SECT. II* COMPOSITE ORGANS. 2OQ 



aggregate a cylinder or circular layer closely invest- 

 ing the wood. If the root or trunk of an annual is 

 taken and treated in the same manner, it will be 

 found to consist of an epidermis, pulp, and inter- 

 spersed fibre. Here then is the termination of the 

 growth of the annual, and of the first stage of the 

 growth of the perennial : how have their several 

 parts or organs been formed ? 



As the pith seems only a modification of the ori- Formation 

 ginal pulp, the same hypothesis that accounts for the of theplU 

 formation of the one will account also for the forma- 

 tion of the other, at least in a*> far as they are found to 

 consist of cellular tissue; the cells and membrane 

 composing them being in both cases alike, though 

 somewhat modified by situation. If the cells of the 

 pith are the largest, it is perhaps because being lodged 

 in the centre they are there the farthest removed from 

 the compression and action of the air : and if the cells 

 of the exterior pulp are the smallest, it is perhaps be- 

 cause being situated towards the circumference they 

 are there the most exposed to the influence of the 

 same causes. But their character is also affected by 

 the character of the juices they contain; the pulp 

 containing a resinous juice, as being mingled from its 

 position with the proper juice of the plant ; and the 

 pith containing a watery juice, as having more com- 

 munication with the ascending sap. Hence the pith 

 and pulp, or parenchyma, are ultimately converted 

 into organs essentially distinct from one another; 

 though phytologists have been much puzzled to Function, 

 assign to each its respective functions. 



VOL. II. p 



