420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910, 



tion, 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 b}^ a column of water 1.65 meters will overcome the surface 

 tension of bubbles having a diameter of 0.02 millimeter, 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 millimeter 

 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 formation of a bubble 

 the sequence of events will be as follows : The water around the bub- 

 ble 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 retreating surface is held by the wall, and as more water is 

 drawn away the bubble can enlarge only longitudinally. At this 

 period the surface tension of the spherical bubble is replaced by the 

 capillary forces of the tubular trachea, and, the capillary forces devel- 

 oped in these tubes being insufficient to withstand the tension, the 

 bubble gradually 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 in the tensile current. 

 No doubt it diminishes the effective cross section of the flow, but, 

 owing to the fact that the conducting tracts are subdivided into such 

 numbers of minute compartments, the development of even a large 

 number of bubbles is unable to wreck the stability of the tensile 

 column of water in the wood. 



The state of affairs in the conducting tissues is illustrated in figure 

 3. For the sake of simplicity a longitudinal section of a conifer's 

 wood is represented. The shaded tracheids are supposed to be filled 

 with water, while the light spaces indicate those containing air bub- 

 bles, which have been expanded by the tension of the transpiration 

 stream till they completely fill the tracheids in which the bubbles 

 occur. It is evident that even when a large number of tracheids are 

 blocked with air the water column in the wood is not broken, but is 

 drawn around the bubbles inclosed in and rendered harmless by the 

 walls of the tracheids. In the figure for example 50 per cent of the 

 tracheids contain bubbles, and yet a considerable volume of water 

 might be drawn up in the remaining tubes. The imbibitional proper- 



