ABSORPTION OF LEAD BY PLANTS 879 



ON THE MODE OF COMBINATION OF LEAD IN THE ROOT 



The question as to whether the assimilated, lead enters into an organic- 

 molecule, or whether it is retained by the plant in the form of a saline 

 compound can easily be decided. In the first case, lead atoms which had 

 once been taken up by the root would not be able to interchange places 

 with other lead atoms, whereas in the second case an active kinetic 

 interchange between the lead atoms bound in the plant and those present 

 in the solution would necessarily take place. 



In order to make the argument clearer, we shall designate the atoms 

 of lead in molecules such as those of lead tetraphenyl as "red" ones, and 

 those which occur in such a form as lead nitrate as "blue" ones. If we 

 dissolve both compounds in the same solvent and then separate them 

 by crystallization, we should find only red atoms in the lead tetraphenyl 

 and only hlue ones in the lead nitrate, since the lead atoms in the lead 

 tetraphenyl are available in an undissociable form. If, on the other 

 hand, we dissolve equi-molecular amounts of lead chloride (with red 

 lead) and lead nitrate (with hlue lead), i.e. two salts in the same solvent, 

 then after separation the two compounds would be composed half of 

 red and half of hlue lead atoms [Hevesy and Zechmeister, 1920]. 

 The distinction between red and hlue corresponds here to radioactive 

 and to inactive lead. 



If the root has taken up active lead and we place it in a solution of 

 inactive lead, then, if the active lead lies stably embedded in organic 

 molecules, no active lead will be able to pass over into the solution, or 

 in other words we shall not be able to displace the active lead with the 

 aid of inactive lead. Now experiment shows that, with the help of a 

 solution which is relatively rich in lead (10^^ ^^^ ^g pg^n remove almost 

 quantitatively the lead taken up by the root, whence we must conclude 

 that the lead in the plant root exists in the form of a dissociahle saline 

 compound, perhaps attached to the cell walls. 



For example, if w^e introduce a Vicia faha (after careful rinsing) 

 which has stood 24 hours in 200 cm^ of an active 10^ N lead nitrate 

 solution into a much more concentrated 10 "^ N inactive lead nitrate 

 solution of the same volume, we find that 95% of the active lead taken 

 up by the root passes over into the 10^^ ^ solution; i. e. the active 

 Pb-atoms are almost completely displaced from their places in the 

 root by inactive atoms, which, of course, preponderate strongly (about 

 20,000 times), from the statistical viewpoint. 



Now a 10~2 iV lead nitrate solution is partially split up hydrolytically, 

 and one might be inclined to ascnibe the inverse dissolving action of 

 lead nitrate to its acid content. However, with the aid of a 10'^ N IINO3 

 solution it was possible to remove only 29% of the lead content of the 

 root, and by the use of distilled water as solvent only 18% could be 



