TOXIC EXUDATES AND SOIL TOXINS. 145 



be assimilated or might prove injurious. He observed that peas 

 grew poorly in water in which peas had previously grown, but that 

 wheat grew readily in it. Perhaps his most striking experiment was 

 one in which one portion of the root-system of Mercurialis was 

 placed in a solution of lead acetate and the other in pure water. In 

 a few days the pure water was found to contain the salt, and he con- 

 cluded that it had been carried through the roots and excreted. 



Roper (1833) called Macaire's results in question on the basis of 

 the difficulty of freeing the roots from soil without injuring them. 

 Unger (1836 : 147) shared this doubt, and believed that the results 

 might also have been due to capillary action. He employed Lemna 

 minor in order to avoid injury to the roots. Plants were placed in a 

 dilute solution of sugar of lead for 8 days, washed in distilled water, 

 and then kept in the latter for 3 days. The most sensitive tests of the 

 latter failed to show any traces of lead, and consequently showed that 

 no excretion had occurred. The experiment was then reversed by 

 placing plants of Lemna for various periods in a salt of ammonium 

 and subjecting them after thorough washing to a concentrated solu- 

 tion of lead acetate. In spite of proof that the salt had been ab- 

 sorbed, no trace of it could be found as an excretion into the solution. 



Daubeny (1835) found that strontium nitrate absorbed by one- 

 half of a root was not excreted by the other half, but when potassium 

 chromate or ferrous sulphate was used, a trace seemed to be secreted 

 into the distilled water. In a later investigation that has become 

 classic (1845), he grew 18 different crops continuously on the same 

 plots and compared the yields with those of crops shifted so that each 

 crop was followed by one of a different kind. There was a gradual 

 decrease in nearly every case, and this was usually greater with con- 

 tinuous cropping. However, the differences between continuous 

 cropping and rotation were insufficient to justify the assumption of a 

 soil toxin. They were attributed to the more rapid removal of the 

 needed nutrients in the plots continuously cropped, and this was 

 borne out by soil and ash analyses, leading to the distinction between 

 available and non-available nutrients in the soil. 



Braconnet (1839) also thought that Macaire's results were due to 

 capillarity or to a siphon-Hke action. His attempt to obtain opium 

 from soil in which poppies had grown was unsuccessful, as was also 

 that of Walser (1838). Both investigators showed, moreover, that 

 the solid excretions found by Brugmans were nothing but the exfoli- 

 ated outer layers of the root. ' 



Boussingault (1844) reached the conclusion that roots do not nor- 

 mally excrete substances, though they may do so in water cultures. 



Garreau and Brauwers (1858 : 186) were of the opinion that the 

 exfoliated matter left in the soil by the growth of roots served to 

 explain the antipathy of certain plants for others. 



