270 PLANT PHYSIOLOGY 



extracellular. The processes are essentially the same but, owing 

 to the source of the food supply, differ in this particular. 



In animals, furthermore, this difference in the method of pro- 

 curing food is accompanied by the presence of a special digestive 

 organ, the alimentary tract, into which digestive enzymes are 

 poured upon the food brought in from outside and in which 

 the foods are made soluble so that they may enter the body of 

 the organism. Plants, on the contrary, have no special digestive 

 organs; but the regions where the food is made and where it is 

 stored must perforce be places where digestion is especially active. 

 In discussing this point, Barnes made the following illuminating 

 statement : 



Misleading comparisons of the leaves to the stomach not rarely occur 

 in primary books, which thus seek to "explain" the work of a leaf. When, 

 as in one notable instance, a leaf is compared to a kitchen, where the dilute 

 "soups" coming up from the roots are "boiled down"; later to a stomach, 

 where the food is made ready; and finally to the lungs, by which the dear 

 little plant breathes, the child would have a truly appalling notion of a 

 leaf were he not usually immune to such bad pedagogy, by reason of his 

 ignorance of at least the stomach and lungs. 



As has been mentioned above, the enzymes which bring about 

 digestion are those which add water and which, for this reason, 

 are called the hydrolyzing enzymes. Since they are classified on 

 the basis of the material worked upon, we shall here consider them 

 as carbohydrases, glucosidases, esterases, and proteases. 



Carbohydrases. — It has been found by Maquenne (1905) and 

 others that starch as stored in plants is not a single substance 

 but is a mixture of amylose and amylopectin. Amylose is soluble 

 in boiling water, turns blue with iodine, and makes up 80-85% 

 of the starch. The amylopectin, which makes up the remaining 

 15-20%, is not colored blue by iodine and does not dissolve in 

 boiling water. In starch paste, the soluble part comes from the 

 amylose, while the gummy, gelatinous material is the amylopectin. 

 In digestion these are both broken down to dextrin by amylase 

 and arnylopectinase respectively. 



The dextrins are intermediate products between starch and 

 sugar. They form gummy solutions and are used in the glue 

 found on postage stamps. The outer crust of bread is rich in 

 dextrins, which have been formed from the starch by the intense 

 heat of baking. These dextrins are formed in three stages, which 



