CHAPTER XXVIII 

 TRANSLOCATION OF SOLUTES 



In most plants a large proportion of the living cells do not contain 

 chloroplasts. All such non-green cells are dependent for essential carbo- 

 hydrates upon the chlorophyllous cells of the plant. Many of the non-green 

 cells are remote from the photosynthesizing cells. The cells in the root 

 tips of trees, for example, are sometimes hundreds of feet distant from the 

 nearest leaves. They, as well as all other non-green cells in the body of 

 a plant, are dependent for their existence upon the simple carbohydrates which 

 are carried to them through intervening tissues from the chlorenchyma. The 

 movement of solutes such as simple carbohydrates from one part of a plant 

 to another is designated as the translocation, transport, or conduction of 

 solutes. Alany other kinds of solutes are translocated in plants besides car- 

 bohydrates, and transport occurs in upward and lateral directions as well as 

 in a downward direction. 



Cell to cell movement of solutes may occur in any part of a plant but 

 the term translocation is generally restricted to the movement of solutes in 

 the specialized tissues of the phloem and xylem in which the distance through 

 which they are transported is usually very great in proportion to the size of 

 the individual cells. 



Anatomy of Phloem Tissues. — Of the various stem tissues only the xylem 

 and phloem possess such a structure as to suggest that a relatively rapid 

 longitudinal movement of solutes can occur through them. Both of these 

 tissues are characterized by the presence of elongated cells and elements 

 which are joined in such a way as to form essentially continuous ducts. 

 Furthermore, it has been shown experimentally that the rate of movement of 

 solutes through other stem tissues such as the pith and cortex is totally in- 

 adequate to account for known rates of translocation through stems. 



The structure of the xylem tissues has already been discussed in Chap. 

 XV. Discussion of the anatomy of the conductive tissues will now be com- 

 pleted by a consideration of the structure of the phloem. Like the xylem 

 the phloem is continuous from the top to the bottom of the plant, the 

 ultimate terminations of the phloem system being in the tissues of the stem 



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