262 THE ANATOMY OF STEMS. 



plants, and consequently their medicinal qualities, are 

 chieh'y deposited. 



The Wood. — Pursuing our investigation in the 

 young stem of the Horse Chesnut ; when the whole 

 of the bark is removed, we find, immediately under 

 and slightly adhering to it, a firmer and more com- 

 pact substance, which, both in a longitudinal and a 

 transverse section, appears to constitute a cylinder, 

 enclosing a column of spongy cellular matter or pith. 

 This is the wood. It has been regarded, 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 analogy 

 does not hold good. It is at first soft and vascular, 

 and is then called Jilburnum ; but it afterwards be- 

 comes hard, and, in some trees, is of a density almost 

 equal to that of iron. In a transverse section of our 

 stem of Horse Chesnut, it appears, to the unassisted 

 eye, a continuous circle of a homogenous structure, 

 of a very light straw color exteriorly or near the bark, 

 and greenish interiorly, or where it is in contact with 

 the pith ; but in some other trees, as the Laburnum 

 and Elder, this circle appears traversed, 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 examined by a magnifying 

 glass, and they are observed in the soft wood, or al- 

 burnum, as well as in the hard and most perfect 

 wood. These two distinct parts, which constitute the 

 wood, may be described under the names of Concen- 

 tric and Divergent layers. 



1. The Concentric layers, in the stem of the Horse 

 Chesnut of one year's growth, when seen through the 



