102 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



Now suppose the interpolated membranes to exert 

 just sufficient resistance to filtration to balance the 

 water-column contained in its own cell above it, then, 

 as Hartig assumes, the movement will depend entirely 

 on the differences of pressure in the air-bubbles. If 

 water is removed so as to diminish the pressure of a 

 bubble in a given tracheide, then the bubble next 

 below exerts pressure and drives water up, and so on. 

 Zimmermann comes to the conclusion that on this 

 assumption also the lifting forces alleged are not 

 sufficient for the purpose, and that Hartig's theory 

 also fails to account for the ascent of water up trees 

 more than thirty feet or so high, for he finds that the 

 suction action can only be propagated for ten meters 

 or so along the system. 



Attention should be drawn to a second paper by 

 Zimmermann, 1 in which he publishes the results of 

 his experiments with a number of Jamin's chaplets, 

 but since the paper deals more particularly with the 

 purely physical phenomena in the glass tubes, it is 

 hardly necessary to discuss it here : it may be noticed 

 that he obtains some curious results with other liquids 

 than water, however, and there can be no doubt as to 

 the importance of the chapelet de Jamin in the plant. 



1 " Ueber die Jamin'ische Kette." Ber. d. deut. bot. Gesellsch., 1883. 

 B. i. p. 384. 



