412 MR. SPENCER ON CIRCULATION AND 
that the tensions of the outer layer of a mass made convex by bending, must, by 
composition of forces, produce at every point a resultant at right angles to the layer 
beneath it—that, similarly, the joint tensions of these two layers must throw a pres- 
sure on the next deeper layer—and so on. Hence if, at some little distance beneath 
the surface of a stem, twig, or leaf-stalk, there exist longitudinal tubes, these tubes must 
be squeezed each time the side of the branch they are placed on becomes convex. 
Modifying the illustration just drawn from the clenched hand will make this clear. 
When, on forcibly grasping something, the skin is drawn tightly over the back of the 
hand, the whitening of the knuckles shows how the blood is expelled from the vessels 
below the surface by the pressure of the tightened skin. If, then, the sap-vessels must 
_be thus compressed, what will happen to the liquid they contain? It will move away 
along the lines of least resistance. Part, and probably the greater part, will escape 
lengthways from the place of greatest pressure, some of it being expelled downwards, and 
some of it upwards. But, at the same time, part of it will be likely to ooze through the 
walls of the tubes. If these walls are so perfect as to permit the passage of liquid only 
by osmose, it may still be inferred that the osmose will increase under pressure; and 
probably, under recurrent pressure, the places at which the osmotic current passes most 
readily will become more and more permeable, until they eventually form pores. At 
any rate it is manifest that where pores and slits exist, whether thus formed or formed 
in any other way, the escape of sap into the adjacent tissue at each bend will become 
easy and rapid. What further must happen? When the branch or shoot recoils, the 
vessels on the side that was convex, being relieved from pressure, will tend to resume 
their previous diameters ; and will be helped to do this by the elasticity of the surround- 
ing tissue, as well as by those spiral, annular, and allied structures which they contain. 
But this resumption of their previous diameters must cause an immediate rush of sap 
back into them. Whence will it come? Not to any considerable extent from the sur- 
rounding tissues into which part of it has been squeezed, seeing that the resistance to 
the return of liquid through small pores will be greater than the resistance to its return 
along the vessels themselves. Manifestly the sap which was thrust up and down the 
vessels from the place of compression will return—the quantities returning from above 
and from below varying, as we shall hereafter see, according to circumstances. But this 
is notall. From some side a greater quantity must come back than was sent away; for the 
amount that has escaped out of the tube into the prosenehyma has to be replaced. Thus 
during the time when the side of the branch or twig becomes concave, more sap returns 
from above or below than was expelled upwards or downwards during the previous com- 
pression. The re-filled vessels, when the next bend renders their side convex, again have 
part of their contents forced through their parietes, and are again refilled in the pan 
way. There is thus set up a draught of sap to the place where these intermittent strains 
are going on, an exudation proportionate to the frequency and intensity of the d 
pes ee na nutrition or thickening of the wood-cells, fitting them to Eme 
GE "gie ee of this action maybe obtained by grasping in one hand 2 p 
di «s g its lower end in water, while holding a piece of blotting-paper 1 à 
upper end, and then giving the sponge repeated squeezes. At each squeeze 
