366 CONSERVATIVE ORGANS. [LKCT. VII. 



Palm, Sagusfarinifera. Dr. Darwin may perhaps, 

 however, be regarded as actually the first who 

 taught that the spiral vessels convey fluids ; and 

 he suggested the idea, which, with some modifi- 

 cations, I have adopted, of the manner in which 

 they carry forward their contents. " It is easy," 

 says he, " to conceive how a vermicular or peri- 

 " staltic motion of the vessel, beginning at the low- 

 " est part of it, each spiral ring contracting it- 

 " self, till it Jills up the tube, must forcibly push 

 " forward its contents without the aid of valves *." 

 He, however, considered them absorbent vessels ; 

 and erroneously supposed that they pass down the 

 trunks of trees from the caudex of each bud, to 

 the roots. Finally, Mirbel-j- regards the spiral 

 vessels as sap vessels ; whilst Mr. Keith considers 

 the reformed opinion of Grew the most correct, 

 namely, that they transmit not only air but sap \. 

 c. The MEDULLA or PITH. Returning to our 

 shoot of Horse Chesnut, we find the tube which 

 is formed by the wood and lined with the medul- 

 lary sheath, as has been already described, filled 

 with a white, dry, very compressible, spongy sub- 

 stance : this is the Medulla or Pith. In the suc- 

 culent state of a stem or a twig, it is turgid with 

 aqueous fluid ; but before the wood is perfected it 



*Phytologia, sect. 11, 8. 



t Eltmens de Phys. veg. l ere partie, p. 205. 



J System of phys. Botany, vol. ii. p. 120. 



