ORGANIC SUBSTANCES AS PLANT FOOD 



207 



The germinating seed does not have at its disposal those agents, 

 such as strong acids and high temperatures, by which the hydrolysis 

 of polysaccharides is brought about in a laboratory. It does, how- 

 ever, have substances capable of bringing about this hydrolytic 

 splitting even at ordinary temperature. These substances are 

 known as enzymes. One of the first substances of this sort to be 

 discovered in plants was the enzyme which brings about the decom- 

 position of starch, known as "amylase," or "diastase." 



nip* 



Fig. 90. — Successive stages in the digestion of a starch grain by diastase (after 



Benecke-Jost) . 



Diastase can be easily obtained from germinating grain, or 

 malt. The finely ground dry malt is soaked in water, the resulting 

 decoction is filtered, and the filtrate then precipitated with alcohol. 

 The white, flaky precipitate is then filtered off and again dissolved 

 in water. When added to a starch paste, this solution will hydro- 

 lyze the paste rapidly. After a short time, the blue iodine coloring 

 characteristic of starch, begins to change : first to violet, then to red, 

 then to a yellowish color, and finally the solution ceases to give the 

 iodine reaction, but, with Fehling's solution, will show a distinct 

 test for sugar. A detailed chemical analysis will show that instead 

 of starch, maltose is present. 



The action of diastase, accordingly, differs somewhat from the 

 behavior of inorganic catalyzers, such as strong acids, which 

 attack the molecule, or, more exactly, the colloidal aggregate of 

 starch and rapidly break it up into the end product of hydrolysis, 

 glucose. Whereas diastase acts more slowly, as is indicated by 

 the gradual disappearance of the color reaction with iodine^ 



