264 BOTANY PART I 



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 



Starch 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 arid 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 xylem 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 numerous molecules of dextrose according to the formula 



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

 a special organic substance called diastase. 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 suscep- 

 tible to high temperatures, and is rendered permanently inactive by 

 heating to about 75 C. It has not yet proved possible to obtain 

 chemically pure diastase; it is always mixed with proteids and was 

 therefore for long regarded as of this nature. .Remarkable views which 

 have more recently been formed as to its chemical nature and its 

 formation still require confirmation ( 40a ). 



Diastase has the same effect 'on starch as sulphuric acid has ; they 

 both act as catalysators. The name catalysators is given to sub- 

 stances which influence the rapidity of a chemical reaction. We are 

 mainly concerned with the acceleration of reactions. The usual 

 method in the chemical laboratory of accelerating a reaction is the 

 application of heat ; the fact that the life of the organism is confined 

 to a narrow range of temperature limits this method. A second 

 method is by the use of inorganic catalysators. Many of these, such 

 as sulphuric acid mentioned above, injure the protoplasm ; it is thus 

 easy to understand why the organism should form special catalysators 

 that are not injurious. These are termed ENZYMES ( 41 ) and occur in 

 both plants and animals. 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 cataly- 

 sator 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 



