74 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



Theodore Hartig's experiment with the piece of stick 

 shows (Boehm thought) that the drop of water causes 

 a long column to move: if, however, the vessels or 

 tracheides are first injected with mercury, it requires 

 great pressure to cause movement of the column, and 

 the experiment fails. Finally, Boehm denies that the 

 vessels or tracheides are ever totally devoid of liquid 

 water ; even when the transpiration is most active 

 there is always some water as well as air present. 



Boehm then puts forward his own hypothesis to 

 explain the water current in tall trees. The cells of 

 the transpiring surfaces (such as the leaf epidermis) 

 have elastic walls, and when they lose water by 

 evaporation, the pressure of the atmosphere tends to 

 drive these walls inwards, whereas their elasticity 

 tends to make them resume their previous shape and 

 positions. Hence an aspirator action is exerted on 

 the cells below, the elastic walls acting like valves. 

 Water is taken from the cells below, and this reduces 

 the pressure on the imprisoned air-bubbles : this being 

 so, the air-bubbles in the cavities still lower in the 

 plant are under slightly greater pressure than those 

 in the cells just considered, and they will expand 



1877, t. xx. pp. 357-389; Jahrb. fur Wiss Bot., 1877, p. 120; Ann. 

 d. Sc, Nat., 1878 ; Warum steigt der Soft in den Bdumen ? Wien, 

 1878; JBot. Zeit. t 1879, p. 225. 



