ts of these
tissues upon soluble proteins. The best-known example of this type of
enzymes is the protease of yeast; but similar ones may be found in
germinating seeds. These vegetable proteases are usually most active in
neutral or only faintly alkaline solutions, and their activity is nearly
always inhibited by even traces of free acids.

Most laboratory studies of proteolytic enzymes are carried on with
preparations of the powerful members of this class of enzymes which are
found in the digestive tract of animals, namely, the pepsin of the gastric
juice, which acts in the acid medium, in the stomach, and the trypsin of
the pancreatic juice, which acts in the alkaline medium of the intestinal
tract. But even these powerful proteases require several hours for the
transformation of an amount of soluble albumin into its amino-acid
constituents which is equivalent to the amount of starch which is
hydrolyzed to maltose by diastase in a very few minutes.

Enzymes which govern oxidative changes, known respectively, as _catalases_
and _oxidases_, are almost universally present in plants. Catalase
decomposes peroxides, with the liberation of free oxygen. It is, therefore,
necessary to the final step in the process of photosynthesis, as elucidated
by Usher and Priestley (see page 26), and serves to prevent the destructive
action of hydrogen peroxide upon chlorophyll. The almost universal presence
of oxidases in plant tissues has been repeatedly demonstrated. They are
present in especially large amounts in tissues which are being acted upon
by parasitic fungi or are combating unfavorable conditions of growth. The
oxidases, in such cases, seem to be the agents by which the plant is able
to stimulate its metabolic activities to overcome the unfavorable
environment for its normal development.

In vegetables and fruits, the common browning, or blackening, of the
tissues when cut surfaces are exposed to the air has been demonstrated to
be due to the catalytic oxidation of the tannins or of certain amino-acids,
especially tyrosine, under the influence of the oxidases which are present
in the tissues. In fact, most pigmentation phenomena are due to changes in
the oxygen content of the chromogens of the cells of the plant, under the
influence of the oxidases which are present in the protoplasm of the cells
in question. Hence, the oxidases may be said to be the controlling agencies
for both the energy-absorbing activities and for respira