142 SEAT OF VEGETABLE LIFE. 



Judging, then, from these changes, about which 

 there can be no doubt (because of them we have ocular 

 proof), we may conceive that the vital envelope is 

 constructed of an indefinite number of distinct con- 

 centric layers, two of which are developed annually ; 

 the inner one (A, Fig. 46) being inflated into alburnum, 

 and the outer one (B, Fig, 46) into a layer of liber. 



Segment of a transverse section of a tree five years old, magnified : 

 a, growth of alburnum first year ; 6, the second ; c, the third ; 

 d, the fourth ; e, the fifth ; /, five layers of liber, ideally mag- 

 nified ; #, epidermis and cuticle. 



The appearance of the structure of the alburnum 

 affords confirmation of the reasonableness of this idea. 

 If we examine it as soon as it is formed, or in any 

 future stage of its existence, we find the longitudinal 

 fibres strongly and distinctly marked,* and the minute 

 vesicles of the cellular fabric between the fibres posited 

 horizontally ; showing that they are enlarged in the 

 same direction that is, advanced from the centre of 

 the tree outwards. (Fig. 2.) 



This hypothesis is only objectionable, perhaps, 

 on the ground of the difficulty of conceiving how 

 such a mass of organisation, forming the extended 

 trunk of a full grown tree, can be contained in 

 such a slender space as that between the liber and 



