FUNCTION.] ORIGIN OF WOOD. 195 



manner of the ordinary ligneous tissue of a Palm stem 

 (fig. 193.), only the bundles do not adhere to each other, 

 and are not embodied, as usual, in a cellular substance. 

 These bundles may be readily traced to the central column, 

 particularly in the younger branches (fig. 191.), and are 

 plainly the roots of the stem, of exactly the same nature as 

 those aerial roots which serve to stay the stem of a Screw Pine 

 (Pandanus) . When they reach the earth, the woody bundles 

 become more apparently roots, divided at their points into 

 fine segments, and entirely resembling on a small scale the 

 roots of a Palm-tree. The central column is much smaller 

 at the base of the stem, than near the upper extremity. This 

 would seem to prove that the woody bundles of the endoge- 

 nous stem are roots emitted by leaves, plunging down through 

 their whole length into the cellular substance of the stem in 

 ordinary cases ; but, in Barbacenia, soon quitting the stem, 

 and continuing their course downwards on the outside. The 

 observation of Du Petit Thouars, that, when Dracaenas push 

 forth branches, each of the latter produces from its base a 

 quantity of fibres, which are interposed between the cortical 

 integument and the body of the wood, forming a sort of plaster 

 analogous to what is found in the graft of an Exogen ; and 

 that, of the fibres just mentioned, the lowermost have a ten- 

 dency to descend, while those originating on the upper side 

 of the branch turn downwards, and finally descend also ; had 

 already rendered the above-mentioned conclusion probable. 



Mirbel, who formerly advocated the doctrine of wood being 

 deposited by bark, has admitted the opinion to be no longer 

 tenable ; and has suggested, in its room, that wood and bark 

 are independent formations, created out of cambium: not 

 meaning thereby merely the viscid secretion found in the 

 spring between the bark and wood of Exogens ; but that 

 universal organic mucus, spoken of at p. 7 of the first volume 

 of this work. In other words, out of what English physio- 

 logists call organisable matter. 



Aware of the difficulties in the way of the old explanations 

 of the formation of wood, Du Petit Thouars, an ingenious 

 French physiologist, who had possessed opportunities of 

 examining the growth of vegetation in tropical countries, 



