232 PHARMACEUTICAL BOTANY 



produced by conjugation, by egg cell and sperm cell union (oospores) 

 oogamous reproduction. Green algae live mostly in fresh water. 

 Some live in brackish water and a few in the sea. Some are asso- 

 ciated with fungi to form lichens. 



1. Order Protococcales or One-celled Green Algae. This order 

 contains nearly all of the one-celled green algae excepting the diatoms 

 arid desmids. 



Family Pleurococcaceae. Pleurococcus vulgaris is a one-celled 

 green alga, millions of which, living together in colonial fashion, con- 

 stitute the so-called "green stain" that is common on the north 

 sides of tree trunks, stone walls and fences. Each organism con- 

 sists of a protoplast surrounded by a cell wall of cellulose. The 

 protoplast contains a chromatophore, cytoplasm and nucleus. 

 Reproduction takes place by the protoplast dividing into two equal 

 parts and laying down a cell wall forming two daughter-protoplasts. 

 These may again divide to form four granddaughter-protoplasts. 

 Still another division may occur as a result of which eight great- 

 granddaughter-protoplasts are formed which frequently adhere to 

 one another forming colonies. 



2. Order Volvocales. This order comprises free-swimming 

 aquatic forms whose vegetative cells are bi-ciliated, green, more or 

 less spherical or compressed. Some of the organisms like Sphcerella 

 and Chlamydomonas consist of single cells bearing a pair of cilia, 

 while others like Pandorina, Eudorina and Volwx show varying de- 

 grees of colony formation. Reproduction sexual or asexual. 



Volvox globator, a typical representative of this order, is found in 

 fresh water pools as a tiny, hollow, spherical, green colony about 

 /-io to J^o of an - i ncn in diameter. When examined under the 

 microscope (Fig. 112), it is found to consist of hundreds of green, 

 more or less spherical cells, united by fine strands of cytoplasm 

 (protoplasmic bridges), the whole being enveloped by a gelatinous 

 sheath. The peripheral cells are provided with cilia, in order that 

 the colony may rotate and roll through the water. In a young 

 colony, all of the cells are alike, each consisting of a mucilaginous- 

 like cell-wall enclosing cytoplasm, a nucleus, a chloroplast and often 

 a red pigment spot. In a mature colony, however, throughout the 

 greater part of its existence, two kinds of cells may be discerned: 



