486 TRANSLOCATION OF SOLUTES 



of ray and parenchyma cells into hard, thick-walled, dead cells known as 

 sto7ie cells. In woody stems further profound modifications occur in the aging 

 phloem tissues as the result of the activity of secondary meristems called 

 cork caTtibiums. 



Downward Translocation of Organic Solutes. — Downward transloca- 

 tion of organic solutes unquestionably occurs for the most part through the 

 phloem tissues. The only possible alternative theory, that downward trans- 

 location occurs in certain xylem elements, has been advocated occasionally and 

 this possibility will be examined briefly in the discussion. 



Most of the evidence indicating that organic solutes (principally car- 

 bohydrates) move toward the basal portions of plants in the phloem has 

 been obtained by ringing experiments, "Ringing," when the term is employed 

 without qualification, refers to the removal of a narrow continuous band 

 of the tissues external to the xylem. Since the ringing entirely encircles the 

 stem all tissues external to the xylem are completely intercepted. This opera- 

 tion is also often called "girdling," Ringing is seldom practiced except on 

 woody stems in which it is equivalent to removing a strip of bark and destroy- 

 ing the cambium. 



In a girdled tree carbohydrates slowly accumulate in the tissues above 

 the ring, while the quantity of carbohydrates in the stem and root tissues be- 

 low the ring slowly become depleted as a result of their utilization in respira- 

 tion and assimilation. Such a result of girdling has also been demonstrated 

 in certain herbaceous plants such as cotton (Mason and Maskell, 1928). 



This indicates that the normal downward translocation of carbohydrates 

 occurs through the phloem tissues. Such a conclusion is also substantiated 

 by experiments in which the cortical tissues external to the phloem are re- 

 moved but the phloem itself is left intact. Downward translocation is not 

 appreciably influenced by treating plants in this manner. 



Horticulturists often practice temporary ringing of the branches of apple 

 trees in order to increase the size of developing fruits. Temporary ringing 

 is accomplished by cutting out such a narrow strip of bark that the gap is 

 gradually bridged by wound tissue. The interception of the phloem persists 

 long enough, however, to interrupt temporarily the movement of soluble 

 carbohydrates to lower parts of the tree. Carbohydrates synthesized by the 

 leaves on the ringed branch mostly remain in that branch or are translocated 

 to the developing fruits. The greater supply of foods favors enhanced 



growth of the fruits. 



Chemical analyses show that the cells of the phloem are relatively rich 

 in carbohydrates and organic nitrogenous compounds. This finding is in ac- 

 cordance with the concept that translocation of organic compounds occurs 



