EXOGENOUS STRUCTURE. 



117 



able that this is the only part of an exogenous stem in which spiral 

 vessels ordinari- 

 ly occur. They 

 may be detected 

 by breaking 



a 



woody twig in 

 two, after dividing 

 the bark and most 

 of the wood by a 

 circular incision, 

 and then pulling 

 the ends gently 

 asunder, when 

 their spirally coil- 

 ed fibres are read- 

 ily drawn out as 

 gossamer threads. 

 They are shown 

 in place in the 

 vertical section, 

 Fig. 156, I. 



195. The Wood 

 (Fig. 156, c) con- 

 sists of proper 



woody tissue, among which the vascular is more or less copiously 

 mingled, principally in the form of dotted ducts (d), or occasion- 

 ally some spiral or annular ducts (e), &c. The dotted ducts are of 

 so considerable calibre, that they are conspicuous to the naked eye 

 in many ordinary kinds of wood, especially where they are accu- 

 mulated in the inner portion of each layer, as in the Chestnut and 

 Oak. In the Maple, Plane, &c., they are nearly equably scattered 

 through the annual layer, and are of a size so small that they are 

 not distinguishable to the naked eye. 



FIG. 154. Longitudinal and transverse section of a stem of the Soft Maple (Acer dasycar- 

 pum) at the close of the first year's growth ; of the natural size. 



FIG. 155. Portion of the same, magnified, showing the cellular pith, surrounded by the 

 wood, and that inclosed by the bark. 



FIG. 156. More magnified slice of the same, reaching from the bark to the pith : a, part of 

 the pith; 6, vessels of the medullary sheath; c, the wood; dd, dotted ducts in the wood; ee, 

 annular ducts; /, the liber, or inner, fibrous bark; g, the cellular envelope, or green bark; ft, 

 the corky envelope ; i, the skin or epidermis; j, one of the medullary rays, seen on the trans- 

 verse section. 



