186 The Rev. P. Keith on the Internal Structure of Plants. 



the plant. As the layers recede from the circumference, the 

 network which they form is finer, though still very irregular, 

 and their texture more compact. Yet the meshes of the dif- 

 ferent layers often correspond, and form, at least in aged 

 trunks, pyramidal apertures, or widen into large gaps or 

 chinks, as in the trunk of the Oak or Elm, exhibiting still the 

 rough traces of the original network. In young trees or shoots 

 the apertures formed by the coincidence of the meshes are not 

 yet left empty, but are occupied by a pulp, somewhat com- 

 pressed, which traverses the longitudinal fibres, and binds and 

 cements them together. 



In all trunks the inner layers are soft, smooth, and flexible, 

 and capable of subdivision till reduced to an absolute film, 

 but not always exhibiting a conspicuous network. The inner- 

 most layer of all is denominated the liber, the Latin term for 

 a book, from its having been used by the ancients to write on 

 before the invention of paper*. It is the finest and most de- 

 licate of the layers, and is often most beautifully reticulated, as 

 in the liber of Daphne Lagetto, remarkable beyond that of all 

 other plants for the beauty and delicacy of its network, and 

 soft and flexible as the finest lace. If the cortical layers while 

 yet young are accidentally injured, the part destroyed is again 

 regenerated, and the wound healed up without a scar ; but 

 if the wound extends beyond the liber, the part destroyed is 

 no longer regenerated. 



The Ligneous Layers. — The ligneous layers, or layers con- 

 stituting the wood, occupy the intermediate portion of the 

 stem between the bark and pith, and are distinguishable into 

 two different sets, concentric layers and divergent layers. 



The concentric layers, which constitute by far the greater 

 part of the mass of the wood, may be seen on the surface of a 

 horizontal section of almost any trunk or branch, as on that 

 of the Oak or Elm, particularly after being for some time ex- 

 posed to the weather. They have been believed to be equal 

 in number to the years of the plant's growth ; but they are 

 not literally or strictly so. Neither are they literally or 

 strictly concentric. On the one side or on the other there is 

 generally an excess of width as well as of number, not ac- 

 cording as it is exposed to, or sheltered from, the light and 

 heat of the sun, as some writers have affirmed, but according 

 to the accidental situation of the great roots and branches. 

 The inner layers are the hardest and the outer layers the 

 softest, and the outermost layer, which is the softest of all, is 



* Is not the converse of this the fact — Was not a book called liber be- 

 cause it consisted of the vegetable substance to which that appellation was 

 originally confined ? — Edit. 



