368 CONSERVATIVE ORGANS. [LRCT. VII. 



places where the leaves are seated, from what it 

 is in the intermediate spaces. In the Lilac, for 

 example, the obtuse tetragon becomes an ellipsis 

 near the insertions of the leaves, which are op- 

 posite; while each angle of the Oleander is length- 

 ened into a horn or process. But besides these 

 diversities of form, the pith 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, nearly the same variations in its diameter 

 are observable. 



The pith, in the majority of ligneous dicotly- 

 ledons, is longitudinally entire; but in some, 

 the Walnut for instance, it consists of a succes- 

 sion of transverse diaphragms intersecting the 

 hollow cylinder of the wood, with the interven- 

 ing spaces empty *. In others the continuity of 

 the medullary column is broken by ligneous plates, 

 which proceeding from the side of the central 

 tube, either partially intersect it or completely 

 partition off portions of it, as in several of the 

 Magnolias ; while in others, again, it is merely a 

 spongy sheath, lining the interior of the cavity, 

 as in the stem and branches of Woodbine, Loni- 

 cera Periclymenum. Where the branches are 



*Vide Plate6, fig. 11. 



