Transpiration and the Ascent of Sap. 03 



20 atm., even if we assume transpiration to be as fast as in lower 

 trees; for we have seen tliat the pressure required to drive water 

 through a stem at the transpiration rate is not more than that 

 exerted by a column of water equal to the length of the stem 

 traversed. When we find osmotic pressures as great as 26 — 30 atm. 

 in low trees like Cytisus and Tilia, we may, in the absence of direct 

 observation, credit the leaves of higher trees with at least as great 

 pressures. 



It will be convenient here to notice some of the objections which 

 have been raised by assailants of the Tension Theory of the Ascent 

 of 8ap. 



1. It has been objected that, although the physical possibility of 

 hanging water-columns in the plant has been demonstrated, yet it has 

 not been shown that sufficient quantities of water to cover transpiration 

 can be transported upwards by the tension developed. Ewart believed 

 that the resistance opposed by the water tracts proved the forces 

 developed in the leaves insufficient to do the work and the tensile 

 strength of water too small to transmit the stress required. Firstly 

 with regard to the energy; the results quoted above show that the 

 energy absorbed by the leaves is several hundred times more than 

 would be required to raise the water in the highest trees in addition 

 to overcoming the resistance of their conducting tissues. Even if we 

 accept the maximum estimate of the resistance, which we have already 

 learnt is certainly much too great, yet the energy absorbed by the 

 leaves is more than sufficient. 



Nor does the tensile strength of water forbid the application of 

 the theory to the highest trees. We have shown reasons to believe 

 that the greatest stresses developed in trees 100 — 150 m high would 

 not exceed 20 — 30 atmospheres. The tensile strength of water has 

 been experimentally shown to exceed 150 atmospheres. This tenacity 

 was proved in samples of water saturated with air. Consequently 

 we need anticipate no difficulty from the fact that the transpiration 

 stream contains dissolved air. 



To set at rest a fear raised by S t e i n b r i n c k ^) as to the 

 adequacy of the tenacity of the water columns in case of a rise of 

 temperature, it may be pointed out that the tensile strength of more 

 than 150 atmospheres was demonstrated over a range of temperature 

 from 25 " C to 80 " C. Temperatures above the latter will not occur 

 in living trees. 



^) C. Steinbiinck, Zur Cohäsionstheorie des Saftsteigen. Ber. d. Deutsch. 

 Bot. Gesell., 1904, Bd. XXII, 8, p. 529. 



