iv COHESION THEORY OF ASCENT OF SAP 91 



being about one-fifth of the diameter of the lumen of the 

 finest tracheids of the pine. Bubbles of this minute size 

 are scarcely ever 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 magni- 

 tude to disappear. A tension anything greater than that 

 exerted by a column of water T65 m. will overcome the 

 surface tension of bubbles having a diameter of 0*02 mm. 

 and they will tend to expand indefinitely under its action. 

 Tensions as great as this must frequently occur in plants. 

 On first thoughts it might appear then, that one bubble 

 having a diameter of 0*02 mm. or more would destroy the 

 possibility of tension in the water of the conducting tracts. 

 A moment's consideration, however, will show that the 

 structure of these tracts sets a limit to the enlargement 

 of the bubble. In the conducting tracts, after the forma- 

 tion of a bubble, the sequence of events will be as follows : 

 The water round the bubble is drawn away by the tension, 

 and the surface of the bubble comes to rest against the 

 wall of the trachea in which it has developed. The retreat- 

 ing surface is held by the wall, and, as more water is 

 drawn away, the bubble can enlarge only longitudinally. 

 At this moment the surface tension of the spherical bubble 

 is replaced by the capillary forces of the tubular trachea, 

 and, the capillary forces developed in these tubes being 

 insufficient to withstand the tension, the bubble gradu- 

 ally pulls out till it completely fills the trachea. When 

 this stage is reached the bubble can enlarge no more ; its 

 surface is restrained on all sides by the walls of the trachea 

 which, as is well known, though very permeable to water, 

 are so fine-grained that their capillary or imbibitional 

 forces are enormous and hold the surface of the water, 

 limiting the bubble close to their inner surface. Sur- 

 rounded thus by the imbibed and rigid wall of the trachea 

 the bubble becomes just like a wetted solid or rigid body 



