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port to younger, acti-vely growing tissues is an important part of their 

 activities. The fate of the mineral nutrients arriving at these two widely 

 different metabolic areas will differ. In young leaves metabolic use will 

 tend to maintain a gradient in favor of continual movement toward 

 these areas. The minerals acquired will be incorporated into new 

 protoplasm. But what of the mature leaves where transpiration is high 

 and metabolic use comparatively low? The transpiration stream will 

 continually make available at the xylem extremities of leaves a supply 

 of minerals. If removal of minerals from the xylem to the mesophyll 

 were as rapid as delivery via the transpiration stream, there would 

 undoubtedly occur an overshooting of metabolic needs in at least the 

 mature leaves. This material must then be subject to re-exportation in 

 order that continued transpiration may not result in unduly high 

 mineral concentrations. Radiophosphorus moves readily into mature 

 leaves and yet concentrations remain below those of the young tissues 

 near the apical meristems. Competition for re-exported minerals for 

 metabolic use within meristems furnishes an opportunity for movement 

 and, provided the mineral in question is mobile within the phloem 

 tissues, a pathway exists for its movement. The older mature and ac- 

 tively photosynthesizing leaves are located between two areas of active 

 growth (roots and stem tips), and apparently supply both with some 

 exported minerals. Since both areas have direct supplies available to 

 them, the amount may be relatively low. However, it can be observed 

 that exported phosphorus and iron reach both growing areas in suf- 

 ficient quantities to be easily measurable. 



Two experimental procedures have been employed to trace export 

 of minerals from leaves. One involves the direct injection of a suitable 

 form of a radioactive isotope into a vein of a leaf and the subsequent 

 following of the migratory material (2). The other method involves 

 the application of a radioactive isotope to the nutrient medium for a 

 period of approximately 4 days, which allows sufficient time for the 

 isotope to be well distributed within the plants. Half of the plants are 

 then removed for autoradiographs and analyses, while the remaining 

 half are placed in a normal nonradioactive nutrient solution for ap- 

 proximately 4 more days. During this latter 4-day period considerable 

 new growth will have occurred; partly at the expense of previously 



