SECT. VI. THE CAUDEX, -s<i 



resemblance M. Mirbel denominates it the tubular 

 sheath.* But as this term indicates no important 

 property belonging to the sheath, and does not even 

 aim at pointing out its use, it will perhaps be 

 better to give it the appellation of the Medullary 

 sheath, or the sheath investing the Pith. 



In some plants the diameter of the pith is large 

 in proportion to that of the stem, as in the Fig, 

 Elder, and Sumach ; while in others it is but very 

 small, as in the Ash ; and in others scarcely per- 

 ceptible, as in the Oak and Elm. It is always 

 most abundant in the young and tender shoot, of 

 which, indeed, it constitutes originally the principal 

 mass. But as the wood increases, the pith gra- 

 dually diminishes, till in old and full grown trunks 

 it totally disappears, and has its place occupied by 

 wood. In the root it is always less abundant, and 

 perhaps also more compact, than in the stem and 

 branches, being discoverable only in the larger 

 divisions, but not in the radicles or fibres ; and dis- 

 appearing also in the mature state of the plant. 

 But in many roots, such as those of the Elm and 

 Laburnum, it is not distinguishable even in the 

 larger divisions themselves. It is distinguishable, 

 however, in the root of herbaceous plants, and in 

 the larger divisions of the root of the Arbor-Fit a, 

 Elder, and Berberry, in which last it is easily de- 

 tected as being of a shade of colour considerably 

 different from that of the rest of the caudex. 



* Phys. Vcg. vol. i. p, 186. 



