128 The Living Plant 



action of peptonizing enzymes : and there are others likewise of 

 rarer sort and lesser consequence. 



The mention of the presence of sulphur and phosphorus in 

 proteins will lead the reader to inquire for the source of supply 

 of those elements. The answer is ready. They are derived from 

 soluble sulphates and phosphates absorbed from the soil by the 

 roots, and are incorporated, through chemical reactions still 

 imperfectly known, with the elements contained in the amides. 

 All soils contain all of the sulphates that plants need, and usually 

 all of the phosphates, though at times the latter are insufficient, 

 and must be added as fertilizers to ensure good crops. 



Class VII. The Regulators of Metabolism, or Enzymes 



It is safe to say that the enzymes (called also ferments) are 

 the most remarkable and least known, although among the most 

 important, of all substances produced by plants, or by animals, 

 either. They are characterized by this remarkable power, viz., 

 they can cause chemical changes, each of one definite kind, in 

 other substances, without themselves entering into the reaction 

 or suffering any appreciable alteration. Because of this mode 

 of action very small quantities of enzymes can alter chemically 

 great quantities of material. Thus the enzyme diastase, which 

 occurs both in the saliva of man and also in the starch-storing 

 organs of plants, can convert (chemically, hydrolyze) great 

 quantities of the insoluble starch by two or three steps into grape 

 sugar, a soluble diffusable material; likewise the enzyme protease 

 (pepsin) occurring in both plants and animals, hydrolyzes in- 

 soluble indiffusable proteins into soluble diffusable peptones; 

 also the enzyme lipase converts insoluble fats into soluble fatty 

 acids and glycerine: cytase converts cellulose of Ivory palm and 

 Date into soluble sugars; and there are many others of lesser 

 prominence. It is these changes which constitute digestion, 

 whether in plants or in animals. By aid of the enzymes the plant 



