322 COMPOSITE ORGANS. CHAP. II. 



same term is made to signify at one time the empty 

 cells only, and at another, both the cells and juice. 

 To the former I shall therefore confine the appella- 

 tion of Cellular Membrane or Tissue ; and to the 

 latter that of Parenchyma ; in which respective ac- 

 ceptations these terms shall be employed throughout 

 the following part of the work. 



SECTION III. 



The Pith. 



Always THE pith, as has been already shown, is a soft and 

 spongy, but often succulent substance, occupying 

 the centre of the root, stem, and branches, and ex- 

 tending in the direction of their longitudinal axis, 

 in which it is enclosed as in a tube. In most plants it 

 is close and compact without any apparent solution 

 of continuity, as in the Willow and Poplar ; but in 

 others it is loose and interrupted, as in the Walnut 

 and Thistle; in the former of which it may be 

 compared to a close and continued succession of 

 transverse plates intersecting the central tube of 

 wood; and in the latter, to a number of fine and 

 membranaceous diaphragms placed at irregular in- 

 tervals throughout the hollow cylinder of the stem, 

 which occasionally burst open in the middle, and 

 form a succession of rings. Sometimes it forms 

 merely a fine and delicate film, of a smooth and 

 brilliant appearance, lining the interior of the 





