126 



ANATOMY. 



M 



686. Cork Oak. 



Horizontal slice showing the development of two woody 

 bundles in a four-year-old branch (mag.)- 





divide the new bundle into two or three parts (fig. 686). These celVilar rays (2, 8, 4), 

 which are termed secondary medullary rays, to distinguish them from the primary 



(1), which start from the pith 

 (M), are thus doubled in each 

 annual ring, and, like the large 

 rays between the h'bro- vascular 

 vessels, form a sort of vertical 

 septa or radiating walls, com- 

 posed of elongated and super- 

 imposed cells ; whence the name 

 of muriform tissue for the me- 

 dullary rays. j/j 



Hence, in its totality the 

 stem presents two very distinct 



systems, the woody (wood), and the cortical (bark). 1. The woody system 

 is formed of the central pith and zones of fibre-vascular bundles, sepa- 

 rated by medullary rays. The innermost of these is the medullary 

 sheath, formed of tracheae and fibres analogous to the liber, and c 

 outwardly composed of woody fibres and rayed, annular, and dotted vessels. The 

 other zones are similarly organized, except that they, never possess tracheae. 

 2. The bark system is formed of the epidermis, the cork, the endophleum, and the 

 bast fibres (liber), external to and amongst which the laticiferous vessels ramify. 

 With age the cells of the pith lose colour, dry, separate, and finally die ; the woody 

 fibres thicken, and usually darken ; of these the heart- wood (duramen) differs from 

 the more recently formed or sap-wood, which is more watery, softer, and brighter 

 coloured. The liber fibres (fig. 686 bis) are more sfender, longer, and more tenacious 

 than the woody fibres ; and are of great use in the manufacture of thread, cord, and 

 textiles. Their bundles descend vertically and rectilinearly in thin concentric plates, 

 whence their name liber (book) j but in some plants, as the Oak and Lime, they form 

 a network, the interstices of which are occupied by the medullary rays. 



From the mode of development of the wood and bark systems, it is obvious 

 that the wood must harden, and the bark decay ; for in all the bark tissues, the later 

 formed are constantly pushing towards the periphery, within which they have been 

 developed ; this produces the exfoliation of the several elements of the cortical sys- 

 tem ; the epidermis first, then the cork-cells, the endophleum, and sometimes the 

 liber. 



It is not necessary to describe any of those anomalous dicotyledonous stems 

 which present peculiar tissues or hyper-development of certain elements, or the 

 absence of others ; except that of Conifers (Pine, Fir, Larch, Yew, &c.), the wood of 

 which, with the exception of a few tracheae in the medullary sheath, is entirely com- 

 posed of regularly dotted fibres. The walls of these wood-fibres (fig. 687) are 

 hollowed into small cups, like watch-glasses, which are arranged in two straight 

 lines, occupying the opposite sides of each fibre. These cups are so placed in con- 

 tiguity that their concavities correspond (fig. 688), leaving an interposed lens-like 



