DIV. II 



PHYSIOLOGY 267 



This Jiydrolytic breaking down of proteids takes place under the 

 influence of " proteolytic " enzymes (proteases) which very probably 

 are closely similar to corresponding enzymes in the animal body. We 

 should therefore have to distinguish 



1. Pepsin, which only breaks down the proteid molecule to 

 albumoses and peptone. 



2. Erepsin, which transforms peptone into amino- acids. It is 

 uncertain whether in addition there should be added 



3. Trypsin, which transforms proteids directly into amino-acids. 



The decomposition products of albumen quickly undergo changes in the plant, 

 and therefore the mixture of nitrogenous organic compounds which one obtains 

 from a plant kept in the dark is not identical with the products of the hydrolysis 

 of albumen outsid^ the plant. In the plant syntheses take place after the primary 

 decomposition, and these lead to the formation of such substances as amides, the 

 most widely spread of which is asparagin. This dominates in Gramineae and 

 Leguminosae (15 g. are present in a litre of sap from bean seedlings) ; it is 

 replaced in Cruciferae and Cucurbitaceae by glutamin, while in the Coni ferae 

 arginin, a di-amino-acid, appears to play the same part. The syntheses proceed 

 still farther in light, when proteid may again be formed from the products of 

 decomposition of albumen. 



B. TRANSPORT OF THE MOBILISED RESERVE MATERIALS 



When the reserve materials have been brought by the aid of the 

 proper enzymes into the soluble form, or have been transformed into 

 substances with smaller molecules, they are capable of being 

 transported; we may speak of them as being mobilised. Their 

 movements are governed by the general principles of translocation of 

 substances. It is especially necessary that a diffusion current should 

 be established and maintained. This is brought about by the active 

 growth of cells at a greater or less distance from the place of storage 

 of the reserve material. As long as this lasts each molecule on 

 its arrival at the place of growth is promptly transformed (e.g. sugar 

 into starch or cellulose), and thus room is made for the molecules that 

 follow. In non- growing organs also (e.g. cotyledons, endosperm) a 

 gradient of diffusion is established by the cells to which the current 

 passes, having a greater power of condensing the sugar (forming 

 starch) than the others. A diffusion current can also be artificially 

 established where a storage structure under proper conditions is 

 placed in relation on one side with a large amount of water. It is thus 

 possible to bring about artificially an emptying of seeds, bulbs, etc. 



When substances have to be transported for considerable distances, 

 the movement of diffusion, since it goes on slowly, is replaced by 

 movement in mass. Thus in spring the reserve materials deposited 

 in the wood of our trees are carried up by the ascending current of 

 water in these vessels ; at this season the fluid in the vessels contains 



