PHYSIOLOGY 223 



is due to the interruption and detention of a flow of nourishing sap 

 through the phloem towards the roots. 



The transfer of the carbohydrates through unbroken cell walls to 

 the various points of consumption can- only be accomplished when they 

 are in solution. In case they are not already dissolved in the cell 

 sap, they must first be converted into soluble substances capable of 

 osmosis. This is of the highest importance for the transfer and utilisa- 

 tion of starch and reserve cellulose (hemicellulose). The former is 

 converted by the influence of DIASTASE into glucose or maltose. 



Diastase belongs to those peculiarly acting substances termed 

 KKUMKNTS or ENZYMES, which possess the remarkable power of 

 decomposing or transforming certain organic compounds without 

 themselves becoming changed or consumed in the process. By virtue 

 of this property they are enabled to transform unlimited quantities of 

 certain substances if the resulting product is continuously removed. 

 According to their most important physiological properties, diastatic, 

 peptonising or proteolytic, and inverting enzymes are recognised, which 

 act on starch, albuminous substances, and sugars respectively. These 

 groups do not, however, exhaust the various ways in which these 

 substances, which play such an important part in the chemical changes 

 taking place in the organism, act. Thus there are trypsin-like 

 ferments which dissolve albumen in an alkaline medium, ferments 

 which dissolve membranes of cellulose, wood, or chitin, those that 

 decompose glucosides and oils, and others that convert urea into 

 ammonium carbonate (ureases). These and other enzymes have been 

 found in plants, and there is no doubt that such bodies play a part in 

 many imperfectly understood processes, especially in Fungi. A point 

 which the enzymes named above have in common is their hydrolytic 

 mode of action. They introduce the elements of water into the 

 substance acted on, starch (C 6 H 10 5 ), for example, being converted 

 into soluble glucose (C H 19 O e ). 



BuCHNER discovered that the expressed sap of the yeast-plant, 

 even after being filtered, is able to transform grape-sugar into alcohol 

 and carbonic acid. He ascribed this property to a special enzyme 

 termed ZYMASE (alcoholase). The name of OXYDASES is given to 

 certain substances which are regarded as acting by introducing oxygen. 



Enzymes are for the most part colloidal albuminous substances which are 

 formed from the protoplasm, and exhibit a so-called catalytic mode of action. 

 They are easily rendered inactive by poisons or by too high a temperature. 

 Inorganic substances (catalysators such as finely divided indium or platinum) 

 exhibit a similar catalytic action to enzymes ; the power the latter possess of exciting 

 fermentation is thus not due to any special vital property. They can act when 

 removed from the organism, and may be precipitated from solution and again dis- 

 solved without loss of power. Besides ferments which hasten chemical reactions, 

 others which arrest the reaction are known (paralysators), others oppose and inhibit 

 the action of catalytic ferments (anti-ferments) ( 44 ). 



