THE FOOD OF PLANTS; ASSIMILATION 163 



frequently contain starch grains, as may be readily shown in 

 the cells of the moss leaf (Fig. 169, C) when colored (stained) 

 with iodine. Photosynthesis is only found in plants containing 

 chlorophyll or other pigments of a similar physiological nature. 

 The sun furnishes the energy in the form of light for the build- 

 ing up of the simplest food products, and the plant cell is the 

 main factory which supplies the food of the w r orld. 



197. The food of plants ; assimilation. All plants with 

 chlorophyll can manufacture their own food by the processes of 

 photosynthesis. Moreover, it is manufactured directly within 

 the protoplasm of the cell and does not have to be absorbed 

 from without, as in the case of the animal cell (see account 

 of Amoeba, Sec. 194). 



As we have already noted, starch is generally the first visible 

 product of this process of food manufacture (photosynthesis). 

 Starch and the related substances, sugars, are the primary foods 

 of green plants, and the most important, but they are merely the 

 starting point for a complex series of processes through which 

 the highly organized proteids of the protoplasm are derived. 

 There are some plants which lack chlorophyll, as the fungi and 

 certain plant parasites, and they, like the animals, depend upon 

 food absorbed from without the body. The food of plants is 

 broken down and recombined in various ways to form the pro- 

 toplasm, as it is in animals, and the breaking down of some of 

 the substances sets free energy in the form of plant heat (corre- 

 sponding to animal heat), as is easily proved in the germination 

 of seeds (see Sec. 5). So the processes of food absorption, or 

 assimilation, in plants are essentially the same as in animals, 

 but the manufacture of food (photosynthesis) is an entirely dif- 

 ferent process and peculiar to plants. 



198. The food cycle. There is a circulation of certain ele- 

 ments (especially carbon, nitrogen, sulphur, and phosphorus) 

 through the bodies of plants and animals which may be called 

 the food cycle (see diagram, Fig. 207). It begins in the plant 

 cell with the manufacture of starch, and related substances 



