370 CONSERVATIVE ORGANS. [LECT. VII. 



rently, uninterrupted from the root to the apex of 

 such stems. Those partitions are almost always 

 present when the pith is composed of distinct 

 plates, as in the Walnut, or of a spongy sheathing 

 membrane, as in Woodbine. 



The colour of the pith, in the succulent shoot 

 or in the young plant, is green, which, as the cells 

 empty, changes to white; but to this there are 

 some exceptions. Thus it is yellow in the Bar- 

 berry, Berberis vulgaris ; pale brown in the Wal- 

 nut, Juglans regia ; fawn-coloured in the Sumach, 

 Rhus Coriaria ; and pale orange in yellow-flow- 

 ered Horse Chesnut, yEsculus flava; but it is 

 more frequently coloured in the caudex of the root 

 than in the stem. 



Placing a thin slice of pith, taken either from 

 a vertical or a horizontal section of our shoot of 

 Horse Chesnut, or of any other plant, under the 

 microscope, it appears to consist of hexagonal 

 cells *, which are larger and more regular in the 

 centre than near the circumference -j~. In very 

 young stems and succulent shoots, these cells are 

 filled with an aqueous fluid, and closely resemble 

 the cellular integument % ; but, in older stems and 

 twigs, they are found empty, or, more accurately 



* The cellular structure of the pith was first pointed out by 

 Grew in his work on the Anatomy of Plants, fol. 1682. 

 t Vide Plate 6, fig. 2, k. 

 J Vide Plate 6, fig. 2, c. 



