ADJUSTMENT TO WATER 55 



the epidermal layers. This strain greatly increases the imbibition 

 and infiltration from the parenchyma cells adjoining the bundles, 

 and explains how the water may enter rapidly enough to supply 

 the constant evaporation from the leaves. 



Infiltration occurs more easily and rapidly through thui walls. 

 Consequently water passes readily into the vessels and tracheids 

 through the thin areas and the pits of the wall. It enters the 

 thickened walls of wood and stone fibers only with much difficulty 

 or not at all. In addition, tracheids are separated by thin end 

 walls, and vessels are continuous for long distances, while fibers 

 are connected by thickened walls. From this it becomes evident 

 that the movement of water upward must take place very largely 

 in the vessels and tracheids, and to a very slight degree, if at all, 

 in the thick-walled fibers. It will be seen later that the sieve 

 tubes of the bundle have a special function, and hence are not 

 available for carrying water to the leaves. 



71. Causes of the movement. The force that arises from the 

 turgidity of the cortical parenchyma is termed root pressure. 

 The latter is often if not regularly sufficient, not only to force 

 the water to filter into the tracheids and vessels, but also to cause 

 it to rise some distance in them. Under exceptionally favorable 

 circumstances, and with vigorous plants, this effect of root pressure 

 may be demonstrated. A stem is cut off near the ground, and a 

 glass tube of small bore is fitted over it tightly by means of a rubber 

 joint. If the plant is absorbing water strongly, the root pressure 

 causes the latter to rise in the tube. Once within the tracheids 

 and vessels, the water is subject to the effect of capillary action. 

 Capillarity, which is merely one form of attraction, is the force 

 which causes a column of water to rise in an extremely fine glass 

 tube when the lower end is placed in water. Capillary action is 

 due to surface tension, i.e., to the attraction between the molecules 

 of the tube and those of the water. It is exerted in the cavity 

 of the vessels, but attraction also causes water to rise in some 

 degree in the walls of both tracheids and vessels. A lifting effect 

 upon the water in the bundles is doubtless exerted by the trans- 

 piration of the leaves. This effect is largely due to the active force 

 of osmosis arising from the increased osmotic pressure in the leaf- 

 cells. The latter is brought al)out by the increased density of 

 the cell-sap caused by evaporation. A similar force is probably 

 exerted by the osmotic pressure of the cells of the stem which are 



