EXOGENOUS STRUCTURE. 



119 



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

 stem grows 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 where 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 annual 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, b), 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 spirally coiled fibres are 

 readily draAvn 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 



icraQcrgiE'l 





ca 



192 



193 



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

 show 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 unbroken plate stretching across 

 the wood, as in this instance. 



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



