THE GREAT GROUPS OF ALG^E 237 
being of this simple kind. It is evident, therefore, that the 
group Chlorophycese begins with forms just as simple as 
are to be found among the Cyanophyceae. 
Pleurococcus is used to represent the group of Protococ- 
cus forms, one-celled forms which constitute one of the 
subdivisions of the Green Algae. It should be said that 
Pleurococcus is possibly not a Protococcus form, but may 
be a reduced member of some higher group ; but it is so 
common, and represents so well a typical one-celled green 
alga, that it is used in this connection. It should be 
known, also, that while the simplest Protococcus forms re- 
produce only by fission, others add to this the other meth- 
ods of reproduction. 
164. TTlothrix. This form is very common in fresh wa- 
ters, being recognized easily by its simple filaments com- 
posed of short squarish cells, each cell containing a single 
conspicuous cylindrical chloroplast (Fig. 205). 
The cells are all alike, excepting that the lowest one of 
the filament is mostly colorless, and is elongated and more 
or less modified to act as a holdfast, anchoring the filament 
to some firm support. With this exception the cells are all 
nutritive ; but any one of them has the power of organizing 
for reproduction. This indicates that at first nutritive and 
reproductive cells are not distinctly differentiated, but that 
the same cell may be nutritive at one time and reproductive 
at another. This plant uses cell division to multiply the 
cells of a filament, and to develop new filaments from frag- 
ments of old ones ; but it also produces asexual spores in 
the form of zoospores, and gametes which conjugate and 
form zygotes. Both zoospores and zygotes have the power 
of germination that is, the power to begin the develop- 
ment of a new plant. In the germination of the zygote 
a new filament is not produced directly, but there are 
formed within it zoospores, each of which produces a 
new filament (Fig. 205, F, G). All three kinds of repro- 
duction are represented, therefore, but the sexual method 
