120 PLANT PHYSIOLOGY 



The student should now make a classification of the sub- 

 stances found in plants, so tabulating them as to show (a) the 

 principal classes or groups, with their names and typical formula; 

 (b) the chemical relations of the groups to one another in order 

 of increasing complexity; (c) places of their occurrence; (d) their 

 meaning in the economy of the plant, whether working materials, 

 skeleton substance, reserve foods, special secretions of ecological 

 significance, or by-products. 



Of the substances in the above classification, some are soluble 

 and some are insoluble in water, the only transporting medium 

 of the plant; yet some of the latter, e.g., starch, are known to 

 be transported, and hence must be converted for the purpose 

 into a soluble form. The precise physics of the translocation 

 will be considered later (under Transport), but it is necessary 

 here to consider how the solution is effected. As a type we may 

 take the solution or hydrolysis (digestion) of starch, well known 

 to be effected by diastase, which presents a study as follows: 



What phenomena are exhibited in the action of diastase upon 

 starch? 



Experiment. Prepare a diastase solution by germinating some 

 20 g. of barley until the sprouts are one-third the length of the grain; 

 then grind it up finely in a mortar (to break the cells), add 100 cc. of 

 water, let it stand, stirring at intervals, for one or two hours, then filter. 

 Prepare a starch paste by adding 1 g. potato starch to 100 cc. water 

 and heating to boiling-point. Mix together 5 cc. of the barley solu- 

 tion and 25 cc. of the starch paste. When mixed place a few cc. in 

 a test-tube, add a few drops of iodine, and note the color; repeat this 

 test at intervals of fifteen minutes, until there is no change of color. 

 Keep in a warm place, the warmer the better, so long as not in excess 

 of 60° 



The test may also be made with diastase (obtained from a chem- 

 ical supply company) dissolved in water, or by using dry malt in place 

 of the germinated barley. 



Diastase is an example of the very important and interesting 

 substances, the enzymes, and upon their chemical nature, known 

 or supposed mode of action, various classes and effects, the 

 student must now inform himself through the literature, noting 

 especially Green's monograph. The hydrolysis of insoluble 

 substances by ferments or enzymes inevitably raises the ques- 



