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 

 command unlimited supplies of the raw materials, 

 carbon-dioxide and water, for the manufacture of 

 carbohydrates, they are by no means in so happy 

 a situation with respect to the raw materials for 

 the synthesis of organic, nitrogen-containing com- 

 pounds. A green plant growing with its roots in 

 the soil, and relying on inorganic salts nitrate of 

 potash, etc. for its supplies of nitrogen, is often 

 hard put to it to obtain enough of these nitrogen 

 compounds wherefrom to manufacture its proteins, 



