202 TRANSLOCATION IN PLANTS 



concentration and not by individual concentration gra- 

 dients. According to the Munch hypothesis any minerals 

 that might diffuse from the xylem across to the phloem 

 would be carried along with the other phloem contents. 

 Mason and Maskell, however, seem to assume that there 

 is no such absorption from the xylem by the phloem. Any 

 such diffusion across, if in appreciable amounts, would 

 seem hopelessly to complicate attempts at interpreting 

 concentration gradients in terms of transport. 



In their first paper on nitrogen transport (1929a) they 

 find that flaps of bark receive nitrogen from above, which 

 is clear proof that the nitrogen has moved down from above. 

 When the bark is connected with the wood, the wood con- 

 tains more nitrogen than when the bark is separated from 

 the wood. They interpret this as proving that the nitrogen 

 has moved from the bark to the wood. Such an interpre- 

 tation is not unreasonable, but an alternative explanation 

 is just as reasonable, that is, when the bark is raised from 

 the wood, the wood fails to receive sugar; therefore it is 

 less likely to absorb nitrogen from the transpiration stream 

 or even retain that which it had originally. The fact that 

 the separated flap does not contain more nitrogen than the 

 normal is in opposition to their contention that less has 

 been transported to the wood, though of course this point 

 might be otherwise explained as resulting from injury. 



36. The Surface Tension Hypothesis of Van den Honert. 

 It has been observed that a substance which lowers the 

 interfacial tension between two immiscible liquids tends 

 to spread rapidly over the interface between them, moving 

 from the region of its application, where the surface tension 

 is lowered, toward a region of its absence where the surface 

 tension is high. Van den Honert (1932) describes an 

 apparatus in which he has observed the rapid spread of 

 potassium oleate over the interface between water and 

 ether. The rate of spread was such as to give a movement 

 approximately 68,000 times that calculated for diffusion. 

 This rate is of the same order of magnitude as that found 

 by Mason and Maskell for the transport of carbohydrate. 



