94 TRANSPIRATION AND ASCENT OF SAP ch. 



the transport of water, the number of air-bubbles is a 

 minimum. 



On the whole, Ewart's estimate of the number of 

 tracheae completely filled with water is lower than that of 

 Strasburger. On the other hand, Strasburger's results 

 are much more numerous, and possibly Ewart's were 

 made when the water content of the branches was extremely 

 low. The methods employed by both investigators seem 

 open to criticism. 



When a branch is cut, even under water, it is possible 

 that bubbles are formed in the tracheae by the act of 

 cutting. Bubbles may be formed anywhere close to the 

 knife, but naturally mostly in the tracheae in contact with 

 the knife on either side, as the knife introduces a discon- 

 tinuity, and the water adheres feebly to it. Probably 

 some of the bubbles observed were thus formed at the 

 moment of making the preparation for examination, and 

 were non-existent when the plant was transpiring. 



In Ewart's experiments the internal and external pres- 

 sures were not given time to come into equilibrium ; conse- 

 quently, supposing 20 per cent, or 30 per cent, of the 

 vessels contained continuous water, while the remaining 

 70-80 per cent, contained gas at reduced pressure, 

 as soon as the branch was cut across, atmospheric 

 pressure would drive the water from the full vessels 

 opening on to the cut surface into those which contained 

 gas at a low pressure, and vessels which had been full 

 and transmitting a tensile stress during transpiration, would 

 appear almost empty after cutting. Ewart himself con- 

 siders that 10 per cent, of the vessels of last year's wood 

 would transmit enough to cover the losses of the most 

 vigorous transpiration. Even if a much larger percent- 

 age than this were completely filled with water at the 

 time of cutting, they would elude observation in this 

 method of investigation. 



Both Strasburger and Ewart have shown that coloured 



