400 



PHYSIOLOGY 



especially in leaves and storage organs. It partly digests starch into 

 maltose, a residue, representing about 20 per cent of the grain, resisting 

 its action for a long time. In the course of decomposition, various 

 dextrins are produced by successive cleavage, presently becoming simple 

 enough to be analyzed. The last member of the series breaks into mal- 

 tose and isomaltose, C 1L .H 2 20 n . There are at least two forms (possibly 

 more), secretion diastase and translocation diastase, differing in the 

 mode of dissolution of the starch grain. The former erodes the surface 

 irregularly, whence narrow canals penetrate the interior, and the grain 

 often falls into fragments; the latter corrodes the grain almost evenly, 

 reducing it gradually in size until it disappears. 



It is probable th.it what is here called diastase consists of at least two enzymes; 

 amylase, which digests starch to a dextrin, and dextrinase, which breaks the dextrin 

 into maltose; this, maltase (see below) cleaves into glucose. 



Inveriase, in like manner, can hasten the hydrolysis of cane sugar 

 into two hexose sugars, glucose and fructose. 



Trehalose and several other enzymes in fungi attack trehalose and other 

 sugars peculiar to them, and digest them into the hexoses of which they 

 were originally built. 



Maltase, an enzyme which is often associated with diastase, carries 

 the process of starch digestion further, cleaving each maltose molecule 

 into two molecules of glucose. 



Inulase likewise attacks inulin, breaking it up into levulins and finally 

 into fructose. Perhaps there is here also more than one enzyme at work. 



Cytase is responsible for digesting hemi-celluloses (chiefly mannans 

 and galactans) of seeds, while enzymes under the same name, but prob- 

 ably different, have been found in wood-destroying fungi, and have been 

 assumed present whenever a tissue is penetrated by a hypha, or by a 

 more massive member, as in the sinking of the foot of bryophytes into the 

 gametophyte (see Part I, p. 108) and in the emergence of the branches of 

 roots through the cortex (fig. 667; see also Part I, p. 250, and fig. 558). 



Fat enzymes. — Lipase, perhaps of several different forms and so 

 deserving distinctive names, has been found in organs where fats are 

 present, especially in seeds and many fungi. Lipase breaks up fats into 

 their components, fatty acids and glycerin, which are then readily dif- 

 fusible. 



Glucoside enzymes. — These are common, setting free glucose from many dif- 

 ferent compounds. Emulsin, for example, breaks amygdalin, a glucoside common 



