THE PITH OF PLANTS. SS 



already described, and the compressed cells termed 

 the silver grain. 



Immediately within the pulp hark, when that can 

 be distinguished, there is frequently another pulpy 

 layer placed over the hard wood or heart wood, which 

 I shall term pulp wood 1 . 



The whole wood of a tree is made up of concentric 

 layers each of which is the growth of one year ; and 

 hence upon cutting a tree across and counting the 

 annual layers or rings of woods, its age may be pretty 

 nearly guessed, if not quite ascertained. 



THE PITH OF PLANTS. 



THE pith 2 is what is popularly termed the heart, 

 because it occupies the centre. It is with few ex- 

 ceptions composed exclusively of cells, rather loose in 

 texture, as in the elder, or compact, as at the nodes 

 of the ash. 



The pith is contained in a cylindrical sheath, termed 

 the pith tube 3 , which, when once formed, is said by 

 Turpin and others never to alter its dimensions. Pro- 

 jections of this pith tube go off to every bud and branch 

 formed on the plant. 



In hemlock, cow-parsnep, and other plants with 

 hollow stems, it is the centre of the pith which is 

 hollowed out. 



(1) In Latin, Alburnum. 

 (2) In Latin, Vagina medullaris. 

 (3) In Latin, Medulla, which is inaccurate. 



