192 MIXED PITH BAKK. [BOOK i. 



supporting this hypothesis, does not justify the conclusion, 

 as has been already shown. 



It seldom happens that any part of the vascular system 

 intermixes with the pith, which is usually composed of cellular 

 tissue exclusively ; but in Ferula and the Marvel of Peru, it 

 has been proved by Mirbel and De Candolle, that bundles of 

 woody fibre are intermixed ; in Nepenthes there is a consider- 

 able quantity of spiral vessels scattered among the cellular 

 tissue of the same part ; and many other cases of a similar 

 kind are now known. In Nyctagos generally, in Pep- 

 perworts (Piperaceee), Cycads, Chloraiiths, &c., this occurs, 

 and has been made by Professor Schultz the character of a 

 division in his Natural System of Botany, called by him 

 Synorgana dichorganoidea ; but such cases may be found in 

 Loranthus, and are not uniform in the orders quoted : in 

 Boerhaavia repanda, for example, the pith contains no bundles 

 of vascular tissue, but is filled with fistulce containing very soft, 

 lax, spheroidal, cellular tissue, surrounded by smaller, harder, 

 and more cubical tissue, which passes into the medullary rays ; 

 a most curious organisation. 



* * Bark. 



The BARK is the coating of the stem immediately above the 

 wood, to which it forms a sort of sheath, and from which it is 

 separable without difficulty at certain seasons. But, although 

 it appears as an independent formation, it is, in reality, 

 organically connected with the wood by the processes of cel- 

 lular tissue, which, under the name of medullary rays, pass 

 through the wood, and lose themselves in the thickness of 

 the bark. Bark is usually distinguished into cortical or 

 cellular integument, under which name is comprehended the 

 whole of the external parenchymatous part, and liber or 

 inner bark, a name used to denominate the fibrous woody 

 portion lying next the alburnum, and this is extremely con- 

 venient for all common purposes. The cortical integument 

 thus defined belongs to the horizontal system, and the liber 

 to the longitudinal, the two standing in the same rela- 

 tion to each other as wood to pith *the cortical integument 



