LECT. VII.] ANATOMY OF STEMS. 369 



given off from a stem, a thread of medulla, in 

 some instances, separates from the central column, 

 and entering the branch, is gradually augmented 

 to a diameter proportionate to that of the 

 branch *. Such a circumstance led M. Aubert Du 

 Petit-Thouars f to describe the medulla at the base 

 of every bud, as an inverted cone, the apex of 

 which originates from the pith of the branch on 

 which the bud appears ; but in general this is not 

 the case, the pith commencing in the bud itself, 

 which originates at the surface of the stem ; 

 and hence no direct union exists between the 

 pith of the branch and that of the stem 

 (see marginal figure). In the annual 

 shoot, the wood shuts up the canal of the 

 pith at its extremity, as soon as it ceases 

 to grow for the season, as is seen in the 

 longitudinal section of our shoot of Horse Ches- 

 nut, immediately under the terminal budj; and 

 thus isolates it from the shoot of the next year. In 

 many plants this forms a kind of woody partition, 

 which marks the limit of the growth of each year 

 in the length of the stem ; but in others it is ab- 

 sorbed, the continuity of the pith being, appa- 



* Vide Plate 6, fig. 6. representing a cutting of the common 

 Elder; in which d. the pith of the branch b. is united by a 

 small thread to e. the pith of the main stem a. 



f Histoire d'un Morceau de Bois, p. 153. 



t Vide Plate 5, fig. 15. 



VOL. I. B B 



