222 PROCESS OF DEVELOPMENT. CHAP. IV. 



now among the external cortical layers ; those that 

 were originally passed through the liber were now 

 imbedded in wood ; and those that were orginally 

 passed between the liber and wood were still more 

 deeply imbedded. The conjecture of Grew, there- 

 fore, is confirmed, at least in substance. For either 

 the layer that is formed separates into two distinct 

 portions, the one tending towards the circumference 

 and forming bark, and the other tending towards 

 the centre and forming wood ; or, two distinct layers 

 are originally generated receding in opposite di- 

 rections, and forming respectively wood and bark : 

 which last part of the alternative is the most likely 

 to be the fact ; because we can scarcely refer two 

 substances so distinct in their character and proper- 

 ties as the wood and bark, precisely to the same 

 origin. 



Buttyim- But although the above experiments are correct 



metiiate j i j 11 r 



generation and convincing, beyond all controversy, as far as 



proper * tne 7 g> 7 et tne y ^ not include the whole of the 

 J utce; case ; for the annual layer, which seejns thus to be 

 formed merely from the bark, is in fact formed only 

 from the proper juice descending from the leaf 

 through the tubes of the liber. The bark then is not 

 to be regarded as the generating cause of the new 

 layer, but merely as the medium of the transmission 

 of the materials from which it is formed. For the 

 proof of this most important fact we are chiefly 

 indebted to the well-known experiments of Mr. 

 Knight, by which he has thrown the highest degree 



