124 BOTANY: PRINCIPLES AND PROBLEMS 



Maltose is also converted into glucose by maltase. Far more 

 important than these changes, however, is the digestion of starch 

 through the action of diastase and other enzymes, with the ulti- 

 mate production of glucose, thus: 



CeHioOo + H2O = C6HX2O6 



The reserve celluloses are broken down by hydrolysis into various 

 sugars through the action of another enzyme, cellulase. Fats, by 

 the agency of lipase and similar enzymes, are broken into gly- 

 cerine and fatty acids, which may be absorbed into protoplasm. 

 Protein-digesting enzymes are necessarily numerous, but two 

 of them, pepsin and trypsin, are especially important. The 

 former converts proteins into water-soluble peptones and pro- 

 teoses; the latter carries the process still further with the ultimate 

 production of amino-acids. It should be noted that all these 

 types of digestion are carried on within the protoplasm of living 

 cells throughout the plant body wherever digestion is necessary, 

 and not in the cavities of special digestive organs. 



Assimilation. — After a food has been digested, it must then 

 enter the protoplasm of a cell and become an integral part of the 

 living substance. About this process, which is known as assimila- 

 tion and which is really the central problem of metabolism, we 

 know very httle. From the activities of protoplasm it is clear 

 that this remarkable substance must be very complex, both 

 chemically and physically. It is highly unstable and is continu- 

 ally undergoing processes of construction and destruction. 

 Although we can trace with some confidence the entrance into 

 protoplasm of certain comparatively simple substances and the 

 departure therefrom of others equally simple, we must plead 

 almost complete ignorance as to the happenings which take 

 place between these two events. It is here that dead matter 

 becomes alive, that inert food substances become endowed with 

 those unique properties of protoplasm which in the aggregate 

 we call life. This change never occurs spontaneously in nature, 

 but is always brought about through the activity of living 

 substance already existing. As far as experience tells us, life 

 always comes from life and in no other way. Although this 

 process is going on continually in every living plant and animal, 

 we have as yet been quite unable to master its intricacies and to 

 imitate it in the laboratory. 



