which is commonly associated with various fungi to form 

 the peculiar group of lichens. 



