DECOMPOSITE ORGANS. CHAP. I. 



the same density throughout, being hardest at the 

 centre and softest at the circumference ; which dif- 

 ference is so striking, that the outer and softest 

 portion has been designated by the name of the 

 alburnum. 



If the transverse section of the wood is now 

 minutely inspected it will be found to consist of an 

 indefinite number of concentric layers of consider- 

 able, but not of equal thickness, intersected by a 

 number of transverse layers diverging from the 

 centre like the rays of a circle; which are often, 

 but not with much propriety, denominated medul- 

 lary rays, upon the supposition that they originate 

 in the pith, a notion that is now exploded. I shall 

 therefore designate them by the more appropriate 

 term of the divergent layers, as being descriptive 

 at least of their distribution and position. But the 

 layers, whether concentric or divergent, that com- 

 pose the intermediate portion of the caudex, are 

 often designated also by the appellation of the lig- 

 neous layers, as constituting the mass of the wood, 

 and in centra-distinction to the cortical layers, or 

 layers constituting the bark. 



The central portion of the caudex is the pith, 

 which is a soft and spongy substance of a whitish 

 colour, surrounded by the wood as by a tube ; or" 

 rather by a cylindrical row of longitudinal fibres 

 closely united togethei: in the form of a tube, and 

 investing it like a sheath; on account of which 



