250 VEGETABLE PHYSIOLOGY 
apparatus, if it possesses one. In such a protoplast we 
may observe at times the storage of such a reserve 
material ag 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 decompositions 
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 onit. 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- 
violet rays being apparently most powerful in effecting 
