42 Henry H. Dixon. 



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

 drawn away, the bubble can enlarge only longitudinallj^ At this 

 period the surface tension of the spherical bubble is i-eplaced by the 

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

 developed 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 inibibitional forces are enormous and hold the surface of the water, 

 limiting the bubble close to their inner suiface. Surrounded 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 eftective 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 

 Fig. 4. 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-bubbles, 

 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 

 round the bubbles enclosed in and rendered harmless by the walls of 

 the tracheids. In the figure for example 50 "/o of the tracheids contain 

 bubbles and yet a considerable volume of water might be drawn up 

 in the remaining tubes. The inibibitional properties of the walls of 

 contiguous water-filled tracheids render the water throughout the stem 

 continuous. Consequently the stress developed above is transmitted 

 round the air bubbles and draws the stream past them, to use 



