190 PRACTICAL BOTANY 



supply water more or less intermittently. Possibly water 

 may be absorbed directly from a moist atmosphere. If dry 

 bark with Pleurococcm on its surface is placed within a moist 

 bell jar, the plants soon become bright green, thus indicating 

 that they are at work. 



Heat, cold, and the extreme drought either of summer or of 

 winter are some of the great extremes which this plant must 

 undergo in order to live. Exposed as it is, it nevertheless is 

 able to pass through such periods and grow luxuriantly within 

 a short time after the return of favorable conditions. If in 

 zero weather Pleurococcus is brought into the laboratory and 

 moistened, within a few hours it begins to grow. 



176. Pleurococcus: reproduction. Pleurococcus cells divide, 

 thus forming new plants directly, as has been seen to occur in 

 Grloeocapsa. This vegetative reproduction in favorable weather 

 results in a rapid multiplication of the plants. Divisions follow 

 one another in such a way that whole colonies, the descendants 

 of one individual, are often found grouped together (Fig. 155). 



Sometimes, in near relatives of Pleurococcus that live in the 

 water, another kind of reproduction occurs. Within the cell 

 wall the protoplasm divides so as to form several (eight, six- 

 teen, or thirty-two) small bodies. Each of these has a nucleus 

 and cytoplasm which are obscured by chlorophyll, an extremely 

 thin cell wall, and two small hair-like extensions, the cilia (sin- 

 gular, cilium). After a time the old plant wall breaks, and 

 these bodies, by means of the active cilia, begin to swim about 

 in the water. Soon they become quiet, the cilia are lost or are 

 withdrawn into the main body of the cell, and the cell begins 

 to grow and develop into a new plant like the one that formed 

 it. Thereafter it may reproduce itself vegetatively or by the 

 process just described. 



These cells that are specially made for the work of repro- 

 duction are called spores. In the study of other kinds of 

 plants we shall find several kinds of spores ; and while they 

 may differ in the ways in which they are formed, they are alike 

 in that all may reproduce the kinds of plants that form them. 



