LECT. VfJ.J ANATOMY OF STEMS. 371 



speaking, filled with air. The cells retain the hex- 

 agonal form in their empty state ; but in some, as 

 the Walnut, this is destroyed in the lamellae, into 

 which the pith then separates; and the same oc- 

 curs in the interior of the medullary sheath of 

 Woodbine, and similar hollow stems. In the' 

 greater number of plants no vessels are percep- 

 tible in the pith ; but in some, entire vessels con- 

 veying proper juice are present, as in the Gum- 

 elastic Fig tree, the proper juice of which is seen 

 exuding from different points of the pith in a hori- 

 zontal section of the stem : and, in all plants, the 

 cells communicate with each other by means of or- 

 ganized pores, which are visible under the mi- 

 croscope. M. Aubert Du Petit-Thouars has lately 

 affected to regard the medulla as deserving that 

 name only after the cells become empty, naming it 

 parenchyma in the early or succulent stage of its 

 existence * : but this is at best a useless refinement ; 

 for, although I am not prepared to admit, with Mr. 

 Keith -f-, that there is an essential difference be- 

 tween the membrane composing the cells of the 

 parenchyma or pulp, and that forming those of the 

 pith, yet the insulated and enclosed situation of 

 the pith is sufficient to obtain for it the dignity of 



* " Ce n'est que par 1'extraction des sues qu'elle (la Mo- 

 " elle) contient qu'elle est devenue Moelle." Essais stir la 

 Vegetation, p. 205. 



f System of phys. Bot. vol. i. p. 324. 

 B B 2 



