Transpiration and the Ascent of Sap. 45 



burger believes, the transpiration current must pass in the vessels 

 almost exclusively. Strasburger records that these vessels con- 

 tained little air although they were not completely air-free. The 

 observations were made in summer. 



Branches of Robinia, Quercus, Acer and Betula which had been 

 frozen hard were examined in winter. All were relatively richer in 

 air then those examined in summer. 



Strasburger deduces from his observations 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 the 

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



Ewart') has also endeavoured to estimate the number of air- 

 containing tracheae. He applied to the bases of cut branches a head of 

 1^/3—2 m of water. The experimental branch was placed horizontally 

 and the distal end cut off. The cut surface of the stump was examined 

 witli a lens. From the cut surface of Wistaria branches, treated in 

 this manner from June to September, water begins to exude almost in- 

 stantaneously from the vessels. If the pressure is raised scattered air 

 bubbles emerge in the stream. In September the number of bubbles was 

 greatest, but even then, Ewart thinks that the number of vessels 

 containing continuous water columns never fell below 10 7o- ^^ the 

 case of the Maple and Poplar, treated similarly, Ewart believes 

 that during June, July, August and September all the vessels 

 contain air at reduced pressure, because at all times it took 20—75 

 minutes before water began to exude from the vessels. He gives no 

 estimate as to the air and water content of the tracheids. He also 

 states that on cutting these stems under mercury the liquid was 

 drawn into all the wider elements. He considers, in contradiction to 

 Hartig's results, that the more distal parts contain most air. 



On the whole Ewart gives a lower estimate of the number of 

 tracheae completely filled with water than Strasburger does. On 

 the other hand Strasburger's results are much more numerous, and 

 possibly E w a r t 's were made when the water content of the branches 

 was extremely low. Both methods 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 discontinuity and the water adheres feebly to it. Probably 

 some of the bubbles observed were thus formed at the moment of 



^) A. J. Ewart, On the Ascent of Water in Trees. Phil. Trans. Roy. Soc. 

 Lend., Vol. 199, (1908), B, p. 362. 



