The Rev. P. Keith on the Internal Structure of Plants. 185 



be added, however, that Mirbel could not account for the 

 fact of the lateral transmission of sap, except through the 

 channel of visible pores ; while Dutrochet, by means of the 

 agency of molecular infiltration, can account for it very well 

 without them*. 



The Pith. — 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 extending in the 

 longitudinal axis of the plant, in which it is inclosed as in a 

 tube. Its structure is similar to that of the pulp, being com- 

 posed of an assemblage of hexagonal cells, containing for the 

 most part a watery and colourless juice. Mirbel regards it as 

 being furnished with pores like those already described; but 

 Dutrochet denies their existence, and contends that the fan- 

 cied pores are merely minute molecules imbedded in the walls 

 of the cells like those of the pulp. He designates them by 

 the name of nervous corpusclesf ; and affirms, besides, that 

 there is no pith in the root J. In many roots, we admit that 

 there is no visible pith ; but let any one examine the root of 

 Acorus Calamus or Berberis communis^ and then let him say 

 what he thinks of the affirmation. 



Yet, if its structure is so very similar to that of the pulp, 

 why, it may be said, is it to be designated by another name? 

 The central situation of the pith is, perhaps, of itself a suffi- 

 cient reason. But there seems to be besides a difference of 

 texture in the membranes composing them. Let the juiceless 

 pith of the Elder or of the Bulrush be compared with the 

 withered pulp or cellular integument of the Lime-tree, and the 

 difference will be obvious. 



The Cortical Layers. — The cortical layers, or interior and 

 concentric layers constituting the mass of the bark, are situ- 

 ated immediately under the cellular integument, where such 

 integument exists, and where not, immediately under the epi- 

 dermis, or they are themselves external. They are distin- 

 guishable chiefly in the bark of woody plants, but particu- 

 larly in that of the Lime-tree, in which they are easily sepa- 

 rated by maceration or exposure to the weather, and in which 

 you may readily count a dozen or more in a branch or trunk 

 of any considerable size. 



In aged trunks the outer layers are coarse and loose in 

 their texture, exhibiting individually a conspicuous and con- 

 siderably indurated, but very irregular network, composed of 

 bundles of longitudinal or cortical fibres, not ascending the 

 stem directly, but winding more or less around the axis of 



* Recherche* Anatomiqucs, p. 48. f Ibid. p. 13. \ Ibid. p. 46. 



Third Series. Vol. 5. No. 27. Sept. 1834. 2 B 



