122 THE STEM. 



inner portion of this cambium-layer is forming wood ; the outer is 

 forming bark. The cells of the first multiply vertically by divis- 

 ion, and then elongate into prosenchyma or woody tissue, a part 

 of them being at the same time commonly transformed into ducts ; 

 thus producing a second layer of wood on the surface of the 

 first, and continuous with the primary layer in the prolongation of 

 the stem and in the branches made the same season. The exte- 

 rior part of the cambium-layer contributes in much the same way 

 to the thickness of the liber, which therefore grows inversely, or 

 by accessions to its inner face. But the bark exhibits such great 

 diversities in growth and structure, that it cannot well be farther 

 considered along with the wood. 



205. Annual Increase Of the Wood, Each successive year a new 

 layer is added to the wood in the same manner ; each layer being, 

 like the first, intersected by the extended medullary rays. A 

 cross-section of such a stem, therefore, exhibits the wood disposed 

 in concentric rings between the bark and the pith ; the oldest lying 

 next the latter, or in the centre, and the youngest occupying the 

 circumference. Each layer being the product of a single year's 

 growth, the age of an exogenous tree may, in general, be correctly 

 ascertained by counting the rings in a cross-section of the trunk. 

 It is obvious, moreover, that the growing parts of an exogenous 

 tree or shrub (and the same applies to the herb) are, 1. The apex 

 of the stem and branches, by buds, which continue the plant up- 

 wards and develope the foliage. 2. The lower extremity of the 

 roots, by which these are advanced from year to year. 3. The 

 cambium-layer, which annually produces a stratum of fresh tissue 

 under the bark, between the buds and the rootlets, over the whole 

 extent of the plant ; its ordinary growth giving rise to new annual 

 layers of wood and inner bark ; while certain cells, taking a spe- 

 cial development, form buds and consequently branches in the axils 

 of the leaves, or, adventitiously (152), from other places, or else, 

 under favoring circumstances, secondary or adventitious roots 

 (130). Lateral buds and roots, although they originate in the 

 cambium-layer, have to grow and break through the bark before 

 they appear externally.* 



* That peculiar state of the wood of the Sugar Maple, called Bird's-eye 

 Maple, is apparently caused by numberless rudimentary adventitious buds, 

 which, failing, to grow, have become involved in the woody layers. 



