SINGLE-CELLED PLANTS AND COLONIES. 



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 algoe. The wall in 

 such as this Pleurococcus is almost 

 uniform and quite thin. 



20. Colonies. The cells are fre- 



,1 j i FIG. 10. Volvo.r. a colony. 



quently associated in colonies, em- The individuals are repre- 



, ill- -11 T<T_ sented by the minute circles, 



bedded in jelly or not. Ine most between which the protopias- 



., . 111 r ^t i mic strands form a network. 



striking and elaborate of these colo- The large bails in the interior 



,. i i TT- 7 / r \ are daughter-colonies to be 



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; Ritssell : Dairy Bacteri- 

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



set free upon the rupture and 

 death of the mother-colony. 

 Magnified about 45 diam. 

 From Bessey. 



