STEM. 21 



This cellular substance only developes hori- 

 zontally ; and it is to it that the peculiar char- 

 acter of different kinds of wood is chiefly due. 



For this reason the wood of the stock of a 

 grafted plant will never become like that of its 

 scion, although, as will be hereafter seen, the 

 woody matter of the stock must all originate in 

 the scion. 



The stem of an Exogenous plant may there- 

 fore be compared to a piece of linen, of which 

 the weft is composed of cellular tissue, and the 

 warp of fibrous and vascular tissue. 



In the spring and autumn a viscid juice is 

 secreted between the wood and the liber, called 

 the Camhium. 



This Cambium appears to be the matter out 

 of which the cellular horizontal substance of 

 the stem is organised. 



In Endogenous stems, such as the Palm, the 

 portion at the circumference is harder than that 

 in the centre ; and there is no separable bark. 



Their stems consist of bundles of woody 

 matter, imbedded in cellular tissue, and com- 

 posed of vascular tissue surrounded by woody 

 fibre. 



The stem is not only the depository of tlie 

 peculiar secretions of species, but is also the 



