266 





VEGETABLE 



bark and pith, are shown by Fig. 234. The texture of these 

 rays is cellular, and hence they were once regarded as pro- 

 cu^-ses of the pith, and were named accordingly, the pith 

 being considered the medulla, or marrow of the plant. But 



Fig. 234. 



that this is an error, is shown by the fact that the first layer 

 of wood next the pith has no medullary rays, though every 

 subsequent circle is crossed by them. 



The Pith is composed entirely of cellular tissue, the 

 cells of which are at first filled with watery fluid, but this 

 is gradually exchanged for air, becoming dry, except near 

 the final buds, before the first layer of wood is perfected. 



The pith is usually white, but in some plants it is colored, 

 being yellow in the Berberry, pale brown in the Walnut, 

 and fawn-colored in the Sumac. 



The diameter of the pith in many herbaceous plants, and 

 in the majority of trees, continues the same in the full grown 

 plant, and the young twig. In a few instances, as in the 

 Elder, it diminishes with the age of the plant, being largest 

 in the young shoot. 



THE ROOT. 



In general, the root may be defined as that part of the plant 

 by which it is fixed to the soil. But there are several excep- 



