244 PHYSIOLOGY OF TREES. 



but when arrived at its full size, which it does 

 in about seven months, it receives the name 

 of white wood, or alburnum. It is the origin of 

 the perfect wood, for ever after occupies the 

 centre of the stem, and retaining' its first form, 

 position, and dimensions unaltered. The lig- 

 neous tissue of which it is composed longitudi- 

 nally, is crossed by rays* of the same substance, 

 which converge from the bark to the pith. The 

 cambium is the seat of the vitality of the plant; it 

 contains the rudiments of both roots and shoots, 

 as well as that specific energy which, under 

 the excitement of surrounding elements, produces 

 all the future expansion of the vegetable being-. 



» Medullary rays is an old term amongst physiologists; 

 but the term and the appearance of the rays on a transverse 

 section have led into error; they have been conceived to 

 be only simple line?, and, as such, have been supposed to be 

 the umbilical tracks or rudiments of buds. But this idea 

 cannot be admitted as probable ; because medullary rays 

 exist in the internodal parts of stems where no buds ever 

 appear; and, moreover, these rays are not simple lines, but 

 vertical partitions of the grain of \vood, extending from the 

 bottom to the top of the stem, dividing the circumference 

 into triangular segments. 



