2J6 PROCESS OF DEVELOPEMENT. CHAP. IV. 



SUBSECTION II. 



Formation Perennials, and their Annual Layers. If a pe- 

 ers of the rennial is taken at the end of the second year and 

 year, nd dissected as in the example of the first year, it will 

 be found to have increased in height hy the addition 

 of a perpendicular shoot consisting of bark, wood, 

 and pith, as in the shoot of the former year ; and in 

 diameter by the addition of a new layer of wood, and 

 of bark generated between the wood and bark of the 

 former year, and covering the original cone of wood, 

 like the paper that covers a sugar-loaf: this is the 

 fact of the mode of augmentation about which phy- 

 tologists have not differed, though they have differed 

 widely with regard to the origin of the additional 

 layer by which the trunk is increased in diameter. 

 According Malpighi was of opinion that the new layer of 

 ghi ancT~ wood is formed from the liber of the former year ; 

 Grew - the layer of liber being by degrees assimilated to the 

 woody substance, and ultimately converted into 

 a layer of wood,, which attaches itself to the layer 

 that was previously formed.* But the defect of this 

 hypothesis is, that it does not account for the forma- 

 tion of the new layer of liber itself. Grew was of 

 opinion that a new ring of sap vessels is first gene- 

 rated on the inner furface of the liber of the former 

 year, which is gradually converted into a new layer 

 of liber that ultimately splits into two portions, 



* Anat. Plant. 11. 



