INTRODUCTION. 



, xiii 



cording to the species, age, and circmxistances of 

 growth of the specimen selected. The centre, 

 called pith^ consists of an uninterrupted column 

 of cellular tissue, mthout any admixture of ves- 

 sels. The cells are hexagonal: in their early 

 stage they are green and filled with fluid ; as the 

 tree advances in age they become bro^ra and dry, 

 and sometimies hard. The pith never alters in 

 size after its first year's growth, and is generally 



TRANSVERSE SECTION OF EXOGENOUS TREE. 



found to be larger in strong lateral shoots than 

 in the main stem, of which the Elder affords a 

 marked example. It is enclosed in a thin tube 

 composed of woody fibre and spiral vessels, called 

 the medullary sheath (from medulla^ marrow, to 



