422 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



factory, yet we have no evidence that the continuity of the whole 

 water column is ever during transpiration interrupted in plants. 



Here it will be interesting to consider the structure of the con- 

 ducting 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 

 waterways 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 evi- 

 dently a most unexpected configuration. It is true that the parti- 

 tions 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 is not inappreciable. This becomes clear from the experiments 

 of Bohm, 1 Elfving, 2 and Strasburger, 3 comparing the conductivity 

 of wood in tangential and longitudinal directions. From them is seen 

 that the pressure required to force water in a tangential direction is 

 immense^ greater than that needed to urge it longitudinally in the 

 wood, although in both eases 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: 



x J. B5hm, De la cause du mouvement de 1'eau et de la faible pression do l'air dans 

 les Plantes. Ann. d. Sci. Nat. Bot.; XII, 1881, p. 233. Idem, Ueber die Ursache der 

 Wasserbewegung und der geringeren Lufttension in transpirirenden Pflanzen. Bot. Ztg., 

 49. 1881. 



2 Fr. Elfving, Ueber die Wasserleitung im Holz. Bot. Ztg.. 42. 1882. 



3 Ed. Strasburger, Ueber den Bau und Verrichtungefl der Leitungsbahnen in den 

 Pflanzen. Jena, 1S91, p. 739. 



