SECT, n PHYSIOLOGY 235 



A. Mobilisation of Reserve Materials 



In the mobilisation of reserve materials we have usually a not very 

 profound change of the nature of a hydrolysis, i.e. a splitting of the 

 substance into smaller molecules with the absorption of Avater. 

 This must be separately considered for the three main types of 

 reserve material the carbohydrates, the fats, and the albuminous 

 substances. 



1. Hydrolysis of Carbohydrates 



Stareh is one of the most important reserve materials in 

 plants. It not infrequently forms the main part of the reserve 

 substance in seeds as well as in tubers and bulbs. In the potato' 

 tuber 25 per cent and in the grain of wheat 75 per cent of the 

 fresh weight consists of starch. It is also present in considerable 

 amounts in the pith, the xylcm parenchyma, the medullary rays and 

 the rind of trees. The starch has to be broken down in order to 

 allow of its translocation. This is effected technically by treatment 

 with acids ; the grape sugar of commerce is obtained by treating 

 potato-starch with sulphuric acid. The molecule of starch is split up 

 into mimerous molecules of dextrose accordina; to the formula 



'& 



In the plant this hydrolysis is effected not by means of acids but by 

 an enzyme called diastase. The enzymes (^^) are the catalysators of 

 the organism ; the name catalysators is given to substances which 

 induence the rapidity of a chemical reaction. We are mainly 

 concerned with the acceleration of reactions. While many inorganic 

 catalysators influence very various chemical processes, the influence of 

 organic catalysators is quite specific ; thus diastase only acts on starch. 

 Since the catalysator either does not enter into the reaction or at least 

 does not do so permanently, a small amount of it is able to hydrolyse a 

 large cjuantity of the substance acted on, if the products of the 

 reaction are continually removed. 



Diastase is found in many parts of the plant, especially in those 

 which contain much starch, such as foliage leaves and germinating 

 seeds. The amount of diastase in an organ is not constant, but is 

 regulated according to the needs of the plant ; farther its action 

 can be arrested by the formation of other enzymes (anti-enzymes). 

 Diastase can be extracted from the organs by water or glycerine, 

 precipitated by means of alcohol from the extract and again dissolved, 

 without any essential change in its properties. On the other hand 

 diastase is very susceptible to high temperatures, and is rendered 

 permanently inactive by heiting to about 75° C. All diastatically 

 active extracts of the plant give a proteid reaction ; it is, however, 



