120 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



those times when according to Boehm their elastic 

 walls should be caving in beneath the atmospheric 

 pressure. 



Godlewski agrees that there is much to support 

 Boehm's view that the ascent of the water takes place 

 in the lumina of the tracheal elements, since the 

 negative pressure necessary for his theory actually 

 exists there, as proved by Von Hohnel's and other 

 experiments. But however near to ten meters high 

 the pressure of the atmosphere could raise the water 

 in a shrub (and we must always remember that the 

 pressure of the air in the uppermost tracheides will 

 never fall to o), Boehm's theory is hopeless when 

 applied to trees. 



Moreover, all Boehm's attempts to explain the 

 support of the water columns (by the resistance of 

 septa to filtration downwards, and the friction of the 

 air-bubbles, &c.) break down before the fact that any 

 resistance to movement downwards will apply to 

 movement upwards as well. 



Finally, Boehm's hypothesis would contradict the 

 principle of the conservation of energy. For if we 

 suppose his system columns of water broken by air- 

 bubbles and septa, and plunged below in root paren- 

 chyma and above in mesophyll to be eleven meters 

 high, and suppose the root parenchyma to be a 



