FUNDAMENTAL OEGANS. 



125 



layer is the suber, which in certain trees attains a considerable development, and 



forms cork. Take now a vertical section of the same branch, and the disposition, 



&c. of the fibres and vessels will appear as in fig. 683. 



The cambium, which does not become organized in annual or herbaceous stems 



like the Melon, in perennial stems becomes highly organized (fig. 684). During the 



second year, this gelatinous 

 tissue undergoes the following 

 changes : outside the woody 



P. L. P. L . P .L.MC. C . F . V . F , V.F.V.T.M. 



684. Maple. 



Horizontal slice, showing the development of a woody bundle in a three- 

 year-old branch, c, cambium layer, separating the wood from the bark. 



1. Pith (M), tracheae (T), punctate vessels and fibres of the first year (v). 



2. Punctate vessels (v) and fibres (F) of the second year. 3. Vessels (Y) and 

 fibres of the third year. Within the bark (s) is seen the cortical layer of 

 the first year (p. L), then that of the second year (P. L), and of the third year 

 (p. L), separated by the cambium (c) from the contemporaneous \voo;ly 

 layer (mag.). 



685. Oak. 



Horizontal slice of a twenty-five years' 

 old trunk. 



fibres interspersed with large vessels (1. v), is formed one fresh cambium layer 

 (2. F, v) ; within the fibres of the liber and of the cortical system another is 

 formed ; these layers become moulded upon the older ones, and the zone of cambium 

 which is transformed to produce them presents a cellular organization at those 

 points, which corresponds to the cells of the medullary rays, so that these continue 

 without interruption from the pith to the cortical layers. 



Each ring of vascular bundles was hence from its earliest condition enclosed in 

 two cambium layers, of which one belongs to the wood, the other to the bark ; each 

 of these vascular bundles, again, is in its turn separated by a cambium layer, which 

 in the third year repeats the process, producing within ligneous fibres (3. F) and 

 large vessels (3. v), and outside liber (L) and cortical parenchyma (p), and so on 

 each year. Now, each wood bundle being composed of two elements, and the large- 

 sized vessels being usually towards the interior of the bundle, we can, by counting 

 their number (which is easily ascertained by the gaping mouths of the large vessels), 

 reckon the number of annual layers, or, in a word, the age of the stem or branch 

 (fig. 685). 



It must be remarked that the secondary ligneous bundles differ from the primary 

 in the total absence of tracheae ; these vessels being confined to the medullary sheath. 



We have said that the medullary rays are not interrupted by the formation of 

 new vascular bundles, because the cambium zone remains cellular at the points 

 corresponding to these rays. If each newly formed bundle was undivided, like that 

 in juxtaposition with it, the number of medullary rays would be always the same ; 

 but this is not the case ; at the circumference of the primitive bundle one or more 

 longitudinal series of cells is developed, which reach to the circumference, and 



