192 



MOTIONLESS COLONIES— PLANKTON 



in which the vegetative individuals are extremely susceptible 

 to adverse conditions. It is this property which enables 

 Pleurococais to exist, and even flourish, in the exposed dry 

 habitats that it usually frequents. Although retaining its normal 

 green appearance throughout the year, its activities are more 

 or less completely arrested during prolonged periods of drought, 

 when its only source of moisture is dew. 



Many of the motionless colonial green forms are common 

 in freshwater pools where they occur entangled among the 

 filaments of the pond-scums or in the shmy growth on the sur- 

 face of larger water-plants. 

 As examples we may mention 

 the four- or eight-celled 

 colonies of Scenedesmus (Fig. 

 loi, A, B), and the character- 

 istic disc-like plates of Pedi- 

 aslritm, in which particularly 

 the marginal cells are often 

 of very distinctive form (Fig. 

 loi, C). In both cases the 

 ordinary course of reproduc- 

 tion consists in the division 

 of the contents of each cell to 

 form a new colony. 



Forms like Scenedesmus, 

 Pediastrum, Endorina, etc., 

 together with many of the uni- 

 cellular Diatoms and Desmids 

 to be mentioned in the next chapter (pp. 206, 2og), very commonly 

 occur in considerable numbers floating freely in the surface-layers 

 of lakes, rivers, etc. (Fig. 103). These floating microscopic 

 plants or vegetable Plankton form the food for many aquatic 

 animals, and may at times occur in such prodigious quantity 

 as to lend a visible colouration even to large pieces of water, 

 a phenomenon known popularly as " water-bloom," and for 

 which the Blue-green Alga; (p. 234) are most frcqucnth' responsible. 

 It may be added that the character of the Plankton usually 

 differs markedly in different seasons of the vear. 



The Vegetable Kingdom taken as a whole comprises sedentary 



Fig. 102. — Pleurococcus. A, Group 

 of cells, under the low power. 

 B, Single cell, and C, pair of 

 cells, under the high power. 

 ch., chloroplast ; n., nucleus ; 

 w., cell-wall. 



