LKCT. VII.] ANATOMY OF STEMS. 367 



becomes dry and spongy ; except near the termi- 

 nal bud, or where branches are given off, in which 

 places it long retains its moisture. 



The form of the pith is regulated by that of the 

 cavity it fills, which in the majority of instances 

 is nearly circular ; but to this there are many ex- 

 ceptions. Thus, in a horizontal section of a young 

 stem or twig of the Elder, Sambucus nigra, and 

 the Oriental Plane, Platanus orientate, we find 

 it circular, but furrowed by the bundles of spiral 

 vessels of the medullary sheath. It is oval in the 

 Ivy, Hedera Helix, and the Ash, Fraxinus ex- 

 celsior ; irregularly oval and furrowed in the Ori- 

 ental Plane, Platanus orientalis ; triangular in 

 the Oleander, Nerium Oleander; pentangular in 

 the Oak, Quercus Robur ; four-sided, with the 

 angles obtuse or tetragonal, in common Lilac, Sy- 

 ringa vulgaris, and yellow flowering Horse Ches- 

 nut, ^Esculus flava ; pentagonal in the Walnut, 

 Juglans regia, and hexagonal in the common Dog- 

 wood, Cornus sanguined. M. de Beauvois is of 

 opinion that the situation of the leaves on the 

 stem regulates the form of the tube which the 

 pith fills, an opinion which M. Mirbel * regards as 

 fallacious ; but it is nevertheless true, that in 

 the horizontal section of many stems, the form of 

 the medulla differs at, and immediately under the 



* Etemens de Phys. v(g. 1 partie, p. 111. 



