94 GERMINATION OF SEEDS: SEEDLINGS 



chemical changes. Consequently, the kinds of enzymes are 

 almost as numerous in the plant as the kinds of substances to be 

 acted upon. Thus for changing starch into sugar there is the 

 enzyme known as diastase which is especially active in seeds, but 

 common in other plant organs and in animal saliva. An enzyme 

 secreted by the Yeast Plant and called zymase acts on sugar, 

 forming besides alcohol, carbon dioxide which puffs up the dough 

 when Yeast is used in bread-making. This enzyme also occurs 

 in seeds, fruits, and other plant organs. Lipase converts fats into 

 soluble fatty acids, and pepsin changes insoluble proteins into 

 peptones and other soluble forms. Then there are oxidases, en- 

 zymes which oxidize substances as the name suggests, and perox- 

 idases which take oxygen away from compounds, and many other 

 enzymes which play an important role in the chemical activities 

 of the plant. The exact chemical nature of enzymes has never 

 been determined because of the difficulty in separating them from 

 other protoplasmic substances which enter into and thus compli- 

 cate the analysis. Nevertheless, there is much evidence that 

 enzymes are protein-like substances. One striking feature of an 

 enzyme is that it does not enter into the chemical action which it 

 causes, and, therefore, a small quantity of an enzyme can keep 

 a chemical action going until a large quantity of a substance is 

 changed. 



Although all hving cells, whether in the embryo or elsewhere, 

 produce enzymes, sometimes, however, certain cells have the 

 secretion of enzymes as their special function, as in Corn, Wheat, 

 and other seeds of the Grass type, where the epithehal layer of the 

 scutellum has for its special function the secretion of the diastase 

 and other enzymes which are necessary for converting the endo- 

 sperm into soluble forms. 



Transportation of Soluble Foods. — After the foods are made 

 into solul)le forms and dissolved in the water present, they pass 

 from one region of the plant to another by the physical processes 

 known as diffusion and osmosis. Diffusion is probably better 

 known among gases where the spread of odors through a house, 

 the fragrance of flowers through gardens, and smoke through the 

 air are everyday illustrations of it. The spread of indigo, ink, 

 or any substance like salt and sugar through the water in 

 which they are dissolving illustrates it. By diffusion substances, 

 whether dissolved in a gas or a liquid, spread farther and farther 



