140 PLANT-ANIMALS [CH. 



which thus settle on and in the capsule, proves that 

 they find in it a favourable medium for growth. 

 Within a few hours, each green cell, having with- 

 drawn its flagella, increases considerably in size and, 

 whilst retaining its green colour, takes on a granular 

 appearance. The eye-spot and pyrenoid become 

 fainter and the cell undergoes division. In the 

 daughter cells thus produced, a series of successive 

 divisions occur till a loose colony of green cells is 

 formed such a colony, in short, as that which enabled 

 us to determine the nature of the infecting organism 

 (p. 120). In egg-capsules, some of the eggs of which 

 have died, the green cells find yet richer supplies of 

 food-material and increase the more rapidly. These 

 observations give us a hint as to the nature of the 

 food-materials contained in the capsules, which serve 

 for the rapid increase in the green cells. For though, 

 as we have learned, green plants have at their com- 

 mand unlimited supplies of the raw materials, carbon- 

 dioxide and water, for the manufacture of carbo- 

 hydrates, they are by no means in so happy a situation 

 with respect to the raw materials for the synthesis of 

 organic, nitrogen-containing compounds. A green 

 plant growing with its roots in the soil, and relying 

 on inorganic salts nitrate of potash, etc. for its 

 supplies of nitrogen-containing, raw material, is often 

 hard put to it to obtain enough of these nitrogen 

 compounds wherefrom to manufacture its proteins, 



