STEM. 17 



135. Cambium is a viscid secretion, which, in the spring, 

 separates the alhurnum of an Exogenous plant from the liber. 

 It is free vegetable mucilage, out of which the new elementary 

 organs (8) are constructed, whether in the form of vessels, or 

 w r oody tissue, or of the cellular tissue of the medullary system, 

 whose office is to extend the medullary plates, and maintain the 

 communication between the bark and central part of a stem. 



136. As Exogenous plants increase by annual addition of 

 new matter to their outside, and as their protecting integu- 

 ment or bark is capable of distension in any degree, commen- 

 surate with the increase of the wood that forms below it, it 

 follows, taking all circumstances into consideration, that there 

 are no assignable limits to the life of an Exogenous tree. 



137. The stem of ENDOGENOUS plants offers no absolute dis- 

 tinction of Pith, Medullary Bays, Wood, and Bark. 



138. It is formed by the intermixture of bundles of vascular 

 tissue among a mass of cellular tissue, the whole of which is 

 surrounded by a zone of cellular and woody tissue, inseparable 

 from the stem itself, and therefore not bark. 



139. It increases by the successive descent of new bundles ^ 

 of fibro- vascular tissue down into the central cellular tissue, 

 curving outwards as they descend. 



140. The vascular bundles of the centre gradually force out- 

 wards those which were first formed, the cellular mass augments 

 simultaneously, and in this way the diameter of a stem increases. 



141. What appears to be bark in these plants is an external 

 layer of cellular tissue, into which the lower extremities of the 

 arcs of fibro-vascular tissue descend obliquely, losing their vas- 

 cularity as soon as they reach the cortical integument, or false- 

 bark. 



142. It is in consequence of this continuity in an oblique 

 direction of the fibro-vascular bundles and the external cortical 

 integument, that the latter can never, in Endogens, be sepa- 

 rated from the wood beneath it. 



143. The diameter of the stem of an Endogenous plant is 

 determined by the power its tissue possesses of distending, and 

 by its hardness. 



144. When the external tissue has once become indurated, 

 the stem can increase no further in diameter. 



145. When the tissue is soft and capable of continual dis- 



