ABSORPTION OF LEAD BY PLANTS 881 



One of the best known cases of this "antagonism" is probably that 

 between CaClg and NaCl. In this case the phenomenon of the suspension 

 of the toxicity of NaCl by CaClg is attributed to the ability of the CaCla 

 to alter the plasma-membrane in such a way that it is less permeable 

 to NaCl [OsTERHOUT, 1912]. Since it has been possible to show that in 

 the case of lead a kinetic dispacement of the assimilated ions by other 

 ions occurs, we shall certainly have to reckon with the possibiUty that 

 the antagonism is in individual cases occasioned by such kinetic effects. 



LEAD ASSIMILATION AND TRANSPIRATION CURRENT 



From the fact that more than 50% of the lead is taken up in 24 hours 

 by the root in very dilute solutions of lead, i.e. a quantity of lead which 

 which was present in more than 100 cm^ of liquid, we can conclude that 

 it is not the transpiration current which transmits the assimilated lead, 

 since the daily loss of water of Vicia faba under present conditions is 

 less than 1 cm^. This independence is also shown in the following experi- 

 ment. In one case the percentage assimilation of lead by the root was 

 determined in the usual way, and in another after the root had been 

 first separated from the stem under water. The volume of water was 

 500 cm^, the lead concentration was 10"^ N, and p^^ — 4. Duration of 

 experiment = 1 hour. 



Root as usual 6.8 and 7.5%. 



Root cut off 6.5 and 7.1%. 



It is seen that the amount of lead taken up was in both cases the same. 

 Moreover, the lack of dependence of the assimilation of salt on the 

 absorption of water by the plant has repeatedly been established [cf. 

 Arrhenius, 1922]. 



THE TOXICITY OF LEAD 



In connection with the experiments described in the previous section it 

 is of interest to note that, as has been shown by Bonnet [1922], the 

 introduction of plants into 10~^ N Pb(N03)2 solution unfavourably 

 influences the transpiration current. In contrast to more dilute solutions, 

 such an appreciable concentration of lead shows distinct toxic effects 

 on the plant(^). Vicia faba which had stood 24 hours in a 10"^ N Pb(N03)2 



^1^ Cf. also Lavison [1911] and older experiments of Philips [1883], Knop 

 [1885], Nolle, Bassleb and Will [1884]. 



5 6 Hevesv 



