174 ORDERS OF GREEN ALGAE 



tide, they are more commonly brown, while in the shade of the 

 brown Algae, or below tidal limits, red forms occur. The 

 Algae appear to have developed along three lines, which are 

 indicated by the green, brown and red colors, although the basis 

 for this classification rests upon structural and reproductive 

 characters. These three classes are: A, Green Algae or Chloro- 

 phyceae; B, Brown Algae or Phaeophyceae ; C, Red Algae or 

 Rhodophyceae. 



Class A. Green Algae or Chlorophyceae 



66. The More Important 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. These plants 

 are doubtless the most primitive survivors of a line from which 

 the higher land plants have been derived. Starting with uni- 

 cellular 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 uni- 

 cellular green Algae; (b) The Zygnematales or conjugating 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 



