THE STEM OF VASCULAR PLANTS. 145 



tissue under the form of a central cylinder ; this is what 

 forms the pith properly so called ; and all the following 

 layers present this cellular tissue under the form of a 

 more or less narrow belt, which separates the belt of 

 fibres of the preceding year from that of the present. 

 All this apparatus is traversed, from the centre to the 

 circumference, by laminae of an analogous nature to the 

 pith, which, in a transverse section, have the appearance 

 of the spokes of a wheel, or the hour-lines of a dial, and 

 have received the name of Medullary Rays (rayons 

 mkduUaires). 



We now proceed to examine, successively, these dif- 

 ferent parts. 



§2.— Of the Central Pith. 



If we cut transversely a stem of the Elder, for example, 

 or of any other Exogenous tree, we observe in the 

 centre, a canal, usually angular, or nearly cylindrical, to 

 which Grew gave the name of the Medullary Cavity, and 

 which is generally called the Medullary Canal (canalis 

 medullaris ; canal meduUaire). This canal is full — at 

 least in the infancy of the branches — of round cellular 

 tissue, to which the name of the Pith (medulla; mo'elle) 

 has been given, because it occupies the centre of the 

 wood, as the marrow fills the central cavity of the long 

 bones of animals. 



The cellules, of which the pith is composed, are 

 usually more regular, larger, more dilated, and of a 

 more spongy texture, than those of the rest of the 

 tissue. In several plants this cellular tissue composes the 

 whole of the pith : in a large number we find a circular 



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