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are covered by layers of cells which protect and shut them off 

 entirely from the influence of intercellular spaces lying more 

 superficially. Thus, in the root, the endodermis or bundle- 

 sheath forms a compact complete layer round the fibro-vascular 

 tissue and cuts it off from the intercellular spaces existing in 

 the cortical parenchyma. This endodermis is present also in 

 some stems in a like position, or if it is absent its place is taken 

 by layers of thickened closely-packed cells which have the same 

 protective function. In consequence of the existence of this 

 negative pressure in the cavities of the vessels, there is a passage 

 of air by diffusion from the surrounding tissues into the cavities 

 of the cells and vessels of the wood. For this negative pressure 

 tends to exercise an attraction for the gases in the other tissues, 

 and to draw them towards the vessel-cavities, thereby setting 

 up currents ; but this attractive power is never sufficiently 

 great to bring the air in the vessels to the normal pressure of 

 the air outside. This negative pressure has also an important 

 influence in promoting and rendering more easy of accomplish- 

 ment than it would otherwise have been the entrance of water 

 from the parenchyma cells adjoining the fibro-vascular cylinder 

 through the walls into the cavities of the vascular system, where 

 it is seen in the case of pruning a Vine in spring. The pas- 

 sage of fluid and of gases from the wood to the stomata must 

 take place by the cell-walls, since the cell-walls can only 

 pass substances in the form of solution. The watery vapour 

 is simply given off as watery vapour from the external moist 

 surfaces of the wood-cells or vessels, and it may be deposited as 

 drops in the plant itself, e. g. } in the case of water found in 

 the cells surrounding the vessels in the tops of high trees ; — or 

 it may pass out of it as watery vapour. Its course is briefly as 

 follows : — The fluid is absorbed by the root-hairs, it passes by 

 diffusion through the parenchyma cells of the root, by filtration 

 under pressure from them into the vessel-walls of the root, and 

 it is conveyed in the thickness of the lignified cell-walls of the 

 woody tissues through the stem to the leaves, and from the moist 

 surfaces of the parenchyma cell-walls of the leaf into the inter- 

 cellular spaces in communication with the stomata, through 



