100 TIMBER AND SOME OF ITS DISEASES, [chap. 



the pressure is equal in two tracheides, that the water 

 does not pass back, through the interstices of the cell- 

 walls, from the higher to the lower one ? Hartig 

 assumes that the closing membranes of the bordered 

 pits only allow water to filter thus easily when 

 they are distended, by the pressure of the gas-bubbles. 

 He thus proposes to reject the "air-pressure theory" 

 altogether, and to substitute for it his " gas-pressure 

 theory." 



Vesque, in 1883, by using branches of Tradescantia^ 

 &c., cut longitudinally, was able^ to sec movements 

 of water and air-bubbles in the vessels and tracheides 

 exposed and placed under the microscope. On 

 stopping up the vessels, the leaves still acting, air- 

 bubbles appeared and grew larger ; on removing the 

 leaves the movements ceased : any slight bend, 

 pressure, change of atmospheric moisture, light, &c., 

 had its effect on the movements. 



Similar observations were also made in 18S3 

 by Capus,^ who used begonias^ dahlias^ &c., and 

 examined the vessels exposed on sections. The 

 method was to remove a slice so as to expose the 



^ Vesque, " Observations directe du mouvement de Teau dans les 

 vaisseaux." Ann. d, Sc, Nat, Ser. vi. t. xv. 1833, ^o- i- 



^ " Sur I'observation directe du mouvement do I'eau dans les plantes." 

 Cumpes Rend,^ 18S3, t. 97, p. 10S7. 



