45] CHAP. II. PHYSIOLOGY OF NUTRITIVE FUNCTIONS. 199 



and further cause the decomposition of peptone into amides, such as 

 asparagin, leucin, and tyrosin. 



The chief importance of the enzymes in tb.e economy of the 

 plant is that by their means the reserve materials, which are 

 accumulated to such a large extent in the form of substances, 

 such as starch, fat, cellulose, proteids of aleuron-grains, which 

 are either not soluble in water, or if soluble are only slightly 

 diffusible, are converted into substances, such as amides and cer- 

 tain sugars, which are both readily soluble and diffusible, and 

 which can therefore travel osmotically from one part to another. 

 For instance, as mentioned above, the excess of carbohydrate 

 formed in the leaves when they are actively assimilating, is com- 

 monly stored up in the form of starch. This carbohydrate is 

 eventually conveyed to other parts of the plant ; but, since starch 

 is insoluble, it cannot be conveyed in that form ; it is, in fact, con- 

 verted into maltose by an amylolytic enzyme present in the leaves, 

 and it is in this form that non-nitrogenous organic substance is 

 conveyed away from the leaf where it has been produced. Other 

 striking illustrations of the importance of enzyme-action are to be 

 found in the chemical changes going on in germinating seeds, 

 bulbs, tubers, etc. When a starchy seed, or a starchy tuber like 

 the potato, germinates, the starch-grains are gradually dissolved, 

 the starch being converted into maltose. When the tuber of the 

 Dahlia or Artichoke, which contains inulin as the non-nitrogenous 

 reserve material, germinates, the inulin disappears and is 

 gradually replaced by grape-sugar. When an oily seed germinates, 

 the oil-drops become less and less apparent, as the oil is gradually 

 decomposed by enzyme-action into glycerin and fatty acids ; the 

 next step is the formation of carbohydrate (sugar or starch), pro- 

 bably from the products of the decomposition of the oil, a process 

 which involves the absorption and fixation of oxygen, since 

 carbohydrates contain a higher percentage of oxygen than does 

 any form of fat or oil ; and then, finally, any starch so formed is 

 converted into sugar. Similarly, the aleuron-grains in a germin- 

 ating seed gradually disappear, the indiffusible proteids composing 

 them being decomposed by the action of a proteolytic enzyme into 

 peptone, and then into amides, in which form they are conveyed 

 osmotically to the growing embryo. Finally, it is obvious that 

 the indiffusible proteids which are conveyed from part to part in 

 the sieve-tissue of vascular plants (see p. 165) must eventually be 



