244 VEGETABLE PHYSIOLOGY 
apparatus, if it possesses one. In such a protoplast we 
may observe at times the storage of such a reserve 
material as starch, and its digestion at the appropriate 
period. 
Even in more complex plants it is certain that the 
living substance of every protoplast is in a constant state 
of change, initiating many decompositions in which its 
own substance takes part, as well as others into the course 
of which it does not itself enter. Among these decomposi- 
tions we must include the various intra-cellular digestive 
processes. 
- Though all protoplasm has this power, it is not usual 
in plants, any more than in animals, to find it exclusively 
relying on it. The work of digestion, at any rate, is 
generally carried out by peculiar substances which it forms 
or secretes for the purpose. We have in plants a large 
number of these secretions, which are known as enzymes or 
soluble ferments. 
The action of these enzymes is not at all completely 
understood. They appear not to enter into the composi- 
tion of the substances which are formed by their activity, 
and they seem to be capable of carrying out an almost in- 
definite amount of such work without being used up in the 
process. They are inactive at very low temperatures, but 
effect the decompositions they set up freely at the ordinary 
temperature of the plant. As the’ temperature at which 
they are working is raised, their activity increases up to a 
certain point, which varies slightly for each enzyme, and 
is called its optimum point. This usually ranges between 
30° and 45° C. If the temperature is raised above the 
optimum point, the enzyme becomes less and less active as 
it rises, and at about 60°-70° C. it is destroyed. The exact 
point, however, varies a good deal in the cases of different 
enzymes. 
Enzymes work most advantageously in darkness or in 
a very subdued light; if they are exposed to bright sun- 
shine they are gradually decomposed the violet and ultra- 
