TKANSPIRATION AND ASCENT OF SAP DIXON. 419 



pressure exerted by the atmosphere. The amounts of water forced 

 up by root pressure are insignificant compared with the losses due 

 to transpiration. Atmospheric pressure can supply the evaporating 

 cells at most only up to a level of about 10.3 meters. "When allowance 

 is made for the resistance opposed by the conducting tracts to the 

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

 raised by these two forces to a height of 10 meters above the roots 

 must be exceedingly small. It follows that the water in the trachese 

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

 transpiration, whenever the loss can not 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 of the 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, wdiile 

 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 neighbors, 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. 



^Miile 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 experi- 

 ments 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 0.01 millimeter is in equilibrium with a pull equal to the 

 hydrostatic head of 1.65 meters. While one having a radius of 0.001 

 millimeter =1 /x could resist the tension exerted by a column of 16.5 

 meters of water. Bubbles having a radius of 1 /* would just be visi- 

 ble with the highest dry objectives commonly in use, their diameter 

 being about one-fifth 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 prepara- 



1 Z. Kamerling, Oberflachenspannung und Cohaslon. Bot. Centralbl., 73. 1898. 



