THE INNER TISSUES OF PLANTS. 285 



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elongated cells, indirectly caused by the continued action of 

 the currents carried. 



§ 281. Here, however, presents itself a further problem. 

 Taking it as manifest that there is a typical distribution of 

 supporting tissue adapted to meet the mechanical strains a 

 plant is exposed to by its typical mode of growth, and also 

 that there goes on special adaptation of the supporting tissue 

 to the special strains the individual plant has to bear; and 

 taking it as tolerably evident that the sap-channels are 

 originally determined by the passage of currents along lines 

 of least resistance ; there still remains the ultimate question — 

 Through what physical actions are established these general 

 and special adjustments of supporting tissue to the strains 

 borne, and these distributions of nutritive liquid required to 

 make possible such adjustments? Clearly, if the external 

 actions produce internal reactions ; and if this play of actions 

 and reactions results in a balancing of the strains by the 

 resistances; we may rationally suspect that the incident 

 forces are directly conducive to the structural changes by 

 which they are met. Let us consider how they must work. 



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 compress 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, 



