420 Br. Bell on the Phyfiology of Plants, 



leaves, and thefe feem necefTary for receiving 

 the air evolved by circulation, which at lengcii 

 pafTes off with the perfpirable matter. But if 

 the air were abforbtd by the leaves, and de- 

 fcended towards the roots, its motion would be 

 oppofite to that of the fap, and, inftead of affift- 

 ing, would obftrudl its progrefs. It is commonly 

 believed, that after the air has entered vege- 

 tables, it is expanded or contraded, according 

 to the variations of the temperature of the at- 

 mofphere, and in this way afllfts the afcent or 

 defcent of the fluids. To this opinion it may 

 be objeded. jft. That the air-veffels in the 

 roots, where the fap is firfi- put in motion, are fo 

 deep feated, that the changes in the heat of the 

 atmofphere cannot afFeit their temperature. 

 2. That the common juice afcends, and the 

 proper juice dcfcends, whether the air be hoc 

 or cold. 3. That the preffurc of the air-vcITels 

 on thofe which contain juice, will not more pro- 

 mote than obftru(ft the motion of the fluids in 

 a given diredion, unlefs the vefllels which in- 

 clude them contained valves, and in this cafe 

 thefe fluids could not have a retrograde motion. 

 Let us look for fome more probable opinion. 



Dr. Hill has demonfl:rated, that the cuticle 

 of plants is an organized fubflrance, containing 

 veflels. In trees and fhrubs, thefe vefl"cls have 

 an external opening; but in the herbaceous plants 

 this is wanting. Trees and fhrubs only are 



poflTeffed 



