ABSORPTION OF LEAD BV PLANTS 



877 



solution (thorium B is an isotope of lead, i.e. a substance showing 

 completely the chemical properties of lead) prepared in this way is about 

 10~i2, a^Q(-[ jf ^,g wish to increase it we only need add to the solution a 

 known amount of lead nitrate. For example, if w-e assume that we have 

 prepared in this manner a 10^ N solution of lead nitrate, antl that 

 after evaporating it to dryness is shows a radioactivity of 10,000 relative 

 units, then each relative radioactive unit would correspond to an amount 

 of 2.10~^mgm. of lead. \\c must of course take account of the fact that 

 the material of the ash of the parts of the plants absorb part of the rays 

 from the contained thorium B, but we can easily eliminate this distur- 

 bance by mixing the preparation used for comparison with the same 

 quantity of ash as is contained in the sample the radioactivity of which 

 we desire to know. 



The following example shows the procedure during an experiment: 

 Vicia faha that had undergone cultivation in a nutrient solution for a 

 fortnight was introduced, after careful washing, into 500 cni^ of a 10^ 

 A^ radioactive solution of lead nitrate, which also contained 1/200 mol. 

 of sodium nitrate. The temperature was 17°. The usual precautions such 

 as screening the roots from light, etc., were also attended to. After 22 

 hours the plant was removed from the solution, and after careful w^ash- 

 ing with distilled w^ater, the various parts — root, fruit, stem and leaves — 

 were dried separately, ignited after the addition of a drop of concen- 

 trated sulphuric acid, and measured electroscopically^^^ The result of the 

 experiment is shown in the following table. 



Experimental (a). Part of plant 



"Weight of 

 ash ia mgm 



"a of the total lead iii 



the solution contained 



in the ash 



Mgm of lead 

 in the ash 



Lead content 



of the ash 



in % 



The purpose of the following experiments was to investigate the 

 manner in which the assimilation of lead in the case of Vicia jaba 

 varies with the lead concentration of the solution. In all of these experi- 

 ments the volume of the lead solution was 200 cm^ and the duration of 

 the experiment was 24 hours. 



The following collection of results shows that the individual experi- 

 ments can be repeated, the agreement being quite satisfactory. Per cent 



^^' Before the the measurement one must wait about six houre in order to be 

 certain that radioactive equilibrium has been established betwen thorium B and 

 thorium C. The reasons lor this are outside the scope of" this paper. 



