40 THE STEM. 



of the bark, bat leaving them connected by the medullary 

 processes ; so that parts, which were in contact in the early 

 stages, become separated, sometimes by several feet, yet a 

 communication is preserved by the medullary rays, which 

 continues as longr as life lasts. 



47. Each of the layers of wood, as we before remarked, 

 is the product of a single year, and by counting these ia3ers 

 the age of the tree at the point of section is readily deter- 

 mined, and by finding the difference in the number of layers 

 between any two points of section, will determine the time 

 that the tree was growing the distance between the sections. 

 For instance, if we should count the layers of a stick of 

 timber, and find the number twenty at one end, and ten at 

 the other, it would show that ten years were required for the 

 tree to increase in length the distance between these points. 

 From these facts, we readily perceive, that trees must be 

 composed of concentric conical sheaths ; the product of the 

 first year forming such a sheath around the pitb, and that of 

 the second year forming a layer around the product of the 

 first year and the pith ; the pith extending through the whole 

 cone. We also observe, that the mode of increase is inverse 

 to that of the bark, for the bark we found increased by an 

 internal layer of cellular integument and liber, and we now 

 find that the wood increases generally by an external layer 

 of cellular matter and woody fibre, the layer of woody fibre 

 and liber always being in contact when the layers are com- 

 pleted. We may readily convince ourselves of the inverse 

 growth of the bark and wood, by inserting two wires, one 

 through the bark, but not so as to touch the M'ood, and in 

 time this wire will fall off, having no deposit made exterior 

 to it ; but by inserting the other wire, so that it shall pass 

 through a slight portion of the wood, instead of falling off, it 

 will become buried deeper and deeper every year by the lay- 

 ers of wood that are formed over it — thus proving most 

 clearly the order in which the wood and bark are formed. 



A remarkable case of the deposition of external layers of 

 exogenous stems is related of the Boabab tree (Adansonia 

 digitata) of the Cape de Verde Islands. In the year 1400, 

 Grew cut his name on two of these trees, and in 3 749 (three 

 hundred and forty. nine years afterwards) Adanson examined 

 the same trees and found the names, with more than three 

 hundred layers of wood deposited over them. If we examine 

 a transverse section of a trunk of a tree we observe, that the 

 wood near the pith and that near the bark present very dif- 

 ferent appearances ; the latter being white and soft, and more 



