98 TRANSLOCATION IN PLANTS 



in the transpiration stream is indicated by the fact that 

 it moves most rapidly toward those leaves that transpire 

 most rapidly, and that it can pass a ringed or dead part 

 of the stem or even through a connecting tube or through 

 a block of gelatin. The fact that the stimulus may be 

 carried in a basal direction is cited as proof that the water 

 current also is often carried backward. Snow (1924, 1925), 

 however, has given evidence that the response to a stimulus 

 is not always due to the transport of a dissolved substance 

 and it is possible that the backward transfer is transmissive 

 and is not due to the movement of a substance. Even if 

 the transfer of a stimulus were demonstrated to be due 

 invariably to the movement of something in the transpira- 

 tion stream, this is not satisfactory evidence that normal 

 solutes, sugars, proteins, and salts from the soil solution 

 are carried in the transpiration stream either upward or 

 downward. Furthermore, Dixon claims that ringing or 

 killing the stem does not interfere with the transport of 

 this hormone, whereas there is abundant evidence that 

 ringing or killing does most decidedly interfere with the 

 movement of these normal solutes. 



For conditions that may be considered normally to bring 

 about a backward flow through the vessels, he mentioned 

 the same ones that Birch-Hirschfeld suggested, that is, 

 rain, dew, and unequal insolation.* He also suggested 

 that the changes in volume that have been observed in 

 leaves might account for a backward flow. So far as I 

 am aware, however, such contractions in volume as he 

 calls upon to account for backward flow occur only when 

 water loss exceeds absorption, and under such conditions 

 water would tend to be drawn from the vessels not pumped 

 back into them, unless one considered the water as drawn 

 back by other competing leaves, but even in that case any 

 solution thus drawn back would tend to be carried to the 

 other leaves and not toward the trunk or roots away 

 from the transpiring top. 



* The same causes were suggested by Hales, in 1727, to account for the 

 ebb of sap through the wood. 



