62 COMPOUND ORGANS OF PLANTS. 



forming a mere lining to the cavity, as in grasses and other 

 herbs. 



A species of jEschynomene, growing in China, has the whole 

 of its stem, which is about an inch thick, composed of a mass 

 of pith, covered by a very thin epidermis. Rice-paper is pro- 

 cured from the herbaceous stem of this plant by the following 

 process. The centre column of pith is cut spirally round the 

 axis with a sharp instrument into a thin lamina, which is then 

 unrolled, and may be made into sheets containing about a foot 

 square. The medullary sheath and the concentric zones of 

 wood are traversed by 



The Medullary Rays. These are numerous thin plates of 

 condensed cellular tissue, which pass from the pith to the cellu- 

 lar system of the bark, and maintain a communication between 

 them. These plates of cellular matter are to be seen on 

 the surface of the cross-section of most exogenous stems, 

 on which they appear as fine lines, radiating from the centre 

 to the circumference, but cannot be traced continuously to 

 any great extent in a vertical direction. The medullary 

 rays constitute the silver grain of the carpenters. They are 

 the remains of the cellular system of the stem, condensed into 

 lines by the adjacent pressure of the woody wedges. The cellu- 

 lar system of the stem is first formed in a horizontal direction, 

 and constitutes the matrix, or bed, into which ascends and 

 descends, in a longitudinal direction, the fibro-vascular or 

 woody system. The wood of the exogen is, in fact, made up 

 of a number of wedges of longitudinal fibro-vascular tissue, 

 embedded in the horizontal cellular tissue of the stem. The 

 base of each wedge is in contact with the inner surface of the 

 bark; the apex is next the pith and its sides are bounded by 

 the medullary rays, which are, as before stated, the remains of 



