CHAP. III. ORIGIN OF WOOD. 259 



hence a section of the wood of that plant has a pale, smooth, 

 homogeneous appearance ; but in the wild cherry the medullary 

 plates are much thicker, they adhere to the bark by deep 

 broad spaces, and are arranged with great irregularity, so that 

 a section of the wood of that variety has a deeper colour, and 

 a twisted, knotty, very uneven appearance. 



In Quercus sessiliflora the medullary rays are thin, and so 

 distant from each other that the plates of wood between them 

 do not readily break laterally into each other, if a wedge is 

 driven into the end of the trunk in the direction of its cleav- 

 age : on the contrary, the medullary rays of Quercus pedun- 

 culata are hard, and so close together that the wood may be 

 rent longitudinally without difficulty ; hence the wood of the 

 latter is the only kind that is fit for application to park- 

 paling. 



As the medullary rays develope in a horizontal direction 

 only, when two trees in which they are different are grafted 

 or budded together, the wood of the stock will continue to 

 preserve its own peculiarity of grain, notwithstanding its 

 being formed by the woody matter sent down by the scion ; 

 for it is the horizontal development that gives its character 

 to the gi'ain, and not the perpendicular fibres which are 

 incased in it. 



The WOOD is at once the support of all the deciduous organs 

 of respiration, digestion, and fertilisation, the deposit of the 

 secretions peculiar to individual species, and also the reservoir 

 from which newly forming parts derive their sustenance until 

 they can establish a communication with the joii. 



Regarding the precise manner in which it is created, there 

 has been great diversity of opinion. Linnaeus thought it was 

 prodviced by the pith ; Grew, that the liber and wood were 

 deposited at the same time in a single mass which afterwards 

 divided in two, the one half adhering to the centre, the other 

 to the circumference ; Malpighi conceived that the wood of 

 one year was prodviced by an alteration of the liber of the 

 previous season. Duhamel believed that it was deposited by 

 the secretion already spoken of as existing between the bark 

 and wood, and called cambium : he was of opinion that this 

 cambium was formed in the bark and became converted into 



s 2 



