CHAPTER XVIII. 



ENZYMES AND THE DIGESTION OF 

 RESERVE-MATERIALS. 



I. The substances stored in seeds, tubers, roots and other organs 

 of plants are chiefly solid, insoluble materials, such as starch and 

 aleuron-grains, which cannot be moved out of the closed cells in 

 which they occur, or are compounds such as oils and fats which, 

 although liquid, are unsuitable for rapid osmotic diffusion. 



Before these reserve-materials can be removed from the tissues 

 in which they are stored to the centres of growth where they are 

 needed, they must be digested or transformed into soluble, 

 easily difTusible substances, which can travel in the ordinary 

 channels available for the translocation of foods. In certain 

 cases the necessary transformation appears to be due to the 

 direct action of the living protoplasm, but in many instances it 

 is accomplished by the chemical activity of substances termed 

 enzymes or unorganised ferments, which are secreted by the 

 protoplasm. 



A considerable number of distinct enzymes are known. They 

 all appear to belong to the protein class of organic compounds, 

 and a very small amount of each is able to transform an almost 

 unlimited bulk of the material upon which it acts without 

 changing or suffering much diminution in the process. Enzymes 

 are inactive at low temperature, and most of them are totally 

 destroyed when their solutions are heated to about 70° C. : the 

 optimum temperature at which they carry on their work best lies 

 beween 30" and 50° C. Their chemical activity is usually greatest 



