100 PLANT PHYSIOLOGY AND ECOLOGY 



plants. Every living cell of the plant possesses the power of 

 digestion, but this is often localized in particular areas. It is 

 inherent in chloroplasts and leucoplasts, and is present to a marked 

 degree in the scutellum and aleurone layer of grains, in the outer 

 cells of embryos surrounded by endosperm, in the digestive cells 

 of insectivorous plants, etc. 



Enzymes possess the characteristic property of acting upon 

 and changing an amount of stored food greatly in excess of their 

 own bulk. The way in which they are formed by the protoplasm 

 is unknown, but the production of a granular substance termed 

 zymogen seems to be an essential step in the process. As a rule, 

 each enzyme is able to digest but a single reserve material, or 

 at most it is able to act upon only a few related materials. No 

 enzyme known at present can act upon a carbohydrate and also 

 upon oils or proteids. The enzymes which digest starch and 

 related substances are termed diastases; that which attacks cellu- 

 lose is called cytase. Invertase changes sucrose or cane-sugar into 

 a simpler sugar. Lipase decomposes fats and oils, while pepsin 

 and trypsin are the chief substances concerned in the transforma- 

 tion of proteids. The chemical changes that take place in diges- 

 tion are highly complex and still largely obscure. Solution is a 

 characteristic step in the case of all solid food materials. Starch 

 and cellulose are converted into sugars, which in their turn may 

 be further changed before use. Starch is dissolved by the action 

 of two enzymes, termed diastase of translocation, and diastase 

 of secretion. The former occurs throughout the plant as well 

 as in the germinating seed. The latter is largely confined to the 

 seed, appearing in it only after germination has begun. Diastase 

 of translocation attacks the starch grain uniformly, while diastase 

 of secretion corrodes it in such fashion as to give it an irregular 

 outline. 



122. Chemosynthesis of digested materials. Food material 

 may be used in different ways by the plant, or even by the same 

 cell. Sugar, for example, may be changed into a proteid and used 

 to make additional protoplasm; it may be used to free energy 

 by means of respiration, or to form cell wall, or it may be con- 

 verted into various carbohydrates. While almost any soluble 

 carbohydrate may- be used directly to release the energy stored 

 in it, it can be assimilated by the protoplasm, i.e., used in the 

 construction of new living material, only through the chemical 



