iv COHESION THEORY OF ASCENT OF SAP 97 



most natural explanation viewed in the light of that 

 hypothesis. 



It has long been recognised that the thickenings found 

 on the walls of the trachea?, viz., the internal supports in 

 the form of annul i, spirals, and networks, are of such a 

 nature that they are pre-eminently suited to resist crush- 

 ing forces. Such strengthenings are quite meaningless 

 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 atmo- 

 spheric 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 would 

 be exposed at most to one atmosphere. 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 needlessly bulky 

 they are disadvantageous because they produce friction 

 and introduce turbulent motion into the upward stream. 

 Ewart finds that, owing to the presence of these thick- 

 enings 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 by Poiseuille's formula. Consequently, for ordi- 

 nary methods of transference assumed in earlier theories, 

 the tracheae of the plant cannot be regarded as 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 when the transpiration removes 

 large quantities of water, and so develops high tensions 

 in the sap. The whole wall is not thickened uniformly, 

 because the permeability of the thinner parts is essential. 

 The thickenings confer on the thin walls the rigidity 

 necessary to support the tensile stresses in the sap. 



H 



