LECT. VII.] ANATOMY OF STEMS. 353 



relative to the origin and increase of the concen- 

 tric zones of wood, observable in the transverse 

 section of any tree or shrub of more than one 

 year's growth, and which has sprung from a seed 



stem or shoot. In the same manner, he supposes the liber is 

 formed by an equal elongation of the fibres df the interior 

 bark of the gem ; so that each gem has thus a double commu- 

 nication with the root of the tree. The latter part of our au- 

 thor's opinion closely resembles that promulgated by Dr. Dar- 

 win, who supporting the theory of the individuality of buds, 

 thus expresses himself: "The bark is only an intertexture 

 " of the caudexes of the numerous buds, as they pass down 

 " to shoot their radicles into the earth" (Phytologia, 4to. 1800, 

 chap. i. 1, 2. 3) ; but he also so far maintained the opinion of 

 Du Hamel, as to suppose that as these caudexes form a new 

 bark over the former one, that of the last year is transmuted 

 into alburnum. M. Aubert Du Petit-Thouars explains the 

 formation of the medullary rays by supposing that the fibres 

 which, according to his doctrine, constitute the new layers of 

 wood and liber, determine the formation of a certain quantity 

 of parenchyma, which is deposited interiorly by the ligneous 

 fibre, and exteriorly by the fibres that form the new liber. 

 (Essais sur la Vegetation consider ee dans le Developpement des 

 Bourgeons, Paris, 1809, 2 e Essai.) I shall not endeavour to 

 point out the very hypothetical nature of this doctrine ; but 

 merely observe, that, with the exception of the remark that 

 each gem is a distinct embryon, the whole is founded upon 

 assumption ; and that it is utterly destroyed by the simple fact, 

 that isolated spots of bark and alburnum are formed on de- 

 corticated stems, which cannot be the roots descending from 

 the gems, unless we suppose that these have penetrated the 

 wood and again protruded at the points where the new patches 

 of bark and alburnum appear. 



VOL. I. A A 



