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 réle 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 living 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 epithelial 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 soluble 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 
