92 TRANSPIRATION AND ASCENT OF SAP ch. 



in the tensile current. No doubt it diminishes the effec- 

 tive 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 illus- 

 trated in Fig. 17. For the 

 sake of simplicity a longitu- 

 dinal section of a conifer's 

 wood is represented. The 

 shaded tracheids are sup- 

 posed to be rilled with water, 

 whilst the light spaces indi- 

 cate those containing air- 

 bubbles, which have been 

 expanded by the tension of 

 the transpiration stream till 

 they completely fill the 

 tracheids in which the bub- 

 bles occur. It is evident that 

 even when a large number 

 of tracheids are blocked with 

 air, the water column in the 

 wood need not be broken, 

 but may be drawn up round 

 the bubbles enclosed 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 consider- 

 water might be drawn up in the 

 The imbibitional properties of the 



Fig. 17. 



able volume of 



remaining tubes 



walls of contiguous water-filled tracheids render the water 



throughout the stem continuous. Consequently, the 



