PRINCIPAL GROUPS OF PLANTS. 21 



more or less to one another, thus forming colonies. The number 

 of individuals in a colony depends upon the number of indi- 

 viduals in the colony when division begins and the extent to 

 which division is carried. Thus if there were four cells in a 

 colony to begin with and division took place in three planes, there 

 would be thirty- two cells in the colony at the end of the period. 

 The development of the green coating on the barks of trees, 

 due to the growth of Protococcus and the protonema of mosses, 

 is usually thought to be more pronounced on the north side. This, 

 however, is a slightly false notion. The fact which determines 

 the position of these plants is the quantity of moisture available. 

 The south and southwest sides of trees in the northern hemisphere 

 are exposed to more light and heat and consequently are apt to be 

 drier, with the result that they are rarely covered with coatings of 



FIG. 10. Protococcus vulgaris. Different stages of division of the cell. After Wille. 



Protococcus and mosses. The under side of slanting trees is a 

 very favorable place, as are also the lower slanting surfaces near 

 the ground of large upright trees, because in these places the 

 water is more likely to be conserved. A careful investigation by 

 Kraemer showed a more abundant growth of green plants on the 

 east and southeast exposure, although the north side of many trees 

 showed good growth also. 



Volvox occurs widely distributed throughout the United 

 States in ponds and pools of fresh water. It is most abundant 

 in warm weather, but also found in midwinter. It appears as a 

 minute spherical colony about y 2 mm. in diameter, made up of 

 numerous cells, the number ranging from several hundred to 

 many thousand (Fig. 11). The cells at the periphery are pro- 

 vided with cilia, so that the colony rolls slowly through the 

 water. Each cell contains a chloroplastid in which starch granules 

 and often a red pigment spot are present. The asexual reproduc- 



