Movement of Materials in Stems 165 



act upon the insoluble forms of proteins and render them soluble. 

 Proteins are mostly inactive storage materials and bear much the 

 same relation to the simpler and more active nitrogen compounds 

 (that is, amides and amino acids, produced by protein digestion) 

 that starch bears to the simple sugars, or fats to the fatty acids. 



It seems probable that enzymes are concerned in the principal 

 activities of all Hving cells. Without them there could be none 

 of the rapid chemical changes in foods that are necessary for the 

 transfer of foods within the plant and for carrying on the other 

 processes. Both the building up of the complex food molecules 

 from simpler ones and the spHtting of these large molecules again 

 is brought about by enzymes that are within the cells. 



Properties of enzymes. It is interesting to know that if an 

 enzyme is put into a test tube with the appropriate food substance, 

 under favorable conditions it will bring about digestion the same 

 as if it were in the living cell. This proves that digestion is not 

 directly carried on by the Hving protoplasm, and that to be 

 digested, foods do not need to be in contact with living matter. 

 It requires but a very minute quantity of enzyme to digest a 

 large amount of the particular food upon which it acts ; for 

 example, a preparation of an enzyme extracted from the pancreas 

 of an animal was found to digest 2,000,000 times its weight of 

 starch. The amount of diastase in a mesophyll cell necessary to 

 transform the starch in that particular cell to sugar is so small 

 that it cannot be measured. Its presence is inferred from the 

 observed changes in the starch. 



In most forms of digestion water chemically is added to the 

 original substances. This seems to weaken the bonds between 

 the different parts of the molecules and to bring about a spHtting 

 into simpler compounds. For example : 



and 



