44 



or less, passes on or exhales, and then the vessels 

 appear empty, that is, filled with air. The same 

 author, however, speaks of horizontal vessels, first 

 noticed by Malpighi, which pass through the bark 

 of trees to the alburnum, and, as he supposes, ad- 

 mit oxygen gas * ; but, besides that these vessels are 

 not proved to extend to the leaves, which alone ope- 

 rate changes on the air, we have seen that the leaves 

 of themselves are able to preserve the life of the 

 plant when the stem is confined in vacm (26.) ; 

 and, on the contrary, that the stem is not sufficient, 

 when deprived of its leaves, or these are smeared 

 over with oil. Lastly, Dr Hales readily caused air 

 and water to pass through a piece of recent stick, by 

 setting one end of it in a cup of water placed under 

 a receiver, and then exhausting the receiver of its 

 air, and these vessels he imagined to perform a re- 

 spiratory function ; but the same vessels were found 

 by Malpighi to be present in the roots, where they 

 are not exposed to the air, and cannot, therefore, 

 serve the purpose of respiration. Neither, as be- 

 fore remarked, have we any proof that any other 

 part than the leaves of the living plant act upon the 

 air in contact with it to an extent necessary to main- 

 tain its life. 



36. Have then these leaves any organization by 

 which an absorption of air can be effected ? It is as- 

 certained that the sap of plants ascends to the leaves, 

 and after undergoing certain changes in them, is 

 again returned to the branches and stern. Dr Dar- 



Phytologia, p. 12, 13. 



