390 CONSERVATIVE ORGANS. [LECT. VIII. 



rectly across the ligneous cylinder of the stem, 

 forms an apparently solid diaphragm ; and swell- 

 ing externally affords a base a. (see 

 marginal cut) on which the bud, 

 and consequently the new branch, 

 are seated. This appearance was er- 

 roneously supposed by Grew* to 

 be occasioned by the shooting of 

 the branch across the pith ; but, it 

 is obviously a process of the medullary sheath; 

 for, when placed under the microscope, it dis- 

 plays the same structure ; and the cells of which 

 it is composed are crowded with small granules, 

 as is generally -the case in those of the medullary 

 sheath. The colour, which is green like the me- 

 dullary sheath, distinguishes it from the pith, 

 which is of a fawn colour in the vine. It is not 

 always, however, thrown completely across the 

 pith; for, when neither a bud nor a tendril rises 

 from the opposite side of the stem, it never 

 stretches entirely across, but leaves 

 a free communication between the 

 pith above and below it, at least 

 one third of the circumference of 

 the ligneous cylinder. The plans in 

 the margin illustrate, more clearly, 

 this fact : thus, where the tendril is 



* Anatomy of Plants^ t, 19. 



