Transpiration and the Ascent of Sap. 47 



ductivity of wood in tangential and longitudinal directions. From 

 them is seen that the pressure required to force water in a tangential 

 direction is immensely greater than that needed to urge it longitu- 

 dinally in the wood, although in both cases the water is free to 

 move through the pits. In the tangential direction, however, in the 

 same distance the number of walls traversed may be hundreds of 

 times greater than in the longitudinal path. It is evident that the 

 persistence of the walls in the development of the water-conduits of 

 plants — introducing, as they are shown to do, an immense resistance 

 to flow — is inexplicable on any view which regards the water as 

 being forced through the stem. Viewed, however, in the light of 

 the tension hypothesis this structure becomes a most beautiful adaptation 

 to confer stability on the tensilely stressed transpiration stream, and 

 one which transforms the water, despite its mobility, into a substance 

 which is stable while sustaining very great stresses, just as if it 

 were a rigid body. True, the tensile stream experiences the resistance 

 opposed by the numerous walls, but the presence of the partitions, 

 conferring, in the manner just pointed out, a new property on the 

 water, renders available such an enormous source of energy at the 

 evaporating surfaces in the leaves for the lifting of the sap, that the 

 amount of energy which is spent in overcoming the resistance opposed 

 by the walls is relatively insignificant. 



The elongated form of the conducting elements secures that the 

 resistance shall be small consistently with the stability of the water; 

 for, of course, if the tension is great, a bubble in a long tube renders 

 a lai'ger portion of the conducting tissues useless than one confined 

 in a short vessel; but on the other hand when the long tube is 

 completely filled it transmits more readily than if it were subdivided 

 into a number of tracheids. Hence we may regard the tissue formed 

 of long vessels as the path of the most rapid part of the transpiration 

 current when the plant has an abundant supply of water, while the 

 tracheids transmit the slowly moving water and continue in function 

 even when supplies are very limited. It is also evident that the 

 small cross section of the tubes, though also introducing resistance, 

 is most essential. In this way each bubble which is formed occupies 

 only an infinitesimal part of the cross-section of the whole water 

 current. 



The structure of the walls themselves is also in complete harmony 

 with the tension hypothesis and finds its most natural explanation 

 viewed in the light of that hypothesis. 



It has long been recognised that the thickenings found on the walls 

 of the tracheae, viz. the internal supports in the form of annuli, spirals 

 and networks, are of such a nature that they are preeminently suited 



