30 WOODY STEM. 



row of the cut cells is seen in the preceding fiornre (Fic^. 24), 

 marked mr. By a longitudinal section made in the direction of 

 these lines, we shall find that they are plates of substance proceed- 

 ing from the pitn and terminating in the bark. They are called 

 mechdlary processes^ or medullary rays, and are composed of cells 

 in the form of thin parallelopipedons. In the embryo, and in the 

 earliest development of the stem, the cellular substance of the 

 bark and pith, are in contact, but immediately vascular and 

 woody fibers are sent down, which pierce the cellular substance, 

 dividing the mass of the pith from the parenchyma of the bark, 

 but leaving them connected by the medullary processes ; so that 

 parts which were in contact in the early stages, become separa- 

 ted, sometimes by several feet, yet a communication is preserved 

 by the medullar}^ rays, which continues as long as life lasts. 



56i Each of the layers of wood, as we before remarked, is 

 the product of a single year, and by counting these layers the 

 age of the tree at the point of section is readily determined, 

 and by finding the diff'erence 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 pith, and that of the second year forming a layej 

 around the product of the first year and the pith ; the pith ex- 

 tending 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 fiber, the layer of 

 woody fiber and liber always being in contact when the layers 

 are completed. We may readily convince ourselves of the in- 

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

 through the bark, but not so as to touch the wood, 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 layers of wood that are 



How are the pith and bark in the earher stages of growth ? — 56. How 

 often are layers of wood produced ? How can the age of a tree be deter- 

 mined ? How are trees composed ? How is the growth of wood com- 

 pared with that of the bark ? How may we convince ourselves of this ? 



