DIGESTION OF STORED FOOD 19I 



their work and shrubs and trees stand bare and apparently 

 inactive it might be conjectured that their store of food would 

 wait unaltered for the return of spring; Ijut this is by no means 

 the case, for part of the food is rendered soluble and appar- 

 ently is used in respiration throughout the dormant period, 

 and the greater part may be changed from insoluble to soluble 

 and back again as the outside temperature falls and rises. The 

 maximum amount of starch is found in the fall, for a large per- 

 centage of starch in the bark is changed to sugar or oil during 

 the winter, and in softwood trees and shrubs the same thing 

 happens in the wood. In hardwood trees the change is not so 

 great in the wood. A rise of temperature during the winter or 

 early spring incites a change back to starch again. 



Digestion of Stored Food. — The forms in which foods 

 are stored are suited as a rule to their safekeeping, but not to 

 their distribution and use. Most foods are stored in an insol- 

 uble form such as starch, oil, and the majority of proteids, 

 and only a few in their storage form are capable both of dif- 

 fusion and assimilation, as glucose and saccharose. The chemi- 

 cal processes by which stored foods are made soluble, diflFusible. 

 and assimilable are called digestion. 



In carrying on digestion the protoplast usually employs to 

 do this work a proteid body known as an enzyme or ferment 

 which it has made apparently by a process of self-decomposi- 

 tion that we call secretion; and of these enzymes the proto- 

 plasts may possibly make as many kinds as there are varieties 

 of food to be digested; and it is also possible that the proto- 

 plasts sometimes incite digestion without the intervention of 

 an enzyme. 



Free oxygen is necessary to the formation of enzymes, and 

 these work best at warm temperatures ranging from 20° C. 

 to 60° C. according to the variety; they are also more effective 

 in the dark than in the light, since light, particularly of the 

 violet end of the spectrum, tends to destroy them. 



It is not known just how the enzymes act in digestion. They 

 incite the necessary chemical changes, but hold themselves 



