THE ASCENT OF SAP 263 



Other objections are the lack of a free surface below the liquid. 

 Pulling the liquid up from a free vessel would be much easier than 

 pulling it out of living cells below. Furthermore, the cells of the 

 wood, although fine and small, are not small enough to raise the 

 water more than a few feet. If capillarity were the important 

 force, it would be reasonable to expect to find small vessels in vines 

 and plants which need much water rapidly. The reverse is found 

 to be the case. Similarly the spring wood, where the most water is 

 carried in a given length of time, contains larger vessels than the 

 fall wood, which is formed in periods of diminished activity. If 

 capillarity were the force involved, the small vessels should be 

 formed in the spring! Plainly to explain the ascent of sap by such 

 forces is out of the question. 



Atmospheric Pressure.— Atmospheric pressure has been sum- 

 moned by some writers to explain the ascent of sap. Their con- 

 clusions were reached after finding a negative pressure when a 

 manometer is inserted into a rapidly transpiring tree. This has 

 been interpreted to mean that the intake of water from the soil 

 by the roots is due to the presence of such negative pressures. 

 The leaves, according to this hypothesis, actively transpire water 

 which tends to create a vacuum below, with the result that more 

 water is forced in from the outside. 



That this argument is fallacious, is easily shown when one re- 

 calls that air pressure can operate only when there is a free surface 

 below. Now such a free surface here does not exist. The lower 

 ends of the vessels are separated from the water in the soil by 

 several layers of active living cells in the root. Before atmospheric 

 pressure could act, it would have to force water through all these 

 cells. Furthermore, the negative pressures obtained have never 

 been more than a fraction of an atmosphere, and even though the 

 pressure inside the wood fell to zero so that the weight of the entire 

 atmosphere were available with a free surface to work upon (which 

 has been shown not to be the case) the highest the water could 

 be raised under such ideal conditions would be only about thirty- 

 two feet. Atmospheric pressure may help some in small plants 

 and herbs, but can such forces avail to raise water 300-400 feet 

 to the tops of the highest trees? 



Pressures less than one atmosphere exist in rapidly transpiring 

 plants as the result of the ascent of sap and not as its cause. The 

 fact that transpiration from the leaves takes place more rapidly 



