INTERNAL STRUCTURE. 75 



tomical structure of a wood}' exogenous stem of a year old is 

 displayed in the Fig. 131-133. Viewing the parts particularly, 

 and in order from centre to circumference, there is, - 



1st. The Pith or Medulla, consisting entirely of soft and rather 

 large thin-walled cells, 1 gorged with sap or other nourishing 

 matter during the growing state, becoming light, dry, and empty 

 when effete. 



2nd. The Layer of Wood, traversed by the medullary rays. In 

 Pines and other Coniferae, the wood is of uniform structure, being 

 wholly composed of a woody tissue with peculiar markings (Fig. 

 125, d, e) : in other wood, ducts of one or more sorts occur ; the 

 most conspicuous being what are termed clotted ducts. These 

 are so large as to be evident to the naked eye in many ordi- 

 nary kinds of wood, especially where they are accumulated in 

 the inner portion of the layer, as in the Chestnut and Oak. In 

 the Maple, Plane, &c., they are rather equably scattered through 

 the annual layer, and are too small to be seen by the naked eye. 

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

 some spiral ducts are uniformly found, and this is the only part 

 of the exogenous stem in which these ordinarily occur. The}' 

 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 coiled fibres 

 are readily drawn out as gossamer threads. As these spiral 

 ducts form a circle immediately surrounding the pith, they have 

 collectively been termed the medullary sheath, but they hardly 

 deserve a special name. The vertical section in Fig. 133 divides 

 one of the woody wedges, and shows no medullary ray ; but there 

 is one at the posterior edge of the transverse section. But, in the 

 much more diagramatic Fig. 134, the section is made so as to show 

 the surface of one of these plates, or medullary rays, passing hori- 

 zontally across it, connecting the pith (p) with the bark (b). 

 These medullary rays form the silver-grain (as it is termed) , which 

 is so conspicuous in the Maple, Oak, &c., and which gives the 

 glimmering lustre to many kinds of wood when cut in this direc- 

 tion. A section made as a tangent to the circumference, and 

 therefore perpendicular to the medullary rays, brings their ends 

 to view, as in Fig. 135, much as they appear on the surface of a 

 piece of wood from which the bark is stripped. They are here 

 seen to be composed of parenchyma, and to represent the horizon- 



1 In rare instances, a few fibre-vascular threads are found dispersed 

 through the pith, presenting a somewhat remarkable anomaly. This 

 occurs in Aralia racemosa, and more strikingly in Mirabilis and other 

 Nyctaginaceae, and in Piperacese. ( 133. foot-note.) 



