xvi INTRODUCTION. 



traces of cellular structure, is supposed to be 

 destined to be converted into the wood and bark 

 of the current year. It probably owes its origin 

 to the newly -formed wood and bark with which 

 it is in contact, but keeps up the connnunication 

 with the pith by means of thin plates of cellular 

 tissue, called medullary rays, or the silver grain, 

 which radiate from the centre to the circumference 

 throughout the whole length of the stem : they 

 are very conspicuous in wainscot oak. 



Exogenous plants derive their name from de- 

 positing successive layers of wood outside that 

 previously formed : in these the wood nearest 

 the centre is the oldest and hardest, and the 

 stem is largest near the base. Endogenous trees, 

 on the other hand, have no central column of 

 pith, nor are the vascular and cellular systems 

 defined ; but every new deposit of wood is made 

 within a trunk, which, when once formed, does 

 not alter in size during any period of its exist- 

 ence : consequently, the stem, is constantly be- 

 coming more and more compact, the most perfect 

 and the hardest part of its substance being near 

 the circumference. Of this class of trees we have 

 no British specimens ; but in many, of the Palms, 

 which attain a great age, the outside becomes so 

 hard as to withstand a blow of a hatchet. 



The structure of the branches of Exogenous 

 trees is precisely similar to that of the trunk, 

 each one consisting of a central colmnn of pith, 

 a medullary sheath, and a number of rings of 

 wood and bark corresponding to the age of the 

 branch, these various parts being severally con- 

 tinuous with the same organs in the trunk. Even 

 the topmost twig is but a repetition of the main 



