THE OPPORTUNITY OF THE AGRICULTURIST 695 



dextro- series ; at one time it was thought that perhaps these 

 alone were preserved and that the plant for its own purposes 

 made use of and destroyed those of the other series ; but of 

 late years this conception has given place to the idea that only 

 members of the one series are formed : in other words, that the 

 course of change is so directed that only df-glucose and com- 

 pounds derived therefrom are built up. The nature of the 

 directive force is even surmised. 



In plants, as well as in animals, the digestion of the more 

 complex compounds such as starch, the biose sugars and the 

 albuminoids — that is to say, their simplification and assimila- 

 tion — takes place under the influence of enzymes. These 

 enzymes are kittle folk to deal with ; at present, we are quite 

 unaware of their precise nature, as they cannot be separated 

 in a state of purity and are very easily destroyed. One of the 

 most typical is the diastase of malt, which is present in all cereal 

 grains and in green leaves : this enzyme acts on starch alone, 

 converting it into soluble sugar (maltose). Invertase, which is 

 present in most yeasts and in growing leaves, has the specific 

 property of converting cane sugar into dextrose and levulose 

 or fructose. In all cases these enzymes act by promoting the 

 hydrolysis of the compound affected ; they, as it were, enforce 

 the absorption of the elements of water and so determine the 

 resolution of the compound into simpler substances. Their 

 activity is altogether wonderful, a minute quantity sufficing 

 gradually to transform a relatively large amount of the hydro- 

 lyte — i.e. the compound hydrolysed. Although the effect they 

 produce is similar to that produced by acids, they are dis- 

 tinguished from these by the fact that, whereas the acids act 

 indiscriminately and only with varying degrees of readiness, 

 the enzymes act selectively — a given enzyme acting only on a 

 particular compound or related series of compounds. The only 

 rational explanation yet given of this specialised behaviour is 

 that the enzyme and hydrolyte are compatible substances, that 

 one is in some way built to fit the other ; the closest approach 

 to a picture of the manner in which an enzyme acts is afforded 

 by a Geometer caterpillar clasping a twig with its many legs 

 while it bites away the adjacent leaf-edge with its mouth : we 

 may think of an enzyme as clinging to its compatible hydrolyte 

 and presenting water at the appropriate centre at which the 

 resolution of the complex molecule is to be effected. 



