THE METHOD OF MOVEMENT 149 



likely to be under greater tension so that cutting into the 

 trunk while taking samples will rupture the columns, which 

 will then be almost instantly replaced by air. Such a 

 replacement of water by air can be easily demonstrated if, 

 during a period of deficient water, the bark is removed to 

 expose the wood and then a notch is cut into the wood. 

 The sudden change in appearance of the wood is due to 

 rupture and withdrawal of the water columns. 



30. Weaknesses in the Hypothesis of Miinch. — 1. This 

 hypothesis would not allow for what seems clearly indi- 

 cated in the experiments reported in Chaps. II and III, 

 that is, that both upward and downward movement of 

 solutes occur simultaneously through the same phloem 

 system. It might account for an upward movement 

 from the leaves at a given level, for example to the apical 

 meristem, and a simultaneous downward movement from 

 the same leaves to the trunk, cambium, and roots. It 

 might account for an upward or downward movement 

 into defoliated regions of the stem as in the experiments 

 described in Sees. 7, 8, and 9, or possibly into the isolated 

 flaps of bark described by Mason and Maskell (1928a), but 

 it would not allow for a simultaneous upward and down- 

 ward movement through the same part of the conducting 

 system, as for example a simultaneous upward movement 

 of nitrogen and downward movement of sugars through the 

 phloem of the trunk. Local reversals in side twigs may be 

 easily accounted for, but even a daily alternation in direc- 

 tion of movement in the main conducting tissues would 

 seem highly improbable. 



In the scheme proposed by Munch, any nitrogen or salts 

 reaching the leaves from the roots must necessarily pass 

 exclusively through the xylem, for there would be a uni- 

 directional flow, backward only, through the phloem of 

 the main stem as long as the leaves were supplying foods 

 to the roots. Under such a system, nitrogen or salts taken 

 into the living cells in the root or stem could reach the top 

 only through the transpiration stream. Eckerson (1924), 

 Thomas (1927), and others have shown that, in some plants 



