62 



COURSE OF THE SAP. 



PT. ir. 



from a viscid substance, to which he gave the name of 

 cambium. But where is the cambium itself formed ? 

 How and where elaborated ? Duhamel thought that 

 the last year's layer of bark was converted into this 

 year's layer of wood. We can see that this is not the 

 case. Du Petit-Thouars thinks that the new layer of 

 wood is formed by the buds (though I believe the 

 origin of this theory is due to Darwin) ; that the fibres 

 in the new layer of wood are the roots of the buds, 

 w^hich, at the bursting of the buds, run between the 

 last year's bark and wood to the ends of the roots. 



Dr. Lindley thus confidently and complacently 

 concludes his statement of the theory of Darwin and 

 Du Petit-Thouars : — ' The elongation of the leaf-bud 

 upwards gives rise to new axes with their appendages ; 

 their elongation downwards increases the diameter of 

 that part of the axis which pre-existed, and produces 

 roots.' 



The argument from the grafted stocks is, I think, 

 stronger against this theory than against the growth 

 from the descending sap ; for though the Doctor may 

 adopt the idea of the lateral flow of sap, and make it 

 necessary to irrigate the roots of the buds, it would 

 scarcely have the power to metamorphose an actual 

 growing fibre of peach-wood into a fibre of plum- 

 wood. 



The pith or Dutrochet and Link bring us back to Hales's doc- 

 and medul- triue of the all-importancc of the pith. Indeed, 



lary rays tit 



or silver Dutrochct would establish the omnipresence of the 



grain. 



