Structure of Wood. 



67 



is a layer of wood, h, which from the first shows a radiating struc- 

 ture the beginning of the " medullary rays." Outside of the wood, 

 there is formed the cambium layer, c, which is to become a Dew 

 layer of wood. Next a "bast" layer, or inner bark, b, beyond which 

 is the bark, r, of coarse cellular structure, and over all an epider- 

 mis, o, covering every part. 



241. When the wood has made two years of growth, the cross- 

 section shows the structure represented in the annexed engraving, 

 in which m (upper side) 



includes the pith m' ', 

 the spongy portion, and 

 m (lower side) the "me- 

 dullary sheath." The 

 wood, h, is the growth 

 of two years, and is sep- 

 arated by the line, jj. 

 The cambium layer is 

 shown at c, and outside 

 of this is the bark. Fig- 

 ures 1 to 7 Show the 51. Structure of Oak at two Years of Age. 

 medullary rays, which are continued outward through the wood and 

 into the bark-J^ The cross-section, Q, the radial section, Sp, and tan- 

 gential section, Sc, show the relation which these rays (called by car- 

 penters the "silver grain") bear to the other parts. In the oak 

 these rays are very conspicuous. They are also very apparent but 

 of much smaller size in the beech, plane tree, maple, etc., while in 

 other trees they are scarcely to be seen. 



242. The medullary sheath and rays are composed of condensed 

 cellular tissue, and although generally the latter extend through in 

 a radial line from the sheath to the bark, and into it, there are many 

 secondary rays that form in the wood, and have no connection with 

 the pith, or with the other rays. In the conifers, these rays become 

 reduced to lines in parallel bauds, too minute to be seen without a 

 microscope of high magnifying powers. 



243. As wood is seen under the microscope, it is made up of 

 elongated cells overlapping each other, and adhering by their sides. 

 They present a great variety of forms, which are often peculiar to 

 the families or orders to which they belong. Among these fibers, 

 there are u umerous ducts and passages. Some COM tain only air, others 



