76 EVOLUTION OF PLANTS 
SUMMARY 
The green algw are probably the most primitive 
of the three classes of Alga, 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 Pheophycee is 
quite probable, as certain unicellular forms, the Peri- 
dinee and Dinoflagellata show a close resemblance 
to the zodspores of the higher brown alge, and may 
represent their ancestral forms. 
Among the green alge the simpler Volvocinez 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 Volvocinez 
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 
Protococcacee. 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, 
