11 



Iodine- 131 can occur in the gaseous state, and in this form it is taken up by 

 both the mesophyll (35 to 40 per cent) and the epidermal tissue (118). The rate de- 

 pends upon the concentration. Stable iodine does not reduce iodine- 131 absorption, 

 but it does reduce translocation. 



The washing effect of rain can reduce the foliar intake of strontium by a factor 

 as large as six (71). Intake of cesium is reduced to a lesser extent. Contaminated 

 dust is satisfactorily removed from leaves by washing with 0. 1 N HCI, but spray 

 contamination is much more difficult to remove (4). Over 50 per cent of the iodine 

 in foliar contamination is removed by washing, 70 to 85 per cent by different ad- 

 hesives, and up to 97 per cent by stripping off the upper and lower epidermis (42). 

 Not only can washing remove surface contamination, but it can also remove ions 

 from within the plant (55). Sodium, potassiura, and manganese are readily leached; 

 calcium, magnesium, sulfur, potassium, and strontium are moderately leached; and 

 iron, zinc, phosphorus, and chlorine are leached with difficulty (130), 



D. Plant-Base Absorption 



Some British workers have given attention to the absorption of fission products 

 from the root mats of long-established grass pastures (107). The root mat is com- 

 posed of roots, basal portions of stems, and organic matter. The strontium- 90 in 

 rainfall and that washed off the plants may be held in the mat long enough for con- 

 siderable absorption to occur. This pathway bypasses soil reactions and may be 

 very important where the root mat is present. 



The proposed plant-base mechanism provides a reasonable explanation for 

 the strontium- 90 concentration in pasture vegetation, which appears far too high to 

 be accounted for from expected soil uptake and foliar absorption. 



E. Distribution in Plants 



Strontium tends^ to accumulate in the aboveground portions of plants (16, 47, 

 67, 78, 106, 123), particularly in the vascular tissues (77), although the root con- 

 centration increases with time (58). The greatest concentration of strontium is 

 usually found in the older leaves (85, 94), with only about one tenth as much in the 

 seeds (78, 94, 120). However, the seeds of a few plants, e.g.. Euphorbia, accumu- 

 late strontium to a greater extent (13). 



Cesium and rubidium are similar to potassium, and therefore it is expected 

 that they are distributed more uniformly throughout the plant than strontium. About 

 10 per cent of the total plant cesium is found in the grain of wheat and oats (47), and 

 other work (67) indicates that there is a slight tendency for both cesium and rubidium 

 to accumulate in the young leaves and flowers. 



In four plant species, the highest concentration of iodine was found in the roots, 

 followed by older leaves and, finally, the younger leaves (119). Other radioactive 

 constituents of fallout, which have not been specifically mentioned, concentrated in 

 the roots, with little translocation to the top (47, 85, 94, 106), 



