CELLULAR STRUCTURE OF TREES. 



changing seasons, and the prevalence of winds ; and that all these act with tenfold 

 greater effect upon a fidl grown than upon a very immature tree. It may therefore be 

 affirmed, that the zones of wood increase in length, and decrease in thickness, as the age 

 of the tree advances, and that both proceed from determinate causes, but that the 

 increase and decrease alike do not follow any rule of universal application. 



Moreover the width of the zones of wood, in the same species of tree growing in 

 different positions, is not the same. Thus the Scotch Fir (Pinus sylvestris), growing 

 at various altitudes, produces rings of wood varying from 0'39 lines, to 10 times that 

 amount. That such must be the case we may readily infer from the fact, that in any 

 plantation trees of the same species, and planted at the same time, attain, within a few 

 years, to very different dimensions. This dissimilarity is far greater when we compare 

 trees of various species ; but yet, in reference to all wooded plants, it may be stated that 

 there is a general resemblance in the size, both in height and thickness, which plants 

 of the same species attain in the course of years. 



Numerous efforts have been made to discover a relation between the height and the 

 thickness of trees ; but whilst there may be an approach to similarity in trees of the 

 same species, there is not a shadow of resemblance in wooded plants as a whole. Thus 

 it has been found, that of two species of Pine the difference was so great, that whilst 

 the relation was as 1 to 5 in one instance, it was as 1 to 120 in another. 



Such speculations may tend to increase a spirit of inquiry, but hitherto they have 

 had no other good effect. 



The foregoing description may suffice for exogenous stems which follow this usual 

 course of development, and therefore for the great majority of trees ; but it is readily 

 conceivable that a difference in figure may exist to a great extent, as in the cells of 

 cellular structure, considered at page 11. 



Fig. 139. 



Fig. 140. 



Fig. 141. 



Fig. 139, representing the section of a tree in which, from the irregular development of the stem, 



there are no concentric zones of wood. 

 Figs. 140 and 140*, showing, in the section of a stem of a Bignonia, four internal deposits of bark, a, 



by which the wood is divided into four wedges. The lines crossing the centre are well developed 



medullary rays. 

 Fig. 141. The stem of a Clematis, in which the medullary rays are greatly thickened, a, the pith ; 



b, the smaller ; and d, the larger wood bundles ; c, the large medullary rays ; e, the bark. 



Thus, whenever the process of growth is so disturbed that it proceeds on one side, 

 whilst it is nearly arrested on the other, it is evident that the figure of the stem will 



