SINGLE-CELLED PLANTS AND COLONIES. 1 3 



Yellow-green algae. 

 19. Single-celled plants with chloroplasts. — Among the 

 single-celled green plants, one of the most common groups 



Fig. i8. — PUurococcus viridis. y4 , a single individual; i?, a colony shortly after 

 division; C, the same after separation. iMagniticd 540 diam. — After Strasburger. 



is that represented by fig. 18, which shows a representative 

 of an extensive series in which the vegetative body consists 

 of a single cell with its wall, cyto- 

 plasm, nucleus, and a few relatively 

 large chloroplasts. In this greater 

 specialization of the protoplasm, these 

 plants show the only advance upon 

 the blue-green algre. The wall in 

 such as this Pleurococcus is almost 

 uniform and quite thin. 



20. Colonies. — The cells are fre- 

 quently associated in colonies, em- 

 bedded in jelly or not. The most 

 striking and elaborate of these colo- 

 nies is formed by Volvox (fig. 19). 



In this plant the colony is a hollow 

 sphere, often large enough to be seen 

 by the naked eye as a minute green ball, composed of thou- 

 sands of individuals, embedded in a common jelly, arranged 

 in a single layer at the surface. Each is connected with its 

 immediate neighbors by strands of protoplasm, and two 



its Dangers, Drinking-water and Ice Supplies ; Russt^ll : Dairy Bacteri- 

 ology ; Frankel (tr. by Linslev) : Bacteriology (medical). 



The individuals are repre- 

 sented by the minute circles, 

 between which the protoplas- 

 mic strands form a network. 

 The large balls in the interior 

 are daughter-colonies to be 

 set free upon the rupture and 

 death of the mother-colony. 

 Magnified about 45 diam, — 

 From Eessey. 



