268 Plant Physiology 



tent, and much doubt is entertained respecting their pre- 

 cise nature. In general, however, they are regarded as 

 protein. They show noteworthy differences among 

 themselves, with regard to solubility, conditions of precipi- 

 tation, and the like. 



The enzymes are products of protoplasmic activity 

 and are not generally regarded as readily diffusible ; that 

 is to say, the work of many of these is primarily within the 

 cell where they may be produced. These are the intra- 

 cellular enzymes. Nevertheless, in a number of cases 

 there may be specialized secretory cells of some impor- 

 tance; and in other cases the digestion, or partial diges- 

 tion, of products prior to absorption, is an indispensable 

 character, as in the case of fungi and higher animals gen- 

 erally. Those enzymes which are active in part without, 

 or beyond, the limits of cells producing them are termed 

 extracellular enzymes. 



The hydrolysis or decomposition of organic bodies like 

 protein, starch, and fat under laboratory conditions (other 

 than by the use of enzymes) is effected only by means of 

 fairly' strong acids, high temperatures, and other intensive 

 agents. Contrariwise, the enzymes effect hydration and 

 decomposition under the conditions of the plant cell or 

 body, although their activity frequently reaches a maxi- 

 mum at 40 C. or slightly above. 



A great majority of the commoner enzymes act by 

 hydration; thus the effect of invertase upon cane-sugar 

 is as follows : 



C^H^On + H^O = CeH^Oe + CeH^Oe. 

 Sucrose Dextrose Levulose 



On the other hand, certain classes of enzymes, not here 



