272 Mineral Nutrition of Plants 



acquired nutrients as the presence of the radioisotope in the new 

 growth will attest. Autoradiographs and analyses are again made and 

 carefully compared with the previous autoradiographs and analyses. 

 Results obtained with phosphorus show that this element is withdrawn 

 from the older tissue and moved to the meristematic areas where it 

 accumulates in relatively high concentrations. The extent of the move- 

 ment of previously acquired P 32 into currently growing stem-tip tissue 

 is shown in Figure 7. It will be seen that roots, stems, and lower leaves 

 have lost considerable phosphorus to the stem tips. This indicates a 

 rather free mobility of P 32 in the "circulatory systems" of the plant. 



The direct injection of a suitable form of a radioisotope into a vein 

 of a leaf located midway up a stem will give a picture of the rapidity 

 and direction of movement (2). Using this method it has been found 

 that a diurnal trend in the movement of phosphorus from mature 

 leaves is evident in bean plants. The most rapid movement occurs in 

 the daylight hours with a strong downward movement extending to 

 the root system in the late afternoon. Considerable movement in an 

 upward direction to leaves higher on the stem and to stem tips fre- 

 quently occurs. However, the extent of movement in an upward direc- 

 tion has proved to be more variable than the extent of downward 

 movement. 



This injection method has been coupled with the stripping method 

 of Stout and Hoagland (9) previously mentioned, except that the stem 

 was "stripped" immediately below the point of divergance of the leaf 

 which was chosen for injection. Using this method with cotton plants, 

 Biddulph and Markle (j) have attained results which show the phloem 

 to be the tissue through which the P 32 moved in leaving the leaf. 



A quantitative estimation of the importance of leaf export of minerals 

 to other areas as compared with the direct delivery from the root cannot 

 be made without a more detailed study. Under certain artificial con- 

 ditions, namely, leaf injection in low salt plants, it appears that move- 

 ment is rapid enough to render this an important source of minerals 

 to growing areas (Figure 8). It may be inferred that the re-export of 

 mineral salts from leaves to roots may have a minor etfect on salt up- 

 take by lessening gradients between root cells and the external nutrient. 

 While the effect of this factor may be measurable, it is difficult to see 

 how it can be an important one. 



