112 BOTANY OF THE LIVING PLANT 



paste, made by heating starch in water, it soon ceases to give the blue 

 reaction with Iodine ; the colour given with Iodine changes to reel, and 

 linally to yellow. This indicates a change from starch to sugar 

 (Dextrose). If starch-giains which have been digested in a solution 

 of diastase be examined microscopically they are seen to have become 

 corroded (Fig. 8 1, 1-4), while the solution gives the reactions of sugar. 

 It is thus seen that under the influence of diastase starch is converted 

 into sugar. What happens is hydrolysis, that is, the splitting up of 

 the substance into smaller molecules, with the absorption of water. 

 Into this process the diastase probably does not itself enter. It acts 

 as a catalyser. It is found that a very small quantity of the enzyme is 

 able to convert a large quantity of starch into sugar, and that the action 

 is more brisk if the product (sugar) be removed as it is formed. This 

 would be the case in the plant-body, where the sugar would be constantly 

 used up. 



Such enzyme-action is common in plants, as it is also in animals. 

 Special enzymes have been extracted, and recognised as carrying out 

 various changes, such as the conversion of cellulose into sugar (Cytase) ; 

 cane-sugar into dextrose and levulose (Invertase) ; inulin into sugar 

 (Inulase) ; or oils into carbohydrate (Lipase). In fact, the action of 

 diastase on starch may be taken as exemplifying many of the changes 

 intimately related to the physiology of the Plant A circumstance 

 which shows that such changes are not only existent but repeated 

 is that so-called " transitory starch " habitually marks the course 

 along which carbohydrate is being conveyed. Numerous small 

 starch-grains are formed in the cells, and there can be little doubt 

 that the conversion of sugar into starch, and again of starch into sugar, 

 may be repeated over and over again. Thus it is seen that in the meta- 

 bolism of the plant chemical change is very active, and that the changes 

 may be reversed. But in order to effect such changes as these, energy 

 must be available. The next question will be Irom what source does 

 such energy come ? 



RESPIRATION. 



The readiest source of energy available for the ordinary purposes 

 of man is by the combustion of fuel, such as coal or wood : that is, 

 its oxidation, the ultimate products being carbon-dioxide and water. 

 The latent energy of chemical separation of the fuel is converted on 

 combustion into kinetic energy. This supplies the motor impulse 

 for engines of various kinds. In a somewhat similar way the living 

 organism, whether plant or animal, may transform energy of chemical 



