I.] GENERAL CHARACTERS AND STRUCTURE. 5 



clear from comparison of what has been said, and of 

 the two figures, that the " annual rings " are simply 

 the expression in cross-section of cylindrical sheets 

 laid concentrically one over the other, the outermost 

 one being that last formed. But on examining the 

 medullary rays in such a piece of timber as that in 

 Fig. 2, it will be noticed that they also are the expres- 

 sion of narrow radial vertical plates which run through 

 the concentric sheets : the medullary rays are in fact 

 arranged somewhat like the spokes of a paddle-wheel 

 of an old type of steamer, only they differ in length, 

 breadth, and depth, as seen by comparing the three 

 faces of the figure. It is to be noticed that the 

 medullary rays consist of a different kind of tissue 

 from that which they traverse, a fact which can only 

 be indicated in the figure by the depth of shading. It 

 is also to be observed that the " annual rings " show 

 differences in respect to their tissue, as marked by the 

 darker shading near the boundary lines on the outer 

 margin of each ring. In order to understand these 

 points better, it is necessary to look at a piece of our 

 block of timber somewhat more closely, and with the 

 aid of some magnifying power. For the sake of 

 simplicity it will be convenient to select first a piece 

 of one of the timbers known as "deal" (firs, pines, 

 &c.), and to observe it in the same direction as we 



