DEVELOPMENT OF PLANTS 155 



CLASS A. GREEN ALGAE OR CHLOROPHYCEAE 



66. The More Imporant Orders of the Chlorophyceae. The 



Chlorophyceae are one of the most interesting groups of the 

 Algae, because they contain primitive forms that are at least 

 suggestive of low animal types and they also exhibit a gradual 

 modification of the plant body and reproductive processes that 

 help us to understand how variations arose and how complex 

 types have been evolved from very simple forms. Starting 

 with unicellular motile forms, the green algae appear to have 

 diverged along several lines. Among the more important of 

 these may be mentioned the following orders : (a) Volvocales, 

 largely unicellular green Algae: (b) The Zygnematales or con- 

 jugating green Algae; (c) Chaetophorales or filamentous green 

 Algae; (d) Siphonales or tubular green Algae. 



67. Order a. Volvocales or Unicellular Green Algae. The 

 lower members of this order represent a very primitive type of 

 plant. Their motility, certain features of their life history and 

 delicate cell walls are suggestive of some of the preceding groups, 

 as well as of low forms of animal life. Sphaerella (Fig. 100) is 

 a familiar example of this group, often appearing in rock hollows 

 as blood-red stains. If some of this red material is examined 

 in water, the plants appear as spherical cells with dense red pro- 

 toplasmic contents and thick walls (Fig. 100, A). This con- 

 dition represents the resting state of the plant. If the plants, 

 after being dried, are allowed to stand in water for a few hours, 

 the nucleus will divide, forming usually from 4 to 16 daughter 

 cells (Fig. 100, B) which escape by the rupturing of the old 

 mother wall and swim actively about by means of two cilia. 

 These cells, or zoospores, remain motile for varying periods, 

 during which time they increase in size, but finally the cilia are 

 retracted and the plants remain in a quiescent state for a short 

 period (Fig. 100, ). A division of the nucleus now results 

 again in the formation of two or more zoospores (Fig. 100, F) 

 which repeat the life history noted above. This method of re- 

 production may occur again and again and thus rapidly bring 

 about a great increase in the number of plants. During the 



