196 LIBER CAMBIUM. [BOOK i. 



The liber does not always, or perhaps often, form a com- 

 pact layer of tubes ; on the contrary, its tissue has a wavy 

 direction, and lies in bundles which touch each other here and 

 there, and can be readily separated. This is in reality the 

 same structure as occurs in the zones of wood, only that it is 

 much more conspicuous, and it arises from the same cause, 

 namely, the necessity for the woody tissue of the liber leaving 

 innumerable passages between its tubes for the free passage 

 of the cells belonging to the horizontal system, and passing 

 from the cortical integument into the medullary rays. 



The tissue of the liber consists of nothing but pleurenchym, 

 of great toughness owing to its very thick sides, and of lati- 

 ciferous vessels. No vascular tissue, properly so called, has 

 been observed in it, with the solitary exception of Nepenthes. 

 The toughness of the liber-tubes is probably given them in 

 order that they may possess the strength, combined with 

 flexibility, which their position near the circumference of a 

 branch renders necessary. 



Anomalous structure in liber has been little observed; 

 the most singular instance of it is that mentioned by 

 Decaisne (Comptes rendus, v. 393.) who says that in Menisper- 

 mads the liber is only formed for the first year, and is after- 

 wards covered over by new wood ; and consequently is found 

 near the centre round the pith, and not at the circumference. 



* * * * Cambium. 



Beneath the liber, and above the wood, is interposed in the 

 spring a mucous viscid layer, which, when highly magnified, 

 is found to contain numerous minute transparent granules, 

 and to exhibit faint traces of a delicate cellular organisation. 

 This secretion is named the CAMBIUM, and appears to be 

 exuded both by the bark and wood ; Dutrochet says only by 

 the former, founding his opinion upon the presence of cam- 

 bium in bark nodules, which, he says, have no communication 

 with the wood of the parent tree; see page 177. 



Although the name of Cambium was originally given to 

 the mucous secretion found in the spring between the bark 

 and wood of Exogens, yet it is in truth nothing more than 

 the organisable matter, or " generative sap," which occurs 



