44 DESCRIPTIVE BOTANY. PART I. 



clearly distinguishable, but the pith and bark are never 

 wanting. 



(48.) Pith. The vesicles of the pith are larger 

 and more regularly arranged than those of other parts. 

 It continues to increase in diameter as long as it re- 

 mains succulent, and in some trees, as the elder, it be- 

 conlPs more than half an inch thick ; but generally it 

 is much smaller. After it has lost its succulency and be- 

 come a dry spongy mass, it scarcely diminishes in size ; 

 but where the branch is much distended, the pith is 

 ruptured, and in some cases appears to be nearly ob- 

 literated. The stems then become hollow, as in many 

 umbelliferous plants. It always forms a continuous 

 mass through the whole stem ; but in some cases it is 

 -o much condensed and hardened as to resemble wood 

 at the places where the leaves are attached, as in the 

 horse-chestnut. 



Although it is generally without any fibres of vascular 

 tissue, such are found in some plants, as in the elder, 

 where they may be seen, in a transverse section, forming 

 a circle of red dots, a short dis- 

 tance within the medullary sheath. 

 In ferula communis there are so 

 many of these dispersed through 

 it, that the stem has the appear- 

 ance of belonging to a monocotyle- 

 donous plant (./?</ 35.). 



(49.) Mediilinry Slieath.The 

 fibres which compose the medul- 

 lary sheath, appear to retain their 

 vitality for a long time after the pith has been exhausted 

 and become dead ; and the trachea? which abound in it 

 may even be unrolled in old and dry wood. 



(50.) Wood. The woody layers seldom, if ever, con- 

 tain perfect tracheae ; but they are composed principally 

 of elongated cellular tissue, traversed by ducts of various 

 kinds. As the tree becomes aged, the innermost layers 

 grow darker and more solid, and are then termed the 

 '' Heart-wood/' or " Duramen." The outer layers, 



