104 TRANSFER OF FLUIDS. [CHAP. 



with the dilatation of the stem, and so retains a tolerably 

 uniform surface. In Birch and Plane the bark separates 

 in successive thin plates. In Scotch Fir both radial 

 fissuring and concentric lamination occur. Here, again, 

 we have opportunity for interesting observation in the 

 field, in trying to ascertain what causes determine the 

 weakest lines of the bark, and so bring about the mode 

 of fissuring peculiar to each species or group of species. 



The inner layer of the bark of many plants and trees 

 in which the fibrous tissue or liber proper is largely 

 developed, is made use of for cordage and in cloth- 

 making. Hemp, flax, and Russia-matting, are all derived 

 from this layer, which, in the plants affording these 

 products, is very tough. 



13. The questions now present themselves Through 

 which of these cells, or systems of cells, in the stem, is the* 

 water, and what it carries with it, absorbed by the roots, 

 conveyed to the leaves ? and after the changes deter- 

 mined by the influence of light in the foliar organs, how do 

 the plastic substances, which thus originate in the leaves, 

 find their way through the plant ? But these questions, 

 reasonable though they seem, it is impossible to answer 

 satisfactorily in an elementary work. 



It is not our business just now to concern ourselves 

 with contested points, so we must be content with a very 

 general and partial explanation. In the first place, we 

 must recall the fact that the entire plant is built up of 

 closed cells and vessels ; consequently, solid substances, 

 even in the minutest state of subdivision and suspension 

 in water, cannot be admitted by healthy uninjured plants. 

 Mistakes as to this point have arisen from using injured 

 or wounded plants for experiment. Therefore only fluids, 

 substances dissolved in fluids, and gases, can be absorbed 

 by the plant ; viz. fluids with solids or gases in solution 

 by the roots, gases and vapour by the leaves. 



The circulation or transference of these fluids and 

 gases from cell to cell can only be by diffusion, a physical 

 process modified and controlled in the living plant by the 

 layer of protoplasm lining the wall of each cell. This 

 process of diffusion depends upon two conditions. First 

 we must have two fluids separated by a membrane of 

 some kind which they can permeate. Second, these 



