LECT. VII.] ANATOMY OF STEMS. 379 



aqueous fluid, which gives them firmness, and 

 enables them to rise to the great height to which 

 they often attain in so short a period, without 

 any curvature; for, on examining the pith, the 

 cells are found not more numerous, but merely 

 larger than in an ordinary shoot. As soon, how- 

 ever, as the first circle of wood is formed, no sub- 

 sequent increase of diameter can be attributed to 

 the pith. If these remarks be correct, the follow- 

 ing must be regarded as the functions of the 

 pith: 1st, to afford the surface necessary for the 

 formation of the first layer of wood ; and 2d, to 

 give a degree of firmness to the succulent stem 

 and recent shoot, which they would not other- 

 wise possess, before the bark and alburnum ac- 

 quire sufficient consistence for that purpose. 



But is the pith of use only in the succulent 

 shoot ? If my idea of its functions be correct, its 

 utility to the plant must cease as soon as the first 

 circle of wood is perfected ; and it is indeed not 

 easy to conceive in what manner it can be useful 

 after it has become dry. Mr. Keith remarks, " it 

 " is essential to vegetation in all its stages * ;" but 

 even admitting that it is, as he and Malpighi be- 

 lieve, an organ of elaboration, we must suppose 

 that all vegetation ceases in every annual shoot at 

 the termination of the season in which it is 



* Syst.of phys. Bot. vol. ii. p. 213. 



