TRANSPIRATION AND ASCENT OF SAP DIXON. 423 



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

 a larger portion of the conducting tissues useless than one confined 

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

 pletely 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 recognized 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 to resist crushing forces. 1 Such strengthenings are quite mean- 

 ingless from the point of view of the imbibition and the various vital 

 hypotheses; and even according to those views which regarded the 

 sap pressed upwards by gas or atmospheric pressure they are need- 

 lessly strong. For it has been shown that it is impossible to crush the 

 tubes of a leaf by an external pressure amounting to 30 atmospheres, 2 

 when, according to the theories just alluded to, they would be exposed 

 to one atmosphere at most. The presence of these thickenings in the 

 tracheae of the leaves forbids us accepting Elfving's view that they 

 protect the tubes from the pressure of the growing tissues. If need- 

 lessly bulky they are disadvantageous because they produce friction 

 and introduce turbulent motion into the upward stream. Ewart finds 3 

 that owing to the presence of these thickenings and to the trans- 

 verse walls, the flow of water through the capillary tubes of plants 

 (viz, trachea?) is only about half what we would expect to find 

 calculating the flow by Poiseuille's formula. Consequently for ordi- 

 nary methods of transference assumed in earlier theories the tracheae 

 of the plant can not be regarded as very efficient. For the trans- 



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



2 As appears from the fact that water is pressed backwards from the leaf cells into 

 the branches by pressures of about this magnitude. Cf. Report of a Discussion on the 

 Ascent of Waters in Trees. Ann. of Bot., vol. 10, Dec, 1898, p. 655. H. H. Dixon On 

 the Physics of the Transpiration Current. Notes from the Botanical School, Trinity 

 College, Dublin, No. 2, 1897, p. 28. 



3 A. J. Ewart, On the Ascent of Water in Trees (First Paper). Phil. Trans. Roy. 

 Soc. Lond., vol. 198, (1905), B, p. 50. 



