IV] VARIOUS THEORIES, &c. 85 



able of transmitting it up the stem ; but he also found 

 that some trees — the birch for instance — never form 

 any true " heart-wood" (duramen) at all, but consist 

 throughout of " sap-wood " (alburnum), the inner 

 layers of which only differ from the outer in being 

 somewhat less permeable to water. 



Hartig also convinced himself that it is the elements 

 with bordered pits, and especially the tracheides, 

 which conduct the water. 



The absorption of the water at the roots has no 

 direct relation to the ascent of water in the stem, 

 being due entirely to the osmotic action of living cells 

 — especially the root-hairs. Only in cases where the 

 imprisoned air expands and exerts pressure, or con- 

 tracts and facilitates the flow of water into a vessel, &c. 

 need we take any account of the root action. But this 

 root-action, which is especially favoured by a rise of 

 temperature in the soil, helps to explain a phenomenon 

 which has been overlooked, viz. that in the summer, in 

 spite of the fact that transpiration is then most active, 

 most of the trees (beech, oak, larch, Scotch pine and 

 spruce) examined, contained their maximum of total 

 water : the birch alone was an exception, because its 

 period of vegetative activity is earlier. 



If the tree contains so much water that the air in 

 the cavities of its tracheides, &c. is at a pressure equal 



