258 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- 



