ver street an d Dean 9 1 



absorbed lead and lead in the culture solution. From this fact he con- 

 cluded that lead is held in the plant in the form of a dissociable saline 

 compound, that is, by electrovalent forces. 



At a much later date (19,20,21) Hevesy and his co-workers carried 

 out similar experiments with radioactive phosphate. They concluded 

 that in the case of a number of plant parts the phosphorus was held 

 by electrovalent forces, presumably as inorganic phosphate, although 

 in the case of a yeast sample it was probably held by electron pair bonds 

 — possibly as hexosephosphate or adenylphosphoric acid. 



In 1938 Jenny and Overstreet (24) studied the rate of the reaction 



R • K* + K + > R . K + K* + 



using potassium clays and potassium chloride solutions for the outside 

 media. They found that although the reaction proceeded at a measur- 

 able rate, only about 10 per cent of the K* initially held by the roots 

 underwent exchange in a period of 24 hours. 



These studies were soon followed by similar experiments with radio- 

 active sodium, rubidium, potassium, bromine, and phosphate by Jenny, 

 Overstreet, and Ayres (26), Mullens and Brooks (jj), Overstreet and 

 Broyer (jS), and Broyer and Overstreet (9). In brief, the experiments 

 showed that all the above-mentioned elements are held by the plant 

 in an exchangeable form, and therefore most likely not entirely by elec- 

 tron pair bonds. On the other hand, a great diversity was noted in the 

 rates by which individual elements underwent exchange. This indicated 

 considerable variation in the strength of binding of the different ele- 

 ments in the plant. 



Recently, the exchange of ions at extremely low concentrations be- 

 tween barley roots and culture solutions has been studied by Overstreet 

 and Jacobson (39) and Jacobson and Overstreet (23). The exchange 

 behavior of Rb+, H L .P0 4 — , Sr++, and I - at concentrations of the order 

 of io -9 molal was studied. The advantages of such solutions are (a) 

 freedom from osmotic effects and (b) extreme degree of sensitivity, 

 enabling experiments to be carried out with single root tips for as short 

 a time as 30 seconds. The isotopic exchange curves on individual apical 

 segments were determined at temperatures near o° C. in order to avoid 

 complications due to metabolic activity. 



The experiments showed that the isotopic exchange curves for the 



