418 MR. SPENCER ON CIRCULATION AND 
dense substance, accumulating from generation to generation by the survival of the 
fittest, will result in an organic habit of forming dense tissue at the required places, 
The deposit arising from exudation at the places of greatest strain, recurring from gene- 
ration to generation at the same places, will come to be reproduced in anticipation of 
strain, and will continue to be reproduced for a long time after a changed habit of the 
species prevents the strain—eventually, however, decreasing, both through functional 
inactivity and natural selection, to the point at which it is in equilibrium with the 
requirement. 
Another side of the general question may now be considered. We have seen how, by 
intermittent pressures on capillary vessels and ducts and inosculating canals, there must 
be produced a draught of sap towards the point of compression to replace the sap 
squeezed out. But we have still to inquire what will be the effect on the distribution of sap 
throughout the plant as a whole. It was concluded that out of the compressed vessels 
the greater part of the liquid would escape longitudinally—the longitudinal resistance to 
movement beingleast. In every case the probabilities are infinity to one against the re- 
sistances being equal upwards and downwards. Always, then, more 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 will return when the vessels are relieved from pressure— 
the force which is powerful in arresting the back current in that direction being the 
same force which is powerful in producing a forward current. Ordinarily, the more 
abundant supply of liquid being from below, there will result an upward current. At 
each bend a portion of the contents 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 we 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 assumption that there is liquid 
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 refilling themselves from other vessels without 
vending 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 ? 
capillary tension upwards; while, on the other hand, the vessels below, expanding after 
m on out, produce a tension both upwards and downwards Br 
o inp ese cay pi the limiting membranes of the vessels impermeable, the mov ge 
E. ji dg un : Pa conditions, soon be arrested. But these membranes are pe ad 
i nding tissues readily permit the passage of air. This state of tensi 
