250 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 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 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 

 80 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 



