THE 1XXER TISSUES OF PLANTS. 269 



When any part of a plant is bent by the wind, the tissues 

 on its convex surface are subject to longitudinal tension, and 

 these extended outer layers cornjress the layers beneath 

 them. Such of the vessels or canals in these subjacent layers 

 as contain sap, must have some of this sap expelled. Part of 

 it will be squeezed through the more or less porous walls of 

 the canals into the surrounding tissue, thus supplying it 

 with assimilable materials ; while part of it, and probably 

 the larger part, will be thrust along the canals longitudinally 

 upwards and downwards. When the branch or twig or leaf- 

 stalk recoils, these vessels, relieved from pressure, expand to 

 their original diameters. As they expand, the sap rushes 

 back into them from above and below. In whichever of 

 these directions least has been expelled by the compression, 

 from that direction most must return during the dilation ; 

 seeing that the force which more efficiently resisted the 

 thrusting back of the sap is the same force which urges it 

 into the expanded vessels again, when they are relieved from 

 pressure. At the next bend of the part a further portion of 

 sap will be squeezed out, and a further portion thrust for- 

 wards along the vessels. This rude pumping process thus 

 serves for propelling the sap to heights which it could not 

 reach by capillary action, at the same time that it incident- 

 ally serves to feed the parts in which it takes place. It 

 strengthens them, too, just in proportion to the stress to be 

 borne ; since the more severe and the more repeated the 

 ^trains, the greater must be the exudation of sap from the 

 vessels or ducts into the surrounding tissue, and the greater 

 the thickening of this tissue by secondary deposits. By 



this same action the movement of the sap is determined 

 either upwards or downwards, according to the conditions. 

 While the leaves are active and evaporation is going on from 

 them, these oscillations of the branches and petioles urge 

 forward the sap into them ; because so long as the vessels of 

 the leaves are being emptied, the sap in the compressed 

 vessels of the oscillating parts will meet with less resistance 



