204 BOTANY PAKT i 



onwards. It is, however, doubtful whether this initial disturbance 

 of the condition of equilibrium is sufficient to effect the raising of 

 the whole mass of water. A sufficient and generally accepted 

 explanation of this much-debated question is still wanting, though 

 progress has been made towards it (-'). 



It has been already explained that the nooT-ruEssuiiE cannot exert such a force 

 during transpiration (p. 201). 



OSMOTIC FORCES act too slowly to be of any value, and, moreover, there is no 

 fixed distribution of osmotic substances that would account for such a current. 



The transpiration current cannot be due to CAPILLARITY. In the first place, con- 

 tinuous capillaries are entirely wanting in some plants (the Conifers, for example), 

 and in the stems of others they are only present for comparatively short distances. 

 Secondly, the concave menisci in the elements of the wood are not in relation with 

 any level or convex surface of water, in which case alone they could have effect. 

 Thirdly, the height to which liquids can rise by capillary attraction and it would 

 be less in the vessels and tracheides than in a glass tube does not approach the 

 height of an ordinary tree ; and, finally, the rate of ascent induced by capillarity 

 decreases so greatly with the increasing height of the fluid, that so copious a flow 

 of water as occurs in plants would be impossible. 



ATMOSPHERIC PRESSURE has, also, been shown not to be the cause of the tran- 

 spiration current. In fact the vessels and tracheides of vigorously transpiring plants 

 contain rarefied air between the short columns of water. This is evident from the 

 way in which stems cut under mercury become penetrated by it. But as the water- 

 courses in plants are all completely shut oft from the outer atmosphere, the external 

 atmospheric pressure could have no effect. The rarefied air within the plants, 

 moreover, shows no such regularity in its distribution that it could possibly give 

 rise to so continuous a flow of water. Further, as the atmospheric pressure can only 

 sustain the weight of a column of water 10 m. high, while sap rises in Sequoia 

 100 m. high, and in Eucalyptus trees of the height of 155 m., the inadequacy of 

 the atmospheric pressure to give rise to such a movement must be .admitted. 



The supposition that the water ascends in the form of vapour through the 

 cavities of the wood, and is afterwards condensed in the leaves, is untenable, as is 

 at once obvious from a consideration of the anatomical structure of the wood, the 

 interruption of its cavities by short columns of water, and the temperature of the 

 plants themselves. And, moreover, the special task of the transpiration current, to 

 transfer the nutrient salts, could not be accomplished if such were the mode 

 of ascent. 



It has also been suggested that all of these processes might be aided by THE 

 CO-OPERATION OF THE LIVING CELLS which are so abundant throughout the wood, 

 and have command of active osmotic forces, to the service of which they could 

 unite a regulative irritability. STRASBURGER'S investigations, however, have 

 shown that poisonous solutions, which would at once kill all living protoplasm, 

 can be transported to the summits of the high trees. Thus the supposition that the 

 living elements at least immediately co-operate in the ascent of water is precluded. 



Recently JOLT, DIXON, and ASKENASY have endeavoured to explain the trans- 

 mission of the suction force of transpiration to the most distant root-tip by the fact 

 of the cohesive force of the water. The occurrence of bubbles of air and vapour in 

 the conducting channels, and the fact that movement of the water interferes with 

 the power of its cohesive force to resist a pull are among the objections to this theory, 

 which have, however, been lessened by recent investigations of STIUNHKINK ( x ). 



