42 EVOLUTION OF PLANTS 
strictly unicellular, the higher ones are multicellular, 
this being most marked in the genus Volvox, where there 
are several thousand cells in each individual, all con- 
nected by protoplasmic threads. We have seen, too, 
that, in the simpler types, all the cells are alike, and 
there is no clear distinction between vegetative and 
reproductive cells, nor between sexual and non-sexual 
ones, while in the higher ones special cells are set apart 
for reproductive purposes, and sexuality is well marked. 
It must be remembered, however, that the evolution 
of the plant body in such specialized Volvocinea, as 
Volvox, is in a direction away from that which leads up 
to the more typical plants, and there is no evidence 
that this peculiar line of development has ever advanced 
beyond such forms as Volvox, which seems to represent 
the highest expression of this type of structure. We 
must regard the whole series of the Volvocinez as an 
offshoot of the main line of development of plants. 
The simplest of the group, such as Chlamydomonas, are 
closely related to the lowest of the typical. green plants, 
the so-called Protococcacez, and may be considered to 
represent a primitive stock which has given rise to two 
branches, one, the Volvocinex, culminating in Volvox, 
the other, the Protococcacez, which leads directly to 
the higher green plants. 
THE PROTOCOCCACEZ 
The Protococcacex, employing this term in its widest 
sense, form a rather poorly defined group of unicellular 
plants, some of which are of doubtful autonomy, since 
many supposed members of this group have been shown 
