iv COHESION THEORY OF ASCENT OF SAP 93 



tension developed above is transmitted round the air 

 bubbles and draws. the stream past them, to use Schwen- 

 dener's figure, like islands in a river. Hence it is evident 

 that it would be impossible to sever the continuity of the 

 water in the conducting tracts, i.e., to prevent evapora- 

 tion above from transmitting a pull to the water in the 

 roots, unless tracheae containing bubbles were to form 

 in some place an unbroken diaphragm across the conduct- 

 ing tissues of the stem. 



Number of air-containing tracheae. From 

 these considerations it appears that, unless an exceed- 

 ingly large number of the conducting tubes contain 

 air and are arranged in a special manner, there is no 

 likelihood of the tensile column being broken. On the 

 other hand, the amount of water transmitted in the stream 

 will be affected by the number of tracheae which contain 

 bubbles and are consequently put out of action in 

 the transmission of water drawn upwards under tension. 

 Hence it is of interest to inquire into our state of know- 

 ledge as to the air-contents of the conducting tracts. 



Results like Hartig's, where the amount of air present 

 is estimated as a percentage of the volume, cannot be 

 utilised here. These results do not tell us how the air is 

 distributed, and it is evident that 10 per cent, of air occur- 

 ring in each trachea might effectively destroy the cohesion 

 of the transpiration current, while 50 per cent, placed in 

 half the tracheae would only diminish the maximum trans- 

 missibility for a given tension to one half. 



Attempts to estimate the number of the vessels and 

 tracheids which contain air have been made on various 

 occasions ; but, unfortunately, all the methods hitherto 

 devised are probably open to error. Of these, Stras- 

 burger's results seem to be the most reliable. 



His general conclusion is that, while a limited amount of 

 air does not make the conducting tracts impassable to water, 

 yet in the peripheral parts, which are principally used in 



