454 MANUAL OF BOTANY 



We have, then, to inquire how these processes of digestion, 

 whether internal or external, are brought about. 



The protoplasm, among its many properties, no doubt has 

 the power of setting up these decompositions, and probably in 

 many of the lowest forms the work is altogether effected by its 

 instrumentality. Indeed, in every cell of a complex plant the 

 living substance is in a constapt state of change, initiating many 

 decompositions in which its own substance takes part, and many 

 others into which it does not enter itself. 



Though all protoplasm has this power, it is not usual in 

 plants, any more than in animals, to find it exclusively relying 

 on it. Usually the work is done by peculiar bodies which it 

 forms or secretes for the purpose. We have in plants a large 

 number of these secretions, which are known as enzymes or 

 unorganised ferments. 



The action of these bodies is not at all completely under- 

 stood. They appear to take no part in the composition of the 

 bodies which are formed by their activity, and they seem 

 capable of carrying out an ahnost indefinite amount of such 

 work, without being used up in the process. They are inactive 

 at a very low temperature, but effect their decompositions freely 

 at the ordinary temperature of the plant. As the temperature 

 rises their activity increases up to a certain point, which varies 

 slightly for each enzyme, and is called its optimum point. It 

 usually ranges between 30° and 45°C. If the temperature be 

 raised above that point, the enzymes become less and less active 

 as it rises, and at about 70°C. they are destroyed. They work 

 best in darkness or very subdued light ; if exposed to bright 

 sunshine they are gradually decomposed. They are often in- 

 juriously affected by neutral salts, alkalies, or acids. 



The enzymes are manufactured by the protoplasm of the 

 various cells in which they occur, being produced from its own 

 substance, in a manner somewhat similar to that of the forma- 

 tion of the cell-wall. Usually their presence is manifested by 

 a marked granularity of the protoplasm, due to the formation in 

 it of an antecedent substance, known as a zymogen, which is 

 readily converted into the enzyme. 



We find various degrees of completeness of differentiation of 

 the cells which produce these enzymes. In the simplest oases, 

 such as the mesophyU of the leaves of most plants, or the seeds 

 of the Leguminosse and other Natural Orders, or the tubers of 

 the potato, the enzyme is found in all the cells which contain 

 the reserve materials, so that an active transformation of the 



