24 TIMBER AND TIMBER TREES. 



casks, if we suppose them put end to end and the parti- 

 tions broken through— it is evident that the prismatic 

 cambium-cell is the primitive form of wood-element. 



Put in this very general way, then, we may say 

 that all wood whatever is formed of elements of the 

 above types derived from the primitive prismatic 

 cambium-cell or its homologue, and in all true timber — 

 the masses of wood of Coniferous and Dicotyledonous 

 trees — the cambium-cells are grouped into a cambium 

 layer, or cylinder, which appears as a ring (the cambium- 

 ring) on a transverse section of the stem. In the 

 Monocotyledons (Palms and Bamboos for instance) and 

 in the Tree-ferns, and in a few other rare instances, 

 however, there is no such cylinder, and although the 

 general principle, above stated is still true, there are 

 certain peculiarities in detail concerning these false 

 timbers, the discussion of which belongs to botanical 

 works..- 



True timber is yielded only by Conifers and Dicoty- 

 ledons, and in all these cases we find 3. pith, medullary 

 rays, and cambium, as well as the mass of wood proper. 

 But it by no means follows that all the derived elements 

 — cells, vessels, tracheids, and fibres — occur in any 

 particular wood, and it is largely due to diff'erences of 

 this order that various woods are so different in structure 

 and quality. 



All woods — whether forming true timber or not — 

 possess at least a few spiral vessels in the earliest 

 stages ; but in Pines, Firs, Cedars, Larches, and a few 

 Dicotyledons, they are only discoverable with the aid 

 of the microscope close to the pith, where they were 

 formed with the first wood, and no true vessels of any kind 

 occur in the main mass of wood. In most Dicotyledons, 

 however, and in the Palms, Bamboos, and Tree-ferns, 



