72 



Structure of Wood, 



and consists of about 44 .4 per cent of carbon, 6.2 per cent of hy- 

 drogen, and^49.3 per cent of oxygen. 



256. However plants may differ in form and structure, they are 

 all the result of cell-growth. These cells form in the interior of 



l\ 



pre-existing cells by subdivision, aud, by pressing 

 upon one another, they elongate into fibrous forms, 

 as already shown on page 68. When macerated, 

 these fibers may be separated, and sometimes they 

 afford differences that enable us to determine the 

 class or group to which the woods belong. In the 

 coniferse, there are thin places along the sides of 

 the cells, which can be readily seen under the mi- 

 59 structure of Co- crosc P e > an d can be found even in the fibers of 



inferous Woods. p aper ma( J e f rom WOO( J. 



257. In some woods, as, for example, the oak, ash, walnut, etc., 

 the cells grow to a larger size, so as to be visible to the naked eye. 

 These form ducts, which are not continuous for great length, but 

 are intermingled in various forms, presenting in cross-sections the 

 appearance of pores. They usually contain air, and at certain sea- 

 sans sap. In ascending among these fibers, the sap passes up be- 

 tween and among the fibers, and through their partitions. In some 

 of the larger ducts, they have a spiral coil 

 of fiber, or the sides are made up of rings, 

 and they are sometimes marked with dots. 



258. In resinous woods, the turpentine is 

 contained in larger cavities, surrounded by 

 smaller cells. The abundance of this product 

 depends in a great degree upon the vigor 

 of growth, and exposure to the air and light. 



259. The annexed cut shows a section of the 

 linden six years old, enlarged about five di- 

 ameters, in which the concentric layers are 

 distinguished by a somewhat denser and 

 slightly colored line along their outer mar- 

 gin, and their relation to the bark is shown. 

 In this figure, a is the medullary sheath, b 

 the liber or inner bark, d the cellular tissues 

 of the bark, e the pith, and / the epidermis 



60. Section of a Llnrton Tree , , 



of .six Years 1 Growth. or outer bark. 



