O. Biddulph 267 



where they ascend via the transpiration stream to the xylem extremities. 

 At these extremities the actively metabolizing cells acquire minerals 

 from a solution which they in turn surround. In other words, two 

 separate absorption processes must occur before the mesophyll cells of 

 leaves acquire minerals, one by the cells at the root surfaces and the 

 other by the cells surrounding the xylem. There is no valid reason to 

 assume that precipitation reactions would not occur in the xylem ex- 

 tremities except perhaps that they may be partially protected by precipi- 

 tation reactions occurring at the root surfaces and so may be partly 

 "screened" from the possibility of reactions of similar intensity. Even 

 so, precipitation reactions can be duplicated in both places. Our evidence 

 is presented as follows. 



Exactly comparable studies of precipitation reactions at root-solution 

 and vein-mesophyll junctions cannot be made for anatomical reasons; 

 therefore, two methods of study are necessary. The root-solution junc- 

 tion has been studied by conventional tracer methods, while the vein- 

 mesophyll junction has been studied by autoradiography. Results are 

 limited to iron and phosphorus. The uptake and translocation of iron 

 by bean plants was measured by the usual colorimetric methods. The 

 roots were not washed prior to analysis and so contained most of the 

 iron which had accumulated both in the roots and on the root surfaces 

 during the growth period. Figure 3 (lower part) shows the iron con- 

 centration of bean roots when grown for 12 days in solutions of various 

 iron concentrations. The upper part of Figure 3 shows the distribution 

 in the leaves of radioiron which was placed in the nutrient solution 

 during two additional days of growth. These data show that the pres- 

 ence of a large amount of precipitated iron on the root surfaces has 

 inhibited the uptake of additional iron (Fe 55-59 ). The composition of 

 the precipitate on the root surfaces has been analyzed and is known to 

 be predominantly ferric phosphate at pH's below 6.0. At pH 6.0 some 

 calcium is present while at pH 7.0 calcium is an important constituent 

 along with the iron and phosphorus. In the nutrient solution used, the 

 phosphorus-to-iron ratio was approximately 10:1. 



With regard to the entry of phosphorus into the plant, the following 

 experiment will show that it is influenced by the presence of iron in 

 the nutrient solution. In this study the phosphorus-to-iron ratio was ap- 



