545 



will become easy and rapid. What further must happen 1 When 

 the brauL'li or shoot recoils, the vessels on the side that was convex, 

 oemg relieved from pressure, will tend to resume their previous 

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

 roimding- tissue, as well as by those spiral, annular, and alhed struc- 

 tures which they contain. JBut this resumption >f their previous 

 €liameters must cause un immediate rush of sap back into them. 

 Whence will it come? Not to any considerable extent from the sur- 

 rcunchng tissues into which part of it has been squeezed, seeing ( hat 

 the resistance to the return of hquid 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 durmg the time when the side of the branch or twig becomes 

 concave, more sap returns from above or below than was expeUed 

 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 rehlled 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 strauis. A rude idea of 

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

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

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

 sponge repeated squeezes. At each squeeze some of the water wih 

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

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

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

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

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

 longitudinal strains also are capable of causmg 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 smiilarly-sti-aight canals running 

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

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

 pressing oue another and structures imbedded in them. It needs 



