88 THE MECHANICAL FUNCTIONS. 



diameter. From the same cause the pith cannot 

 increase in size ; and is even found to diminish 

 by the pressure of the surrounding wood. Thus 

 the vertical elongation of the entire stem con- 

 tinues during the whole of the second year, and 

 the trunk becomes sufficiently strengthened by 

 the addition of the second layer on its outside 

 to bear this increase of its height. 



While this process is going on in the wood, 

 corresponding changes take place in the bark, 

 and a new layer is added on its inner surface, 

 or that which is contiguous to the wood. This 

 layer constitutes the liber. All these new depo- 

 sitions must of course tend to stretch the outer 

 portions of the bark, which had been first formed, 

 and which yield to this pressure to a certain 

 extent ; but, becoming themselves consolidated 

 by the effects of the same pressure, they acquire 

 increasing rigidity ; and, the same cause con- 

 tinuing to operate, they at length give way, 

 in various places, forming those deep cracks, 

 which are observable in the bark of old trees, 

 and which give so rugged an appearance to 

 their surface. The cuticle has, long before this, 

 peeled off, and has been succeeded by the con- 

 solidated layers of cortical envelope which form 

 the epidermis. But the epidermis, which is con- 

 tinually splitting by the expansion of the parts 

 it encloses, itself soon decays, and is constantly 

 succeeded by fresh layers, produced by the 



