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MEDULLARY RAYS.—BARK OR CORTEX. 89 
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year, which extend from the rings of those years respectively 
to the bark ; these are called secondary medullary rays. In the 
Cork-oak both kinds may be well seen in a transverse section 
(fig. 187, 1, 2, 3, 4). 
The medullary rays are composed of flattened six-sided cells, 
which are placed one above the other in one or more rows, like 
the bricks in a wall, hence the tissue which they form is termed 
muriform parenchyma ( figs. 185, B, 7, %, and 94 ). Itis a 
variety of tabular parenchyma, as already noticed (page 48). 
The tissue formed by the medullary rays is not continuous 
from one end of the wood to the other, but the rays are 
more or less interrupted by the passage between them of 
the fibro-vascular tissue forming the wood, so that they are 
Fic. 187. Fic. 188. Fie. 189. 
HL iltl 
(rayon) 
Fig. 187. Transverse section of a portion of the stem of the Cork-oak 
(Quercus Suber), four years old. m. Pith. 1. Medullary ray of the first 
year’s growth. 2, 3, 4. Medullary rays of successive years. pe. Liber and 
cellular envelope. s. Cork layers. Fig. 188. Surface of the stem of a 
Dicotyledonous tree from which the bark has been removed. Fig.189, 
Vertical section of a branch of the common Maple, perpendicular to the 
medullary rays. ji, fl. Fibro-vascular tissue forming the wood. 7m, 7m, 
Medullary rays. 
split up vertically into a number of distinct portions (jigs. 
188 and 189, rm). This arrangement may be observed by 
examining the surface of a stem from which the bark has 
been removed (jig. 188), or still better by making thin sections 
of the wood perpendicular to the rays,—that is, tangential to 
the circumference of the stem (fig. 189). In some stems, such 
as those of the species of Aristolochia, and also in many plants 
of the natural order Menispermacez, and in other orders, the 
medullary rays are very conspicuous, forming large plates 
between the wedges of wood. In other plants, such as the Yew 
and Birch, they are comparatively small. The medullary rays 
constitute the silver grain of cabinet-makers and carpenters, as 
it is to their presence that many woods, such as the Plane and 
Sycamore, owe their peculiar lustre. 
6. The Bark or Cortex. —The bark is situated on the 
outside of the stem, surrounding the wood, to which it is 
