645 



wiil become easy and rapid. What further must happen ? "When 

 the branch or shoot recoils, the vessels on the .side that was convex, 

 oeiug relieved from pressure, will tend to resume their previous 

 diameters ; and will be helped to do this by the elasticity of the sur- 

 rounding tissue, as well as by those spiral, annular, and allied struc- 

 tures 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 tha 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 not all. 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 prosenchyma 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 compression. 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 same way. There is thus set up a draught of sap 

 to the place where these intermittent strains are going on, an exuda- 

 tion proportionate to the frequency and intensity of the strains, 

 and a proportionate nutrition or thickening of the wood- 

 cells, fitting them to resist the strains. A rude idea of 

 this action may be obtained by grasping in one hand a damp 

 sponge, having its lower end in water, while holding a piece of 

 blotting-paper in contact with its upper end, and then giving the 

 Bponge repeated squeezes. At each squeeze some of the water will 

 be sent into the blotting-paper ; at each relaxation the sponge will 

 refill from below, to give another portion of its contents to tlw 

 blotting paper when again squeezed. 



But how does this explanation apply to roots ? If the formation 

 of wood is due to intermittent transverse strains, such as are pro- 

 duced in the aerial parts of upright plants by the wind, how does it 

 happen that woody matter is deposited in roots, where there are no 

 lateral oscillations, no transverse strains? The answer is, that 

 longitudinal strains also are capable of causing the effects described. 

 It is true that perfectly straight fibres united into a bundle and pulled 

 lengthways would not exert on one another any lateral pressure, and 

 would not laterally compress any similarly-straight canals running 

 ilong with them. But if the fibres united into a bundle are variously 

 bent or twisted, they cannot be longitudinally strained without com- 

 pressing one another and structures imbedded in them. It needs 



