330 CONSERVATIVE ORGANS. [LET. Vlt, 



pears to constitute a cylinder, enclosing a co- 

 lumn of spongy cellular matter or pith. This is 

 the wood. It has been regarded 1 , in reference to 

 the vegetable, as answering the same end as bone 

 in the animal body ; but, except in its property of 

 giving firmness and support to the plant, the ana- 

 logy does not hold good. It is at first soft and 

 vascular, and is then called Alburnum ; but it af- 

 terwards becomes hard, and, in some trees, is of 

 a density almost equal to that of iron. In a trans- 

 verse section of our stem of Horse Chesnut, it ap- 

 pears, to the unassisted eye, a continuous circle 

 of a homogeneous structure, of a very light straw 

 colour exteriorly or near the bark, and greenish 

 interiorly, or where it is in contact with the pith ; 

 but in some other trees, as for example the La- 

 burnum and the Elder *, this circle appears tra- 

 versed, at nearly regular distances, by rays of an 

 evidently different structure. These are found, 

 however, to exist also in the stem of the Horse 

 Chesnut, and in every other woody dicotyledon 

 when it is examined by a magnifying glass; and 

 they are observed in the soft wood, or alburnum, 

 as well as in the hard and most perfect wood. 

 These two distinct parts, which constitute the 

 wood, may be described under the names Concen- 

 tric and Divergent layers ^. 



* Vide Plate 6, fig. 3. 4. 5. 



f These names are adopted by Mr. Keith (System of physio- 

 logical Botany ) y and are more expressive of the parts they are in- 

 tended to designate, than any others which have been employed. 



