9 o TIMBER AND SOME OF ITS DISEASES. [CHAP. 



system, and all movement of the water ceases the 

 leaves and cortex in the upper parts of the tree dry up 

 and death may ensue. In the case of the spruce 

 mentioned this occurred when the lumina of the 

 tracheides contained liquid water to the extent of 75% 

 of the whole volume. 



The more slowly water is being absorbed by the 

 roots below, as compared with energetic transpiration, 

 the more the air becomes rarefied, even in the lower 

 parts of the tree ; and the difference of pressure 

 between the air above and that below may become so 

 small that the water ascends only very slowly. On 

 the other hand, the more energetic the root-action, the 

 denser the air in the lower tracheides becomes, being 

 pressed by the water behind. When active transpira- 

 tion follows upon this state of affairs (as in the early 

 summer) we have the greatest difference in the 

 pressures set up transpiration rarefies the air-bubbles 

 above, and root-pressure compresses them below 

 whence the water ascends rapidly. Moreover this 

 conduces to a continuance of rapid transpiration, for 

 leaves transpire more freely when turgid. 



On the other hand, the ascent and transpiration of 

 water act in no appreciable way on the process of 

 absorption : the osmotic activities at work are 

 practically independent of such pressures and strains 



