160 TRANSLOCATION IN PLANTS 



seem incapable of controlling the type of material moving 

 toward it through the phloem, but could control only the 

 rate of flow of the mixed contents through a control of the 

 pressure gradient. 



5. The explanation given by Miinch accounting for the 

 removal of water from the receiving cells seems possible 

 when the latter are in the position of the cambium. In 

 this case the receiving cells lie between the phloem which 

 supplies the food solution and the xylem through which 

 the water may be removed. The receiving cells at the 

 apical meristem, however, are not in such an ideal situation 

 in this respect, and an explanation of the removal of water 

 seems more difficult. Furthermore, for cortical storage 

 tissues a possible mechanism for removing the excess water 

 is not obvious; for the phloem is between the receiving 

 tissues and the tissues which might remove the water. 

 In an exposed stem it is possible that evaporation from 

 the outer surface might be effective, but in many plants 

 the cortical storage tissues are subterranean, where water 

 loss from the surface may become very shght. It is con- 

 ceivable that solutions may enter through plasmodesma 

 and water may move backward through the walls, but the 

 rate of such movement must be rather low. Movement to 

 tissues of this sort, therefore, seems difficult of explanation 

 according to the hypothesis of Miinch. 



6. Another serious objection is that this hypothesis 

 seems not to allow for a simple explanation of the inabiUty 

 of leaves to empty themselves when they are removed from 

 the stems and their petioles placed in water. If Munch's 

 hypothesis is correct, one would expect that such isolated 

 leaves, or cut stems with leaves, would rapidly empty 

 their sugar through the cut phloem into the water. The 

 greatly shortened channel for transport and the increased 

 water supply through the xylem should increase the steep- 

 ness of the pressure gradient and empty the leaves of their 

 sugar much more quickly than when left attached to the 

 plant. The observations of SaposchnikofT (1893), Deleano 

 (1911), and the unpublished findings of Miss Skaer (1931) 



