118 



THE STEM. 



196. The vertical section in Fig. 156 passes directly through 

 the middle of one of the woody plates that collectively compose 

 the layer ; and therefore the medullary rays do not appear. But 

 in the much more magnified Fig. 157, the section is made so as to 

 show the surface of one of these plates, and one of the MEDULLARY 

 RAYS passing horizontally across it, connecting the pith (p) with 

 the bark (Z>). These medullary rays form the silver-grain, (as it 

 is termed,) which is so conspicuous in the Maple, White Oak, Red 

 Oak, &c., and which gives the glimmering lustre to many kinds 

 of wood when cut in this particular direction. But a section made 

 as a tangent to the circumference, and therefore perpendicular to 

 the medullary rays, brings their ends to view, as in Fig. 158; 



much as they appear when seen on the surface of a piece of wood 

 from which the bark is stripped. They are evidently composed of 

 condensed parenchyma merely, and their origin has already 

 been explained (191). They represent the horizontal system of 

 the wood, or the woof, into which the vertical woody fibre, &c., or 

 warp, is interwoven. . The inspection of a piece of oak or maple 

 wood at once shows the pertinency of this illustration. 



197. The Bark, in a stern of a year old, must next be more atten- 

 tively considered. At first it consisted of simple cellular tissue, or 

 parenchyma, undistinguishable from that of the pith, except that 

 it assumed a green color when exposed to the light, from the pro- 

 duction of chlorophyll (87) in its superficial cells. But during the 

 formation of the proper wood, an analogous formation occurs in 



FIG. 157. 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 (b) : magni- 

 fied. But a section can seldom be made so as to show one unbroken plate stretching across the 

 wood, as in this instance. 



FIG. 153. A vertical section across the ends of the medullary rays ; magnified. 



