128 PROCESS OF NUTRITION. CHAP. ill. 



set of valves, and so on till it got to the top of the 

 plant. 



BorelH. This theory was afterwards adopted by Borelli, 

 who endeavoured to render it more perfect by 

 bringing to its aid the influence of the condensation 

 and rarification of the air and juices of the plant 

 as a cause of the sap's ascent. And on this prin- 

 ciple he endeavoured also to account for the greater 

 force of vegetation in the spring and autumn ; be- 

 cause the changes of the atmosphere are then the 

 most frequent under a moderate temperature ; while 

 in the summer and winter the changes of the at- 



o 



mosphere are but few, and the air and juices either 

 too much rarified or too much condensed, so that 

 the movement of the sap is thus at least prejudi- 

 cially retarded, if not perhaps wholly suspended. 

 But as this theory, with all its additional modifica- 

 tions, is still but a combination of the theories of 

 Grew and Malpighi, it cannot be regarded as af- 

 fording a satisfactory solution of the phenomenon 

 of the sap's ascent. 



DuHamel. With this impression upon his mind, and with 

 the best qualifications for the undertaking, Du 

 Hamel directed his efforts to the solution of the 

 difficulty, by endeavouring to account for the phe- 

 nomena from the agency of heat, and chiefly on 

 the following grounds : * Because the sap begins to 

 flow more copiously as the warmth of spring re- 

 * Phys. de* Arb. liv. v. chap. ii. 



