ALG 51 
vessel in which they are placed. After a short period 
of active movement, they settle down, become invested 
with a cell-membrane, and enter what may be called 
the Protococcus stage, in which they sometimes remain 
for a long time, giving rise to large colonies of unicellu- 
lar plants by repeated fission and separation of the cells. 
Indeed these unicellular stages of many alge have been 
given special names, under the mistaken impression that 
they were really autonomous forms instead of simply 
transitory stages in the development of a filamentous 
alga. Usually, however, the zodspore, after coming 
to rest, elongates, and, by the formation of a transverse 
wall, becomes two-celled, and, by further elongation 
and repeated cross-divisions, assumes the filamentous 
form of the adult plant. (Fig. 9, D, E, F.) 
THE CONFERVACEZ 
The order which seems to be most directly connected 
with the Protococcacee is that known as the Confer- 
vacee, especially important in a study of the evolution 
of plants, as it probably represents, more nearly than any 
other existing group, the direct ancestral forms of the 
higher plants. 
The lowest members of the order are simple un- 
branched filaments, composed of perfectly similar cells 
(Fig. 8, A). Somewhat higher in point of develop- 
ment are a number of common forms, e.g. Cheetophora, 
Cladophora (Fig. 8, B), which are branched, while many 
of these, as well as such of the unbranched forms as 
are attached, often show a modification of the basal cell 
into a root-like organ. Where this is the case of course 
