388 EVOLUTION OF PLANTS 
the higher plants. These are known as the Volvoca- 
cee, or Volvocines, and have been claimed by zoélo- 
gists as animals, although there seems no question of 
their close relationship with the lower green plants. 
It is true that they are actively motile, and show other 
animal properties, but they usually possess a cellulose 
membrane, and the characteristic green chromatophore 
of the typical plant cell, and inasmuch as they are con- 
nected with unmistakable plants by a complete series 
of intermediate forms, there seems to be no valid 
reason for not considering them as low plants. It 
is interesting to note, however, that the lower mem- 
bers of the series of Volvocinee are very much like the 
animal flagellate infusorians and also the swarm-spores 
of the slime-moulds, from which they differ mainly in 
the presence of a green chromatophore. The frequent 
recurrence of this free-swimming, flagellate type among 
both the lower animals and plants suggests some similar 
forms as the ancestral type for both of the great series 
of organic beings, which here converge. 
The simplest of the Volvocinee are round or oval 
cells, which in their ordinary condition are actively 
motile, swimming by means of two delicate cilia. In 
the younger stages these cells are quite destitute of a 
membrane, but older cells usually have a distinct cellu- 
lose wall, with openings through which the two cilia 
protrude. The structure (Fig. 6, B) is that typical 
.of the lower green plants. The green chromatophore 
(chloroplast) has the form of a cup and fits into the 
lower part of the oval cell-cavity. Within the hollow 
of the chromatophore is included a mass of protoplasm 
in which is imbedded the nucleus. The forward end 
