448 MANUAL OF BOTANY 



and if a leaf is cut off from the stem while construction is 

 going on, it can very soon be detected in the mesophyll cells as 

 well. 



Proteids are probably gradually broken down into amides, 

 such as asparagin, which, being a readily diffusible body, can 

 pass through the cell-walls. 



These elaborated materials, in their passage from the leaves, 

 travel by the bast and not by the wood. The carbohydrates 

 and the amides probablj^ go by the parenchj^ma. The sieve - 

 tubes differ from the latter in containing a quantity of proteid 

 material, and it is at least possible that proteid as such may 

 travel by them. If so, it is probably reconstructed in the sieve- 

 tube from the amides, &c., which are being transported from the 

 mesophyll. A certain amount of carbohydrate also may go this 

 way. 



That the stream of elaborated food is transported by tissues 

 external to the wood appears clear from the fact that if an 

 incision is made round a branch or trunk of a tree and a small 

 ring be removed, extending inwards as far as the wood, the 

 growing parts below the wound dwindle and die, while those 

 which leave the axis above the incision become more luxu- 

 riant. 



Even along the path of the stream we find the same ten- 

 dency to temporary deposition which we noticed in the chloro- 

 plastids. If the progress of the stream is checked from any 

 cause, we find a temporary accumulation of starch near the 

 point of stoj)j)age, which disappears when the flow again asserts 

 itself. 



The direction which the food substances take is dependent 

 upon many circumstances. When gi-owth is active in any 

 part, whether an apical meristem or a cambium layer, the stream 

 is directed towards such seat of consumption. This is due to 

 a process of diffusion set up in consequence of the removal of 

 constructive material from the stream by the growing cells, and 

 the consequent weakening of the solution there. When there is 

 more produced than is sufficient to supply an immediate demand, 

 the surplus is deposited in some part or other of the plant, to 

 supply future needs. There is thus continually going on during 

 active life a storage of surplus food substances in places which 

 we may call reservoirs of reserve materials. In some cases 

 they are again very speedily removed, but in others they remam 

 there for a considerable time. 



The nature of these reservoirs is very varied. Seeds, tubers, 



