EXOGENOUS STRUCTURE. 



119 



ea) it is early separated into a series of horizontal plates. As the 

 stem gi-ows older the pith becomes dry and light, its cells filled with 

 air only ; and then it is of no further use to the plant. 



212. The Wood consists of proper woody tissue (53), among which 

 the vascular is more or less copiously mingled, principally in the 

 form of dotted ducts (Fig. 191, d), or occasionally some 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 wher© they are accumulated in the inner portion of 

 each layer, as in the Chestnut and Oak. In the Maple, Plane, &c., 

 they are rather equably scattered through the amiual layer, and are 

 of a size so small, that they are not distinguishable by the naked eye. 

 — Next the pith, i. e. in the very earliest formed part of the wood, 

 some spiral ducts are uniformly found (Fig. 191, 5), and this is the 

 only part of the exogenous stem in which these ordinarily occur. 

 They may be detected by breaking a woody twig in two, after divid- 

 ing the bark and most of the wood by a circular incision, and then 

 pulling the ends gently asunder, when their spiraUy coiled fibres are 

 readily drawn out as gossamer threads. As these spiral ducts form 

 a circle immediately surrounding the pith, they form what has been 

 termed the Medullary Sheath. This is no special organ, and 

 hardly requires a special name, since it merely represents the earli- 

 est-formed vascular tissue of the stem. 



213. The vertical section in Fig, 191 divides one of the woody 

 wedges ; and therefore the medullary rays do not appear. But in 



FIG. 192. Vertical section through the wood of a branch of the Maple, a year old ; so as to 

 Bhow one of the medullary rays, passing transversely from the pith {p)to the bark (6) : mag- 

 nified. But a section can seldom be made so as to show one unbrolcen plate stretclLing across 

 the wood, as in this instance. 



FIG. 193. A vertical section across the ends of the medullary rays ; magnified. 



