THE INNER TISSUES OF PLANTS. 277 



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 physio- 

 logical 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 verte- 

 brate axis ( 254-7), it was pointed out that in a transverse- 

 ly-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 cylin- 

 der, 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 dicoty- 

 ledonous axis is such a cylinder so strained. The main- 

 tenance 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 



