418 MANUAL OF BOTANY 



The filtration when set up tends to relieve the pressure in the 

 cortex, and additional water can then be absorbed as before. 

 The water is thus made to rise gradually in the axial stele, for 

 the root-hairs and the cortical tissue exert together in this way 

 a kind of pumping action, forcing it along the axis. This force, 

 due to the turgescence set up as described, is known as root- 

 pressui'e, and is one main factor in the transport of water 

 through the plant. The turgescence not only leads to the rise 

 of sap in the axial stele, but spreads throughout the whole of 

 the cortical tissue of the plant, reaching indeed every cell into 

 which osmotic diffusion can take place. 



The stele of the root we have seen to be directly continuous 

 with that of the stem, the woody elements being in contact 

 throughout. The stream of water consequently^ passes up the 

 woody tissue of the stem so long as the cells are living. The 

 hard dead wood of the duramen takes no part in the conduction, 

 which is confined to the alburnum. Some discussion has taken 

 place as to the exact path followed, but that the one indicated is 

 that which is taken has been proved bj^ cutting incisions round 

 the trunks of trees to various depths. So long as the alburnum 

 is not cut the stream is not interrupted, but if the wound passes 

 through that region it stops, and all parts of the tree above the 

 incision droop and die. 



The water has been said by some observers to pass along the 

 cell- walls of the woody vessels and tracheids, and not through 

 their interior. Other writers hold that the main stream travels 

 along the interior of the cells. To this point we shall return 

 later. In either case the water ascends through the trunk, 

 branches, and twigs of the tree till it reaches the points at which 

 the foliar meristeles leave the central axis. It passes along the 

 woody elements of these, entering the leaves thus by the fibro- 

 vascular bundles or veins. From these it makes its way to the 

 parench^'matous mesophyll of the leaves, the cells of which are 

 thus made turgid. Osmotic diffusion, interrupted in the axis of 

 the plant, is resumed in the leaves, and a general equihbrium of 

 turgescence is reached in them. 



We have seen that the parenchyma of the leaves, and par- 

 ticularly that of the lower half of each is abundantlj^ supplied 

 with intercellular spaces {fig. 1183). The cells which abut upon 

 these spaces are furnished with very delicate walls and readily 

 allow a process of evaporation to take place, watery vapour 

 passing into these passages. The surface of the leaf we have 

 seen to be furnished witli a number of openings or stomata. 



