Darwin, in the organs of attachment of climbing plants. Speaking of _Solanum jasminoides_ he says:--“When the flexible petiole of a half-or a quarter-grown leaf has clasped any object, in three or four days it increases much in thickness, and after several weeks becomes wonderfully hard and rigid; so that I could hardly remove one from its support. On comparing a thin transverse slice of this petiole with one from the next or older leaf beneath, which had not clasped anything, its diameter was found to be fully doubled, and its structure greatly changed.... This clasped petiole had actually become thicker than the stem close beneath; and this was chiefly due to the greater thickness of the ring of wood, which presented, both in transverse and longitudinal sections, a closely similar structure in the petiole and axis. The assumption by a petiole of this structure is a singular morphological fact; but it is a still more singular physiological fact that so great a change should have been induced by the mere act of clasping a support.” If there is a direct relation between mechanical stress and the formation of wood, it ought to explain for us the internal distribution of the wood. Let us see whether it does this. When seeking in mechanical actions and reactions the cause of that indurated structure which forms the vertebrate axis (§§ 254–7), it was pointed out that in a transversely-strained mass, the greatest pressures and tensions are thrown on the molecules of the concave and convex surfaces. Hence, supposing the transversely-strained mass to be a cylinder, bent backwards and forwards not in one plane but now in this plane and now in that, its peripheral layers will be those on which the greatest stress falls. An ordinary dicotyledonous axis is such a cylinder so strained. The maintenance of its attitude either as a lateral shoot or a vertical shoot, implies subjection to the bendings caused by its own weight and by the ever-varying wind. These bendings imply tensions and pressures falling most severely first on one side of its outer layers and then on another. And if the dense substance able to resist these tensions and pressures is deposited most where they are greatest, we ought to find it taking the shape of a cylindrical casing. This is just what we do find. On cutting across a shoot in course of formation, we see its central space either unoccupied or occupied only by soft tissue. That the layer of hard tissue surrounding this is not the outermost layer, is true: there lies beyond it the cambium layer, from which it is formed, the phloëm, and the cortex. But outside of the soft phloëm there is frequently another layer of dense tissue now known as the pericyclic fibres, having frequently a tenacity greater even than that of the wood--a layer which, while it protects the cambium and offers additional resistance to the transverse strain, admits of being fissured as fast as the cylinder of wood thickens. That is to