SINGLE-CELLED PLANTS AND COLONIES. 



13 



Yellow-green algae. 



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

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



Fig. 18.— Pleurococcus viridis. A, a single individual; B, a colony shortly after 

 division; C, the same after separation. Magnified 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 algae. The wall in 

 such as this Pleurococcus is almost 

 uniform and quite thin. 



20. Colonies. — The cells are fre- 

 quently associated in colonies, em- Fl The I9 i'r^iv?duaTs are C re° P n rel 



1 1 j 1 • -n i-t-ii sented by the minute circles, 



bedded in jelly or not. The most between which the protoplast 

 striking and elaborate of these colo- The large baibin the n ?nte°ior 



c 1 1 T7- 7 / cl \ are daughter-colonies to be 



nieS IS formed by VolVOX (fig. 19). set free upon the rupture and 



In this plant the colony is a hollow Magnified 'aLT^s r "diam?— 

 sphere, often large enough to be seen rom essey ' 

 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 ; Russell : Dairy Bacteri- 

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



