374 CONSERVATIVE ORGANS. [jLECT. VII. 



air is introduced into hollow ligneous stems ; but, 

 probably, in those in which the pith is sheathing, 

 as in the Woodbine, the union between the utricles 

 may be less intimate in the centre, and the air 

 insinuating itself between them, while the cells 

 are emptying themselves, compresses their sides 

 together and separates them ; and extending itself 

 in the length of the stem, forms it into a hollow 

 tube. In others, as the Walnut, it may be intro- 

 duced laterally, in separate quantities, at dif- 

 ferent points at the same time, and these dilating, 

 compress the horizontal portions of the emptying 

 cells between them, so as to produce the medul- 

 lary plates, which characterize that and similar 

 stems. 



Phytologists have not differed more on any 

 subject than in assigning the functions of the pith. 

 When we reflect that Caesalpinus, who lived in 

 the sixteenth century, taught that the pith is less 

 essential to the life and growth of a tree than 

 the bark *, it is astonishing that writers of so re- 

 cent a period as Dr. Darwin and Sir J. E. Smith, 

 should be found maintaining the accuracy of the 

 ancient opinion, that the pith is to the vegetable 

 what the brain and spinal marrow are to the 

 animal body-}-. Darwin's imagination led him 



* De Plantis. Flor. 1583. 



f Such was also the opinion of Linnaeus, Amcen. dead. 

 vol. iv. p. 372. 



