254 STRUCTURE AND LIFE HISTORIES 
wall (Fig. 184). We have here, in fact, a unicellular 
organism. 
244. Reproduction. The reproduction of Pleurococcus 
consists merely of the processes of cell-division, and the 
final separation of the daughter-cells. The latter may 
adhere until several divisions have occured, but eventually 
the middle layer of the cell-wall common to the two 
adjacent daughter-cells is dissolved, probably by an 
enzyme. The cells then separate from one another, and 
become independent plants, increasing in size, and soon 
repeating the simple reproductive process just described. 
245. Simplicity of Pleurococcus. The structure, life- 
processes, and life-relationships of plants could hardly 
find a more simple expression than in Pleurococcus. 
Morphologically the green plant is here reduced to its 
lowest terms. There is no differentiation into parts 
no hold-fast, roots, or rhizoids, no shoot, no special 
reproductive organs. From the standpoint of physiology, 
every essential function is performed by the one cell- 
absorption of water and dissolved nutrient substances 
by osmosis through the cell- wall and plasma-membrane; 
photosynthesis, with entrance of carbon dioxide and 
exit of oxygen by diffusion through the membrane and 
cell- wall; respiration, with the accompanying exchange 
of gases in the same way; digestion, assimilation, and 
growth, resulting finally in reproduction by the division 
of the entire plant body into new individuals. So far 
as known, such processes as cell-fusion (in fertilization 
or conjugation), reduction, and alternation of genera- 
tions are entirely absent. Pleurococcus is a generalized 
plant, with almost no division of physiological labor. 
At one time the entire plant body functions vegetatively, 
