Mil. MAINS VEGETABLE PHYSIOLOGY. 



409 



" At the end of September, and in many kinds of trees much sooner, the sections 

 appear as beneath. 



Sections of a stem at the end of the fifth year. The envelope and layers of liber are too 

 thin to be shown by the pencil. 



" Here we observe that a new concentric layer of alburnum has been added 

 during the fifth summer, and also an additional layer of liber has been parted off, 

 and placed close to that of the preceding year, and lined on the inner side with an 

 almost imperceptible membrane or coating of gelatinous matter, which is the vital 

 envelope, and from which the new growths of wood and liber of the next, and all 

 succeeding years will be produced. 



" Judging, then, from these changes, about which there can be no doubt (because 

 of them we have an ocular proof), we may conceive that the vital envelope is con- 

 structed of an indefinite number of distinct concentric 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. 



Fig. 46. 



Segment of a transverse section of a tree five years old, magnified -. a, growth of alburnum 

 first year ; b, the seeond ; c, the third ; d, the fourth ; e, the fifth ; /, five layers of liber, ideally 

 magnified; g, 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 

 VOL. i. NO. ix. (SEPTEMBER, 1833.) G G 



