14 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



107. It carries upwards the oxygen liberated by the de- 

 composition of carbonic acid and water, and conducts it into 

 the leaves. 



108. The WOOD lies upon the medullary sheath, and con- 

 sists of concentric layers. 



109. It is formed by the successive deposit of organized 

 matter descending from the buds, and by the interposition 

 of the medullary system, here called medullary rays, con- 

 necting the pith and the bark 63 b b . 



110. The first concentric layer lies immediately upon the 

 medullary sheath and pith, and consists of woody and vasi- 

 form tissue 63 *. 



111. Each succeeding concentric layer consists <tf woody 

 and vasiform tissue, which either form themselves intoMistinct 

 strata, in which case the latter is innermost, or are con- 

 founded together. 



112. When there is any material difference between the 

 compactness of the tissue of the two sides of a concentric 

 layer, zones are formed in which the woody tissue is outer- 

 most ; but when the vasiform and woody tissues are equally 

 intermingled, no apparent zones exist. 



113. A concentric layer, once formed, never alters in di- 

 mensions. 



114. Each concentric layer, which is distinctly limited, is 

 usually the produce of one year's growth. 



115. Therefore, the age of an Exogenous tree should be 



