iv COHESION THEORY OF ASCENT OF SAP 95 



liquids rise most rapidly in the trachea? containing un- 

 broken columns. 



It will be seen that although our knowledge as to the 

 actual proportion of trachea) containing bubbles during 

 transpiration is very unsatisfactory, yet observation 

 supports the view that always during transpiration there 

 are continuous tracts of tracheae free from air of 

 considerable cross section. It is also to be remembered 

 that the periodic flooding of the tracheae with water forced 

 upwards by root-pressure will bring the bubbles into 

 solution and will re-establish the conditions for tension 

 throughout the water-tracts. 



Evidence from structure. Here it will be interesting 

 to consider the structure of the conducting tracts, and 

 to see how far their details bear out the theory of the 

 tensile sap. 



The salient feature of this structure is the subdivision of 

 the water-ways by an immense number of longitudinal 

 and transverse partitions into minute compartments the 

 vessels and tracheids. For a system the function of 

 which is to conduct fluids, this is evidently a most unex- 

 pected configuration. It is true that the partitions are 

 permeable to water ; but when a considerable distance is 

 to be traversed, the sum of the resistances opposed by the 

 walls to the flow becomes important. This becomes clear 

 from the experiments of Bohm, Elfving, and Stras- 

 burger, comparing the conductivity of wood in tangential 

 and longitudinal directions. From their experiments it is 

 seen that the pressure required to force water in a tangential 

 direction is immensely greater than that needed to urge it 

 longitudinally 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, 



