90 TRANSPIRATION AND ASCENT OF SAP ch. 



of leaves, at lower levels also, is in a tensile state. This 

 tensile state is no less inevitable at the top of 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 trachea?. 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. 



Effect of bubbles. 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 

 sap of plants these bubbles will behave in exactly 

 the same way as we have seen bubbles behave in the 

 experiments on tensile liquids. If they are sufficiently 

 minute they will have a very small radius of curvature, 

 and the surface tension forces preventing them from en- 

 larging 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. Kammerling has shown 

 that a bubble having a radius of 0*01 mm. is in equili- 

 brium with a pull equal to the hydrostatic head of T65 m., 

 while one having a radius of O'OOl mm., = 1/a, could resist 

 the tension exerted by a column of 16*5 m. of water. 

 Bubbles having a radius of 1/* would just be visible with 

 the highest dry objectives commonly in use, their diameter 



