262 VEGETABLE PHYSIOLOGY 



decompose respectively carbohydrates, proteids, glucosides, 

 and fats or oils. In nearly every case the action of these 

 enzymes is one of hydration, the body acted upon being 

 generally made to take up water, and to undergo a subse- 

 quent decomposition. . 



Of those which act upon carbohydrates we have two 

 varieties of diastase, which convert starch into maltose, or 

 malt-sugar ; inulase, which forms another sugar, levulose 

 or fructose, from inulin ; invertase, which converts cane- 

 sugar into glucose and fructose ; glucase or maltase, which 

 produces grape-sugar from maltose ; and cytase, which 

 hydrolyses cellulose. Another enzyme, which does not 

 appear to be concerned with digestion so directly as the 

 others, is known as pectase ; it forms vegetable jelly from 

 pectic substances occurring in the cell-wall. 



The members of the second group act upon proteid 

 substances, and are technically known as proteolytic 

 enzymes. The principal members of this group are pepsin 

 and the various trypsins, which convert albumins and 

 globulins into peptones, the trypsins also decomposing 

 certain peptones into amido-acids. Allied to these is rennet, 

 which converts the caseinogen of milk into casein, the 

 characteristic proteid of cheese. It occurs in a great 

 many plants, but its function in vegetable metabolism 

 is unknown. 



The enzymes which act upon glucosides are many ; the 

 best known are emulsin and myrosin ; others of less frequent 

 occurrence are erythrozym, rhamnase, and gaultherase. 

 Those which decompose fats have not been so fully inves- 

 tigated : they are known as Upases, but whether there 

 are many different varieties or not has not at present been 

 ascertained. 



Diastase appears to exist in two varieties, distinguished 

 from each other by their mode of action on the starch 

 grain. One, called diastase of translocation, dissolves the 

 grain slowly from without inwards, without altering its 

 general appearance ; the other, diastase of secretion, dis- 



