262 PHYSIOLOGY. BOOK II. 



had died; but the scion had continued to grow, and had 

 emitted from its base a sort of plaster formed of very distinct 

 fibres, Avhich surrounded the extremity of the stock to some 

 distance, forming a kind of sheath ; and thus demonstrating 

 incontestably that fibres do descend from the base of the scion 

 to overlay the stock. The singular mode of growth in Pan- 

 danus is equally instructive. In that plant the stem next the 

 ground is extremely slender, a little higher up it is thicker 

 and emits aerial roots which seek the soil and act as stays upon 

 the centre. As the stem increases in height it also increases 

 notably in diameter, continuing to throw out aerial roots. As 

 it really grows, the stem, if the roots were pruned awa}', would 

 be an inverted cone ; but if we add to the actual thickness of 

 the base of the stem the capacity of the aerial roots at that 

 part, the two together will be about equal to the capacity of 

 the stem at the apex ; showing that, unless the roots descend 

 among the wood, the stem will not increase in diameter. 



Mirbel, who formerly advocated the doctrine of wood being 



deposited by bark, has, with the candour of a man of real 



science, fairly admitted the opinion to be no longer tenable ; 



and he has sugsjested in its room that wood and bark are in- 



dependent formations, — v,'hich is no doubt true, — but, he 



adds, created out of cambium, in which it is impossible to 



concur; for this reason. All the writers hitherto mentioned 



or adverted to have considered the formation of wood only 



with reference to exogenous trees, and to such only of them as 



are the common forest plants of Europe. Had they taken 



into account exotic trees or any endogenous plants, they would 



have seen that none of their theories could possibly apply to 



the formation of wood in that tribe. In many exogenous 



plants of tropical countries, wood is not deposited in regular 



circles all round the axis, but only on one side of the stem, or 



along certain lines upon it : were it a deposit from the bark, 



or a metamorphosis of cambium, it would necessarily be 



deposited with some kind of uniformity. In endogenous trees 



there is no cambium, and yet wood is formed in abundance ; 



and the new wood is created in the centre, and not in the 



circumference : so that bark can have, in such cases, nothing 



whatever to do with the creation of wood. 



