no TIMBER AND SOME OF ITS DISEASES. [CHAP. 



it, we find it to consist of parts each bounded above 

 and below by an air-bubble, and Elfving's contention 

 in opposition to Zimmermann's, is that the friction 

 of these bubbles and capillarity completely support 

 these parts. 



Calculation shows that if the tracheides measure 

 0*02 mm. in diameter, and have the same capillary 

 ascension as glass (which of course we do not know) 

 then the capillary ascension would = 1*5 meter a 

 number amply sufficient for the purpose. 



Hence, says Elfving, we may safely assume that 

 each short water-column in the series, between its two 

 air-bubbles, exerts no pressure downwards it is a 

 weightless segment, so to speak whence the columns 

 may be maintained at any height likely to come into 

 dispute. As to the origin of the air-bubble there is 

 no difficulty. The tracheides, &c. are cells containing 

 protoplasm and other contents when young, but 

 when the walls are thickened and the bordered 

 pits, &c. completed, the living contents disappear, 

 leaving sap only in the cavities. As water is withr 

 drawn a tendency to form a vacuum is instituted, 

 but this is prevented by vapour, the tension of which 

 increases as the withdrawal of water proceeds. Since 

 the water absorbed into the plant contains air, how- 

 ever, the reduction of pressure causes the sap to part 



