35 



XXVI. " Organic Plant Poisons." I. — " Hydrocyanic Acid.'* 

 Winifred E. Brenchley. Annals of Botany, 1917. 



31, 447-456. 



Experiments have been carried out to test the action of hydrocyanic 

 acid on plants when applied to the roots in water culture, and com- 

 parisons were made of the effects of formic acid and sodium cyanide. 

 The results showed that prussic acid is very toxic to peas and barley. 

 All strengths up to and including 1/100,000 kill peas outright, either 

 immediately or after a short interval of poor growth. All strong 

 concentrations kill barley, but with 1/100,000 a period elapses during 

 which no growth occurs, after which a little progress is made, though 

 the plants never attain any size. 



The peas killed by prussic acid shrivel from the cotyledons upwards 

 and the roots contract so intensely that they are often completely 

 withdrawn from the nutrient solution. Barley roots decline to enter 

 strong solutions at all, but often put out laterals, which stop short at 

 the surface of the solution and develop the bunchy habit characteristic 

 of growth in the presence of poison. 



Formic acid is comparatively harmless to barley, except in very 

 strong concentrations, whereas sodium cyanide is quite as toxic as 

 prussic acid. 



No trace of stimulation in peas or barley has been obtained with 

 any of the compounds tested. 



XXVII. " Organic Plant Poisons.'' II. — " Phenols." Winifred 

 E. Brenchley. Annals of Botany, 1918. 32, 259-278. 



Experiments on similar lines were carried out with various phenols, 

 phenol itself, the three cresols, resorcinol, pyrocatechol, pyrogallol, 

 phloroglucin, and orcinol. 



The general action of these various phenols upon barley and pea 

 plants grown in water cultures is very similar, though the individual 

 substances exercise specific actions at somewhat varying concentrations. 

 In every case a solution containing one per cent, of the molecular 

 weight in grams per litre (M/ioo) of the phenol proves to be fatal, 

 death usually occurring within a very short time after the plant 

 comes in contact with the solutions. Occasionally, as with resorcinol 

 and orcinol on peas, the shoots continue to make a certain amount of 

 growth for a few days, even though the roots are killed. Apparently 

 the toxin in these cases is not conveyed to the leaves at once, so 

 that they are able to grow for a time at the expense of the food stored 

 up in the seeds. More usually, however, the growth of the shoots is 

 checked simultaneously with that of the roots, though the leaves retain 

 their green colour for a long time before they wilt. 



The difference in the relative toxicity of the phenols is well shown 

 by the action of solutions one-fifth as strong as the above (M/ioo x 

 1/5). Marked toxic action is evident at first in every case, and the roots 

 are often killed and discoloured, o-cresol, pyrocatechol and pyrogallol 

 kill peas outright at this strength, but with the other substances the 

 roots make an attempt to right themselves after some time has elapsed. 

 New laterals are pushed out, which frequently refuse to enter the 

 solutions, so that the recovery is only partial. Peas make only very 

 slight recovery from the effects of m-cresol, rather more from those of 

 ^-cresol, phenol and phloroglucin ; while in presence of resorcinol and 



