700 PART lY. — THE PHYSIOLOGY OF PLANTS. 



optimum activity of transpiration, that is to say, a certain activity 

 of transpiration which promotes to the utmost the formation of 

 organic substance ; so that if the average activity of transpiration 

 fa.lls short of, or exceeds, this optimum, the nutrition of the plant 

 suffers, as shown by a diminished formation of organic substance. 



§ 8. Distribution of Water and otiier Substances. It is 

 clear that, when the plant-body is so far difFerentiated that only 

 certain parts of it are in a position to absorb water and substances 

 in solution from without, there must be a distribution of the ab- 

 sorbed substances from the absorbent surfaces to the other partis. 

 Further, when the plant-body is differentiated into parts which do, 

 and others which do not, contain chlorophyll, there must be a 

 distribution of the produced organic substance from the former to 

 the latter. 



In plants of relatively low organisation, the distribution takes 

 place entirely by diffusion; by simple diffusion when the plant is 

 a coenocyte ; by diffusion through the cell-walls, that is by osmosis, 

 when the plant is multicellular: and even in the highest plants 

 diffusion plays an important part. 



With regard, first, to the distribution of water and substances 

 absorbed in solution from without, in the more highly organised 

 plants. In these plants, as already stated, the conducting tissue is 

 the wood or xylem of the vascular bundles, extending from the 

 roots, the absorbent organs, to the leaves, the transpiring organs. 



With regard to the mechanism by which the water absorbed by 

 the roots is conveyed to the leaves, it must, in the first place, be 

 clearly understood that, as already mentioned (p. 678), the xylem 

 does not communicate directly with the atmosphere, but is a com- 

 pletely closed tissue-system. The mode in which water and sub- 

 stances in solution are introduced into this closed tissue-system in 

 the root is as follows : — The root-hairs absorb water from the 

 soil; the absorbed water passes by osmosis from the root-hairs 

 into the adjacent cortical parenchymatous cells of the root ; these 

 cells become highly turgid, and when a certain degree of 

 turgidity is attained, the water escapes by filtration under 

 pressure from the innermost parenchymatous cells into the xylem- 

 vessels upon which they abut. The water is thus forced into the 

 xylem under considerable pressure, which is termed the root- 

 pressure. In the leaves, water is withdrawn from the xylem by 

 the adjacent cells which absorb it osmotically, and from these in 

 turn by those which are actually transpiring. 



