206 THE MOVEMENTS OF FLUIDS IN STEMS. 



water from cell to cell in the other parts of the tree. This rapid 

 current is of the greatest interest, and will require special con- 

 sideration further on. 



The fluid concerned in the rapid current in the xyleni moves in 

 one direction, upwards from the root to the leaves. The slow 

 current on the other hand moves in any direction; the parts 

 needing water take it from the parts in their immediate neigh- 

 bourhood, and ultimately from some external or internal supply. 



III. The movement of water in the stem. 



From the description already given it will be evident that as 

 the cell- walls are all more or less saturated with water (the cell- 

 sap in its widest sense), and as all parts of the plant are cellular, 

 there must be a continuous sheet of water from the tips of the 

 roots to the extremities of the loftiest branches. In addition 

 most of the cells contain cell-sap in the cavities of the protoplasm, 

 or in the centre of the cell after the protoplasm has disappeared. 

 The movement of the water is either slow or rapid, depending on 

 certain external and internal conditions, and from experiments 

 can be shown to move in any direction up or down or transversely. 

 The water enters by the root, being absorbed chiefly by the root- 

 hairs above the apex. The absorption is due to endosmose, and 

 gives rise at the early part of the season to the root pressure, so 

 long known for the surprising force it exerts, and from being the 

 cause of the bleeding of trees from wounds or cut branches. The 

 root pressure does not, however, last long (as shown by the cessa- 

 tion of bleeding), and after the leaves are in vigorous action it 

 ceases altogether. Hence, although the roots are still absorbing 

 water, yet it is so rapidly got rid of by the leaves, that no pressure 

 is exerted. The root-pressure, as shown by the experiments of 

 Hofmeister, varies in different plants. The following are some of 

 Hofmeister's results : 



Urtica urens, . . . 283 millimetres of mercury. 

 Vitis vinifera, . . . 804 ,, ,, 



Through the xylem of the fibro-vascular bundles, and in the 

 walls of the wood-cells, parenchymatous and prosenchymatous, 

 the fluid of the rapid current moves. The vessels at this period 

 contain air, and as has been shown by Hoehnel, the air is in a 

 state of diminished tension. This is shown by the fact that mercury 

 will rush into the vessels of a shoot when cut under that metal, 

 and rise in the vessels of the stem to a height proportional to the 



