THE STEM. 58 
_cambium as soif, which unite and form the new concentric 
cylinder ; while the medullary rays extend outwards at 
the same time, and connect the new layers with the interior 
parts. 
171. M. Mirbel and others were of opinion that the cam-. 
bium annually forms a new layer of alburnum and a new lay- 
er of bark. We know that the cambium can repair the bark 
when it has been injured ; and, as the new layers of wood 
and bark are formed where this fluid is found, it is not un- 
_ reasonable to suppose that it acts an important part in this 
_ process, ; 
172. M. Du Petit-Thouars’ theory is, that the successive 
formation of woody layers is caused, by the development 
of buds, from which, in spring, issue numerous fibres, which 
_ descend in the cambium between the liber and the albur- 
num. In gliding downwards they meet the fibres which de- 
_ scend from other buds, and form a layer of greater or less 
_ thickness, which soon becomes solid, and forms a layer of 
wood. og 
173. Each bud is regarded as a separate system of vege- 
tation. The buds are considered so many individuals 
placed upon a common stock, and elongating in two ways— 
upwards, forming new stems and branches, leaves, &c.— 
and downwards, forming roots ; the descending fibres being 
the roots which the buds send forth, and the cambium bear- 
ing the same relation to the roots of the bud as the soil does 
to a germinating seed. M. Thouars considers buds as ana- 
logous in structure and mode of development to the embryo 
of the seed, which in germinating produces a young stem 
analogous to the scion prodtced by the growth of a bud. _ 
He calls the latter a fixed or adherent embryo, while he de- 
nominates that within the seed a free embryo. Thus the — 
wood and bark are considered as formed of the roots of the — 
174. Grew, Malpighi, and Duhamel, supposed that the 
buds which are annually developed on the surface of the vege- - es 
cambium forms annually a layer of liber, which separates e | 
