40 



LETTEKS ON TREES. 



3. The accompanying figures are plans of hori- 

 zontal sections of the stems of Exogens, and shew the 

 wood, the bark, and the cellular tissue, together with 

 the central medullary (or pith) cavity and the medul- 

 lary rays. Figure 2 is the 



scheme of a shoot one vear old. 

 In the centre (a) is the medul- 

 lary or pith cavity ; around this 

 are blocks or bundles of woody 

 fibres (cZ d d), having the appear- 

 ance of truncated wedges, and 

 forming the woody stem of the 



shoot. Proceeding from the me- 

 dullary cavity, and passing between the bundles (which 

 they separate and isolate), are the medullary rays 

 (c c c) : encircling both these and the bundles is a ring 

 of cellular tissue (b h b), which may be termed the 

 medullary ring ; and encircling the whole is the bark 

 (eee e). The medullary cavity, the medullary rays, 

 and the medullary ring, are everywhere continuous, 

 form but one and the self-same tissue — the cellular, 

 and have their distinctive appearances given them 

 simply by the woody bundles passing down through the 

 common mass of tissue. Take away these bundles, and 

 nothing will appear save an unbroken mass of cellular 

 tissue, enclosed within a ring of bark. 



4. This cellular tissue, which, as I before remarked, 

 appears to be the basis or matrix of the whole stem or 



