185*7.] upon Views Entertained. 247 



In the earlier stages of growth in the young exogen, its structure 

 is entirely cellular ; after a short time we perceive wedge-shaped 

 bundles, edges of which point toward a common center, arranged 

 around a central cellular mass, called pith, which is connected with 

 the outer bark by means of cellular processes called medullary rays. 

 At first the pith is. large, the medullary rays, also, are of cousidera- 

 able thickness. Subsequently, by the increase of the old formations, 

 and the development of new ones between them, the medullary rays 

 become more restricted. Such is the young tree during the first 

 year ; at the end of the second year, the shoot is found to have in- 

 creased in diameter, by the foundation of a zone of vessels consist- 

 ing of porous and woody tissue, and a zone of fibrous bark, the 

 medullary rays still continuing outwards. Thus the young tree is 

 yearly increased, one layer or zone after another. We will here 

 briefly notice functionally, these several parts. First, the medullary 

 tube, occupying the center of the stem and lining the innermost 

 layer of the wood containing the pith. Its walls are formed of long 

 parallel vessels disposed longitudinally. The pith, or central por- 

 tion, consists of a cellular tissue, the cells diminishing toward the 

 circumference. The pith is generally of a greenish color at first, 

 and full of fluid ; subsequently this disappears, leaving a light col- 

 ored, spongy, dry mass. The office of the pith is not fully under- 

 stood, but is supposed by some to aff'ord nourishment to the youno- 

 buds, being often filled with dextrine, or starch, convertible into 

 sugar by the process of vegetation. According to the celebrated 

 Hales and Dutrotchet, the pith performs a very important part 

 in the phenomena of vegetable growth, and the bark and the pith 

 have been generally considered the source of the buds; but the in- 

 genious experiments of Mr. Knight have settled both of these hy- 

 potheses, and have established, as far as the present state of science 

 will permit, the doctrine that the buds derive their origin from the 

 albuminous portion of the tree. The alburnous vessels terminat- 

 ing upward, inwardly join the central vessels, which, deriving their 

 origin from the alburnous tubes, convey nutriment, and probably 

 give existence to new buds and leaves. The medullary sheath con- 

 sists of fibro-vascular or spiral vessels, which sheath includes the 

 pith. A few woody fibers are occasionally intermingled with these 

 spiral vessels. This sheath, according to some physiologists, is in 

 direct communication with the leaf buds, and leaf veins, and carries 



