PHYSIOLOGY OF TREES. 243 



lively large in young shoots, but gradually be- 

 coming less, and in very old trees hardly percep- 

 tible. It appears in the crowns of the roots as 

 well as in the stem, and extends through the 

 branches into every the minutest twig. As an 

 organ, its use is not apparent, as it is the first 

 to decay ; and stems and even young shoots live 

 very well and long after being deprived of it. It 

 most probably only acts as a support to the 

 infant stem, and as a reservoir of moisture to 

 sustain it against drought. 



Of the Jibrous cylinder,— This principal organ 

 of the stem surrounds the pith, and is composed 

 of a closely united body of ligneous fibres (em- 

 bedded in cellular membrane, the cellules of 

 which are arranged horizontally), forming, of and 

 among each other, various sized tubes and in- 

 terstices, perpendicularly arranged, and extending 

 from the collet or bottom of the shoot to its utmost 

 height ; and from the collet downwards over 

 every root. At the end of the summer's growth 

 it forms a cone, embracing the pith already de- 

 scribed. During its growth it is called cambium ; 



R 2 



