9 6 



bark, and more particularly to the elements in it known as the 

 sieve tubes. 



I have shown that almost the entire amount of the fluid 

 absorbed by the roots is expelled in the form of watery vapour 

 by the leaves in the process of transpiration, and that when 

 this supply is either insufficient to meet the demand occasioned 

 by transpiration, or is interfered with, the phenomenon of with- 

 ering takes place. Also, that the descent is determined by local 

 circumstances ; that its course is regulated only by the physical 

 laws of diffusion rendering it necessary that it should pass 

 through the parenchyma tissues ; and the interpretation to be 

 placed upon the latter part of the observation which I have 

 mentioned, alone remains to be noticed. The alkaline reaction 

 is due to the presence of a proteid substance which is capable 

 of being assimilated by the protoplasm. 



It seems, then, that proteids, as such, and otherwise than in 

 the form of nitrogenous crystalline bodies, are conveyed by the 

 plant. These proteids are stated to be formed by the combination 

 of starch with the body known as asparagin, since this latter 

 body is only absent when active formation of starch is going 

 on in the cells of the plant. If the plant be placed in the dark, 

 we get formation of asparagin in the cells of the plant, but no 

 formation of starch. Asparagin is regarded as formed by a 

 very complex series of degradation changes from the original 

 nitrogenous protoplasmic material of the plant, and in this way, 

 in combination with starch, it is regarded as being rendered 

 again capable of being used up and brought into the substance 

 of the protoplasm once more. It is this body, then, that passes 

 in the form of solution through the sieve-tubes, which consist 

 of cells laid end to end, the transverse partition walls between 

 which cells have become perforated by canals, so that the 

 contents of long lengths of cells communicate with one another ; 

 and thus afford a means of conveyance for proteids, as such, from 

 one part of the plant to another. They appear to pass mainly 

 by diffusion through their walls to any point where fresh 

 material of this kind is needed to sustain the life of the plant, 

 in consequence of a consumption of them at that point, otherwise 

 there is no current at all. 



