46 Henry H. Dixon. 



making the preparation for examination and were non-existent when 

 the plant was transpiring. 



In E wart's experiments the internal and external pressures 

 were not given time to come into equilibrium ; consequently supposing 

 20 7o or 30 % of the vessels contained continuous water while the 

 remaning 70 % — 80 % contained gas at reduced pressure, as soon as 

 the branch was cut across atmospheric pressure would drive the 

 water from the full vessels opening on to the cut surface into those 

 which contained gas at a low pressure, and vessels which had been 

 full and transmitting a tensile stress during transpiration, would 

 appear almost emptj^ after cutting. Ewart himself considers that 

 10 "o of the vessels of last year's wood ') would transmit enough to 

 cover the losses of the most vigorous transpiration. P^ven if a much 

 larger percentage than this were completely filled with water at the 

 time of cutting, they would elude obtervation in this method of 

 investigation. 



Both Strasburger and Ewart have shown that coloured 

 liquids rise most rapidly in the tracheae containing unbroken columns. 



It will be seen that although our knowledge as to the actual 

 proportion of tracheae containing bubbles during transpiration is very 

 unsatisfactory, 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 

 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 unexpected configuration. It is true that the partitions 

 are permeable to Avater; 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 Böhm^), Elfving^) and Stras burger^), comparing the con- 



') A. J. Ewart, On the Ascent of Water in Trees. Phil. Trans. Roy. Soc. 

 Lond., Vol. 199, (1908), B, p. 362. 



■-) J. Böhm, De la cause du mouvement de l'eau et de la faible pression de 

 l'air dans les Plantes. Ann. d. Sei. Nat. Bot., XII, 1881, p. 233. Idem, Ueber die 

 Ursache der Wasserbewegung und der geringeren Lufttension in transpirirenden 

 Pflanzen. Bot. Ztg., 49. 1881. 



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



*) Ed. Strasburger, Ueber den Bau und Verrichtungen der Leitungsbahnen 

 in den Pflanzen. Jena 1891, p. 739. 



