76 EVOLUTION OF PLANTS 



SUMMARY 



The green algae are probably the most primitive 

 of the three classes of Algae, and may have given rise 

 to the other two, although an independent origin of 

 the red and brown forms from unicellular ancestors is 

 not impossible, and in the case of the Phaeophyceae is 

 quite probable, as certain unicellular forms, the Peri- 

 dineae and Dinoflagellata show a close resemblance 

 to the zoospores of the higher brown algae, and may 

 represent their ancestral forms. 



Among the green algae the simpler Volvocineae prob- 

 ably represent the most primitive forms from which the 

 others have sprung. These actively motile plants also 

 show possible affinities with such low animals as the 

 flagellate Infusoria. 



With this free-swimming cell as the starting-point, 

 specialization has apparently proceeded in several direc- 

 tions. First of all, within the group of the Volvocineae 

 themselves there has been specialization in two ways, 

 first, the production of a multicellular plant body ; sec- 

 ond, a high degree of differentiation of the reproductive 

 parts which reaches its most complete expression in the 

 genus Volvox. The series of forms leading up to the 

 latter is very complete, every grade of development 

 being represented by existing genera. 



The second line of development is illustrated by the 

 Protococcaceae. By the loss of motility in the vegeta- 

 tive cells, and the formation of a continuous cellulose 

 membrane, these have lost their power of locomotion. 

 Within this series are also found multicellular plants, 



