LKCT. VII.] ANATOMY OF STEMS. 363 



may maintain their irritability, and consequently 

 their contractility, for two or more seasons, or as 

 long as the medullary sheath remains succulent ; 

 although, as it is not necessary for the progression 

 of the sap that they should act by alternate con- 

 traction and dilatation, after the alburnous or lig- 

 neous vessels are completed, it is probable that 

 they lose these properties, after the first year of 

 the life of the stem or of the twig. 



If we inquire what are the opinions of other 

 phytologists respecting the functions of the spiral 

 vessels, we find Malpighi regarding them as bron- 

 chia, or air-vessels *, and the same opinion is 

 supported by Grew*f~, Hales |, and Du Hamel. 

 Grew, however, believed that they acted as sap- 

 vessels in the wood ||, in which, as I have already 

 stated, he fancied he had detected them ; and Du 

 Hamel once suspected that they might contain 

 highly rarefied sap. The supposition that they are 

 air-vessels, probably originated from their always 

 appearing empty when examined ; but on the same 

 account the animal arteries were regarded as air- 

 vessels by the ancients and their followers, until 



* Anal. Plantarum. 

 f Anatomy of Plants, fol. edit. 

 \ Vegetable Statics, chap. v. 

 Physique des Arbres. 



|| His words are, " in the wood the sap ascendeth only by 

 4< the air-vessels." Veg. of Trunks, chap. i. 



