Transpiration and the Ascent of Sap. 53 



renders the water column continuous across tracheae, which had been 

 put out of action by the development of bubbles. 



Again the gradient of pressure which determines the flow of 

 w^ater into the root may be maintained in part or even very largely 

 by atmospheric pressure. This can only act when absorbing cells are 

 surrounded by liquid water which the atmospheric pressure may 

 force through the tissues of the plant. When, however, the liquid 

 surface is moved into the cell-walls atmospheric pressure ceases to 

 be effective, as it is then withstood by the menisci formed in the 

 walls and can urge the water no further. 



The effectiveness of atmospheric pressure when the roots are 

 surrounded with water has been experimentally demonstrated by 

 Vesque.^) Curiously enough he found that the increase in the rate 

 of upward movement was less marked when the roots of herbaceous 

 plants were exposed to external pressure than when the subject of 

 experiment was a shrubby plant. 



When root -pressure and atmospheric pressure are unable to 

 advance water upwards to the evaporating cells sufficiently quickly 

 to make good evaporation losses, then it is evident that the tension 

 in the water of the plant rises. This tension is either sustained by 

 the menisci in the absorbing cell-walls of the root or, if water is 

 present on the outer sides of the cells, it is transmitted through the 

 walls, and causes a flow of water inwards. 



It is interesting to consider the behaviour of the water in the 

 soil when acted upon by the absorption of the plant and to compare 

 it with that of the water in 'the conducting tracts. 



Whenever the root hairs come into contact with liquid water 

 its entry will be determined by the sum of the effects of (1) the 

 tension in the sap, transmitted from the leaves, (2) the osmotic 

 pressure of the solutions in the tracheae and (3) atmospheric pressure 

 acting on the water in the soil round the roots. The first two 

 are of the nature of a drag oi- traction on the water while the third 

 is a vis a tergo. 



It might be thought — as Böhm suggested 2) — that when the 

 first two forces draw more water into the root than is being con- 

 stantly pressed forward by the atmosphere, that the water in the soil 

 would pass into a tensile state. Only the very feeblest tensions can 

 exist in this way in the soil water. For it does not fully occupy any 

 rigid envelope which would be able to prevent the water changing its 



^) J. Vesqiie, Recherches sur le mouvement de la sève ascendante. Ann. d. 

 Soi. Nat. Bot., XIX, 1884, pp. 157 et seq. 



") J. Böhm, Capillarität und Saftsteigen. Ber. d. Deutsch. Gesell., XI, 1893, 

 pp. 203 et seq. 



