124 GENERAL BOTANY 



It will occur to the student that the oxygen liberated by 

 photosynthesis during the day may be 'used by the green leaf 

 cells for respiration, and that the carbon dioxide liberated in 

 respiration may likewise be built by photosynthesis into sugar 

 and starch. While this is true, it does not change the funda- 

 mental distinction between the two processes as to their nature 

 and use in the living plant organism. In addition to photo- 

 synthesis there are other processes, notably fermentation and 

 what is termed anaerobic respiration, which are closely related 

 to normal, or aerobic, respiration. These processes will be con- 

 sidered more appropriately, however, in connection with the life 

 of the fungi, in a later chapter. 



DIGESTION AND ASSIMILATION 

 DIGESTION 



The nature of the digestive process is the same in both plants 

 and animals, and consists in the conversion of foods from an in- 

 soluble into a soluble condition fitting them for circulation and 

 final assimilation by the tissue cells of the body. The starch 

 which is stored in leaves, wood rays, tubers, and seeds is a good 

 illustration of an insoluble food which must be converted into 

 soluble sugar by digestion before it can be circulated or used by 

 the living cells of growing parts. In like manner the gluten or 

 protein of wheat and the fat of seeds like flax and the castor bean 

 are insoluble and unusable until they are digested at the time of 

 seed germination to form soluble proteins and fats for the growth 

 of the embryo. The place where digestion occurs is very different 

 in the higher plants and animals. Since there are in plants no 

 specialized digestive organs like the alimentary canal of animals, 

 digestion takes place in the cells of storage organs in any part 

 of the plant body where reserve foods exist. In the mesophyll 

 cells of leaves digestion is probably going on at all times, since 

 starch formed by photosynthesis is continually being converted 

 into sugar for immediate use by the leaf cells or for transport 

 along the veins and the phloem of the stem to the wood-ray cells 

 and other storage tissues of the stem. In germinating seeds 



