264 THE MAINTENANCE OF THE INDIVIDUAL 



Protein digestion is l^rought about by a different group of enzymes, 

 called proteases. These enzymes are found in abundance in leaves and 

 germinating seeds of plants and to a lesser extent in practically all 

 plant tissues. In the living plant, the digestive enzymes carry on a 

 necessary and important work. If plants make foods in the green 

 leaf, and they do, and if they store foods in the root, stem, fruit, and 

 seed, then there must be some way to transfer the foods made in the 

 leaf in a soluble form to those parts of the organisms where the food 

 is finally used. This work of changing insoluble foods to soluble foods 

 is obviously performed by enzymes. A still more interesting phenom- 

 enon sometimes takes place. Many of these enzymes under certain 

 conditions are capable of reversing their actions, that is, of converting 

 a soluble substance like sugar into an insoluble one such as starch, 

 or of changing proteins to soluble forms so that they can be transported 

 through the vascular system of the plant and stored in insoluble form 

 in seeds, nuts, and roots. 



The changes from sugar to starch may take place in leaves wherever 

 certain plastids known as amyloplasts exist. These bodies have 

 the power to form starch in the presence of a series of enzymes which 

 first bring about the transformation of simple sugars to more complex 

 sugars, and then to an intermediate substance between sugars and 

 starches, called dextrin. Dextrin is changed into soluble starch by the 

 enzyme, amijlase, and finally the soluble starch is converted into 

 insoluble starch by the enzyme, coagulase. Thus we see that the 

 work of enzymes is absolutely essential to the life of the plant. Al- 

 though plants and animals obtain their foods in different ways, they 

 probably assimilate it in much the same manner, for foods serve 

 exactly the same purposes in plants and in animals, namely, they are 

 oxidized to release energy and they build up living matter. 



How Food Is Used in the Plant Body 



Although, basically, the uses of food are production of energy and 

 making of protoplasm, certain substances are produced by plants 

 which are not found in animals. For example, the plant cell is charac- 

 terized by its cellulose wall which in old cells is strengthened by the 

 addition of a complex substance, known as lignin. This forms the 

 useful substance we call wood. In addition, other products charac- 

 teristic of plant activity should be mentioned : the fatty substances, 

 known as cutin and suherin, as well as waxes which give the "bloom" 

 to certain fruits ; the essential oils in resins, such as lemon, pepper- 



