552 



Another side of the general question may now be con sidered. We 

 have seen how, by intermittent pressures on capillary vessels and 

 duots and inosculating canals, there must be produced a draught of 

 sap towards the point of compression to replace the sap squeezed out. 

 Eut we have still to inquii-e what will be the effect on the diotribu- 

 tion of sap throughout the plant as a whole. It was concluded that 

 ont of the compressed vessels the greater part of the liquid would 

 'Jbcape longitudinally — -the longitudinal resistance to movement being 

 [eaiit. In every case the probabilities are infinity to one against the 

 res'stances being equal upwards and downwards. Always, then, 

 moi e sap will be expelled in one direction than in the other. But in 

 whichever direction least sap is expelled, from that same direction 

 most sap wiU return when the vessels are reheved from pressure — tlw 

 force which is powerful in arresting the back current in that direction 

 being the same force which is powerful in producing a forward cui'- 

 rent. Ordinarily, the more abundant supply of liquid being from below, 

 there will result an upward current. At each bend a portion of the con- 

 tents will be squeezed out through the sides of the vessels — a portion 

 will be squeezed downwards, reversing the current ascending from the 

 roots, but soon stopped by its resistance ; while a larger portion will 

 be squeezed upwards towards the extremities of the vessels, where 

 consumption and loss are most rapid. At each recoil the vessels will 

 be replenished, chiefly by the repressed upward current ; and at the 

 next bend more of it will be thrust onwards than backwards. Hence 

 wc have everywhere in action a kind of rude force-pump, worked by 

 the wind ; and we see how sap may thus be raised to a height far 

 beyond that to which it could be raised by capillary action, aided by 

 osmose and evaporation. 



Thus far, however, the argument proceeds on the asumption that 

 there is hquid enough to replenish every time the vessels subject 

 to this process. But suppose the supply fails — suppose the roots 

 have exhausted the surrounding stock of moisture. Evidently the 

 vessels thus repeatedly having their contents squeezed out into the 

 surrounding tissue, cannot go on refiUing themselves from other 

 vessels without tending to empty the vascular system. On the one 

 hand, evaporation from the leaves causing a draught on the capillary 

 tubes that end in them, continually generates a capillary tension up- 

 wards ; while, on the other hand, the vessels below, expanding after 

 their sap has been squeezed out, produce a tension both upwards 

 Mnd downwards towards the point of loss. Were the limiting mem- 

 branes of the vessels impermeable, the movement of sap would, under 

 these conditions, soon be arrested. But these membranes are perme- 

 able; and the surrcunding tissues readily permit the passage of air. 

 This state of tension, then, wfll cause an entrance of an- into the tubes ; 

 the columns of liquid they contain will be interrupted by bubblct. 

 It seems, indeed, not improbable that this entrance of air may tftki- 



