56 VEGETABLE PHYSIOLOGY. [LESSON 20. 



these rays are large, but in the more advanced woody stem, they ap- 

 pear as lines only. This tissue constitutes the " silver grain " of ma- 

 ple, and other trees, when cut in the direction from the pith to the 

 bark. 



329. They do not proceed in a continuous line, however, from the 

 top to the bottom of the tree, but pass through the woody fibres in 

 such a way as to be interrupted in their course. 



330. The medullary rays in some plants (Clematis) are large and 

 broad, while the woody wedges are comparatively small ; in most exo- 

 genous plants these rays are complete, but in the Cork-oak, and 

 others, they only extend partially through the stem. 



331. Cambium Layer. This layer is found between the wood 

 and the bark, and has been originally connected with both. 



332. It is composed of a layer of nucleated cells, formed in a 

 mucilaginous fluid called Cambium, and they are concerned in the 

 formation of the woody tubes of the inner bark, and in the addi- 

 tions made to the cells of the medullary rays. 



333. In the Spring of the year, during the flow of the sap, these 

 cells are actively engaged in the process of growth, at which time the 

 bark may be easily separated from the wood. 



334. The Bark. Originally this tissue is composed of uniform 

 cellular tissue, resembling that of the central part of the stem ; trans- 

 formations take place, however, in the progress of growth, by which 

 fusiform (spindle shaped) tubes are formed in the inner portion of 

 the bark next to the woody circle. 



335. This portion is called the inner bark ; it consists of woody 

 fibre, and some lactiferous vessels ; it is the fibrous part of the bark, 

 and is frequently called Bass, or Bast tissue. 



336. These fibres are long and tenacious, and are employed ex- 

 tensively for economic purposes.; those of the Lime-tree, Hemp, 

 Nettle, and Daphne canndbina, are employed for different articles 

 of useful manufacture. 



337. Sometimes the fibres separate, so as to form meshes, as in 

 the Lace-bark tree ; at other times they form a continuous layer, as 

 in the Horse-chestnut. 



338. The most remarkable fact in connection with woody fibre, 

 is its immunity from decay ; worn to rags, in an apparent state of 

 thorough disintegration as linen, it is doomed again to meet our gaze 

 in a new form as paper. 



339. The value and importance of woody fibre, as applied to do- 

 mestic manufactures, cannot be overrated, and appears to have been 



