TKANSLOCATION OF NUTEITIVE MATERIALS 227 



cells, so that very soon the whole of the parenchyma of the 

 constructive region is saturated with the sugar. This par- 

 enchyma abuts however on other cells which contain no 

 chloroplasts, especially the sheaths and the bast of the 

 nbro-vascular bundles. Diffusion of sugar into these takes 

 place, and proceeds from cell to cell, especially among the 

 delicate bast tissue, so that a stream of sugar is soon diffusing 

 all along the bast. As a rule it does not penetrate very 

 far beyond this tissue, owing largely to the anatomical 

 arrangements of the parts and the great facility which the 

 structure of the bast affords for this diffusion. So long as 

 the manufacture goes on, therefore, there is an outflow of 

 the manufactured carbohydrates from the region of its forma- 

 tion, the ultimate and even the temporary direction of the 

 stream being determined by other factors which we shall 

 consider later. 



This removal of sugar from the leaf can be proved by 

 several observations. We find but little of it in the meso- 

 phyll of the leaf, though we know it is being continually 

 produced there. We find it fairly easily in the bast of the 

 veins, and if a leaf is cut off from the stem while construc- 

 tion is going on, so that it cannot be transported away, it 

 can very soon be detected in the mesophyll cells as well. 



This, however, is not all. The process of diffusion is a 

 slow one and does not serve to remove the sugar as fast 

 as it is formed. The excessive formation of sugar would 

 soon lead to such saturation of the sap as would at any 

 rate temporarily inhibit its construction, were it not for 

 another agency at work. The chloroplasts are endowed 

 with another property than that so far described, which is 

 now called into play. This is a peculiarity of the body of 

 the plastid, and is quite independent of the colouring 

 matter, being shared by other quite colourless plastids 

 which occur in other parts of the plant. These structures 

 have the power of converting sugar into starch, a power 

 which we must examine more fully in a subsequent chapter. 

 The transformation is apparently a process of secretion. 



Q 2 



