ANATOMY OF STEMS. 269 



by the wood and lined with the Medullary Sheath, 

 filled with a white, dry, very compressible spongy sub- 

 stance : — this is the Medulla or Pith. In the suc- 

 culent state of a stem or a twig, it is turgid with 

 aqueous fluid ; but before the wood is perfected, it 

 becomes dry and spongy ; except near the terminal 

 bud, or where branches are given oft', in which places 

 it long retains its moisture. 



The form of the pith is regulated by that of ihe 

 cavity it fills, which in the majority of instances is 

 nearly circular ; but to this there are many excep- 

 tions. Thus in the horizontal section of a young stem 

 or twig of the Elder (Sambucus) and the Plane (Pla- 

 tanus), we find it circular, but furrowed by the bun- 

 dles of the spiral vessels of the Medullary Sheath. 

 It is oval in the Ivy, and the Ash ; irregularly oval and 

 furrowed in the Plane ; triangular in the Oleander 

 (Nerium Oleander) ; pentangular in the European 

 Oak ( QjLiercus Robur) ; four-sided, with the angles 

 obtuse, or tetragonal, in the common Lilac, and yellow 

 flowering Horse Chesnut (JEsculus flava) ; pentago- 

 nal in the Walnut (Juglans regia) ; and hexagonal 

 in the Red-twigged Cornel ( Cornus sanguinea). The 

 situation of the leaves on the stem regulates the form 

 of the tube which the pith fills. But besides the di- 

 versities of form which the pith presents, it varies in 

 diameter in other respects. In the young tree, of a 

 few inches in height, it is smallest at the basis of the 

 stem, largest in the middle, and smaller again at the 

 summit ; and in the growth of each future year, near- 

 ly the same variations in its diameter are observable. 



The pith, in the majority of ligneous dicotyledons, 

 is longitudinally entire ; but in some, the Walnut, for 

 instance, it consists of a succession of transverse dia- 

 phragms intersecting the hollow cylinder of the wood, 

 with the intervening spaces empty. In others the 

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