2 54 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, 



