Transpiration and the Ascent of Sap. 41 



motion of water in them, we must conclude that the supply of water 

 raised bj^ these two forces to a height of 10 m above the roots must 

 be exceedingly small. It follows that the water in the tracheae 

 above this level is at all times in tension, and, in times of vigorous 

 transpiration, whenever the loss cannot be made good by the lifting 

 pressure of the atmosphere, the water in the tracheae of leaves at 

 lower levels also, is in a tensile state. This tensile state is no less 

 inevitable at the top ot a column of water unsupported at the base, 

 such as is found in a high tree, than is the state of compression at 

 the bottom of a deep vessel filled with water. The former is caused 

 by the weight acting against the cohesive forces of the water, while 

 the latter is necessitated by the weight acting against the resistance 

 of the water to crushing. 



Owing to the permeable nature of the walls, the water in one 

 trachea is continuous with that in its neighbours and, consequently, 

 the tension in one is transmitted to the water in adjacent tracheae. 

 Thus the tension applied at the mesophyll cell-surfaces is transmitted 

 downwards, through the water in the tracheae of the leaf and of 

 the petiole, to the water in those of the stem. 



While air bubbles are found extremely rarely in the tracheae of 

 the vascular bundles of the leaf, investigators seem agreed that they 

 are of common occurrence in the conducting tissues of the stem. It 

 is evident that in the tensile water in plants these bubbles will 

 behave exactly in the same way as we have seen bubbles behave in 

 the experiments on tensile fluids. If they are sufficiently minute they 

 will have a very small radius of curvature and the surface tension forces 

 preventing them from enlarging will be correspondingly great. When 

 these forces balance, or are greater than, the tension in the water, the 

 tension will be transmitted past the bubbles, and, if the bubbles adhere 

 to the walls of the tracheae, the tensile stream will be drawn past 

 them. Kamerling^) has shown that a bubble having a radius of 001 mm 

 is in equilibrium with a pull equal to the hydrostatic head of 1"65 m. 

 While one having a radius of 0*001 mm = 1 // could resist the tension 

 exerted by a column of 16'5 m of water. Bubbles having a radius 

 of 1 ^L would just be visible with the highest dry objectives commonly 

 in use, their diameter being about \'5 of the diameter of the lumen 

 of the finest tracheids of the pine. Bubbles of this minute size are 

 almost never observed in the tracheae of plants. In fact the methods 

 of preparation, involving as they do the relief of the existing tension, 

 or even the exposure to atmospheric pressure, would cause bubbles of 

 this magnitude to disappear. A tension anything greater than the 

 pull exerted by a column of water 1-65 m will overcome the 



*) Z.Kamerling-, Oberflächenspannung und Cohäslon . Bot. Centralbl. , 73. 1 898. 



