54 TRANSLOCATION IN PLANTS 



of a number of woody plants will open only for a short time 

 each day, or may show almost no opening at all, while 

 those on normal branches will open more widely and remain 

 open longer. There is also a possible effect of high carbo- 

 hydrate on leaf structure, thickness of cuticle, thickness of 

 wall, etc., and also perhaps a slight effect of the high con- 

 centration on vapor pressure, though the latter effect is 

 probably very slight. 



That a reduction in transpiration should result in a 

 reduced solute transport through the xylem does not neces- 

 sarily follow. I know of no experimental evidence showing 

 that the normal upward transport of nutrient salts is in 

 any way interfered with when transpiration is reduced to a 

 minimum by high humidity. It has been suggested that 

 the minimum transpiration necessary for transport has not 

 been reached in experiments at high humidity. The 

 amount of transpiration from the leaves of the ringed stems, 

 however, has almost certainly exceeded this minimum, yet 

 in some instances the increase in nitrogen or ash of these 

 leaves has been exceedingly small as compared with the 

 increase in the check leaves. Since in most cases the 

 ringed branches were in competition for water with check 

 branches on the same plant, one might postulate that a 

 slight reduction in transpiration from the ringed stem might 

 greatly favor flow to the check stem. In other experi- 

 ments, however, in which single main stems were ringed 

 in some cases while one of a forked pair was ringed in 

 others, no differences in response were evident between the 

 branches having competition and those having none. 

 When one realizes that any solutes passing the ring must 

 be trapped in the tissues above, whereas much of the 

 material passing into the check leaves may be transported 

 out again, it is surprising that the tissues above a ring do not 

 accumulate more than they do, even if most of the solutes 

 are carried in the phloem and only small amounts leak 

 into the xylem. If, on the other hand, the transport is 

 chiefly through the xylem, I see no satisfactory explanation 

 for the failure to accumulate above a ring, even assuming 

 that the rate of transpiration is greatly reduced. 



