48 Heury H. Dixon. 



to resist crushing forces.^) Such strengthenings are quite meaningless 

 from the point of view of the imbibition and the various vital hypo- 

 theses; and even according to those views which regarded the sap 

 pressed upwards by gas or atmospheric pressure they are needlessly 

 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 -), 

 when, according to the theories just alluded to, they w^ould be exposed 

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

 the tracheae of the leaves forbids us accepting Elf ving's view that 

 they protect the tubes from the pressure of the growing tissues. 

 If needlessly bulky they are disadvantageous because they produce 

 friction and introduce turbulent motion into the upward stream. 

 Ewart finds ^) that, owing to the pressence of these thickenings and 

 to the transverse walls, the flow of water through the capillary tubes 

 of plants (viz. tracheae) is only about half what we would expect to 

 find calculating the flow bj' Poiseuille's formula. Consequently 

 for ordinary methods of transference assumed in earlier theoi'ies the 

 tracheae of the plant cannot be regarded as very efficient. For the 

 transmission and stability of a tensile stream, however, these thickenings 

 are essential. And their strength, so far from being superfluous, is 

 probably often tested severely in times w^hen the transpiration removes 

 large quantities of water and so develops high tensions in the sap. 

 The whole wall is not thickened uniforml}', because the permeability 

 of the thinner parts is essential. The thickenings confer on the thin 

 w^alls the rigidity necessary to support the tensile stresses in the sap. 

 It is interesting to find that we often have indications that the 

 unsupported wall would not in itself have sufficient rigidity to bear 

 the crushing forces it is exposed to. These indications are particularly 

 frequent in the protoxylem.^) Here commonly, when elongation has 

 widely separated the rings and spirals, the thin part of the walls 

 of the vessels is drawn in as a constriction between the spiral or 

 annular supports, and often the whole vessel is collapsed if the 

 supports have become too oblique. That this is not due to the pressure 

 exerted by the growth of the surrounding tissues follows from the 

 fact that these instances are most frequently found in leaves. 



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



^) As appears from the fact that Avater is pressed backwards from the leaf cells 

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

 the Ascent of Water in Trees. Ann. of Bot., Vol. X, 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. 



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

 Roy. Soc. Lond., Vol. 198, (1905), B, p. 50. 



'') H. H. D i X n , Physics of the Transpiration Current. Notes from the Botanical 

 School, Trinity College, Dublin, No. 2, 1897, p. 14. 



