Transpiration and the Ascent of Sap. 33 



lumiiia are known to transmit the major part of the current and it 

 seems improbable, even where we can invoke such great forces as 

 the tensile strength of water, that they could suffice to drag an 

 adequate water supply through the fine grained cell-walls. Yet we 

 were able to show by experiment that even when the lumina are 

 rendered impassable for water, some small amount of water is trans- 

 mitted in the walls by this process.^) 



When we found ourselves compelled to give up these hypotheses. 

 the one, as assuming conditions inimical to the transmission of tension 

 in the water, and the other, because it did not agree with the 

 accertained fact that the water moved in the lumina, it was an 

 easy transition to arrive at the conclusion that the water passed up 

 in the lumina in a state of tension. How, in the lumina of the 

 conducting wood, the necessary conditions for the production of tension 

 are fulfilled, we shall now proceed to enquire. 



Even in Text Books of Physics the cohesion of liquids is seldom 

 alluded to, and the conditions necessary to produce a state in which 

 liquids may transmit a tensile stress are not adequately treated. 



Donny-) in 1846 showed that it was possible for a column of 

 sulphuric acid 1'255 m high to hang in a vertical tube closed at its 

 upper end, when atmospheric pressure was not allowed to press the 

 liquid upwards from below. He compares the phenomenon to the 

 well known experience that the mercury of a barometer may be 

 retained above the actual barometric height, if the tube, filled by 

 inclining it, is raised gradually to a vertical position. He further 

 states that this phenomenon has been explained by Laplace^) as 

 being due to the cohesion of the mercury and to its adhesion to the 

 glass. Donny also looked for the cohesion or tensile strength of 

 water. He appears, however, to have failed to demonstrate it in the 

 same way which had been successful in the case of sulphuric acid. 

 He observed, however, the tensile strength of water in the following 

 less direct manner: If a vertical glass tube one metre long partially 

 filled with water and sealed at both ends is struck vigorously on 

 the lower end with the palm of the hand, bubbles open in the liquid 

 and instantly close again with a metallic click. A blow on a tube, 

 which has been similarly set up, but from which the air has been 

 removed by careful exhaustion, produces no bubbles; nor is a click 



') H. H. Dixon and J. J 1 y , The Path of the Transpiration Current. Ann. 

 of Bot., Vol. IX, 1895, p. 404. 



-) J. Donny, Sur la cohésion des liquides et sur leur adhésion aux corps 

 solides. Ann. de Phys. et Chira., Sér. III Tome 16, 1846, pp. 167 et seq. 



^) Laplace, Mécanique céleste. Supplément au X° libre, p. 3. Quoted by 

 Donny, loc. cit. 



Progressus rei botanicae III. 3 



