ESTABLISHING THE TOXIC LIMITS. 21 



METHOD OF ESTABLISHING THE TOXIC LIMITS. 



Before going into details of the results obtained in the simple 

 solutions, the methods of determining- the limits of endurance of each 

 variety to each salt will be explained. At one time the writer had 

 thought of fixing the limit of endurance in toxic salt solutions at the 

 concentration in which none of the marked radicles would survive at 

 the end of twenty-four hours. A few experiments, however, showed 

 that this was not the proper method, for occasionally one of the root- 

 lets would be sound and healthy at the end of twenty-four hours in a 

 concentration far above that which would permit the roots of a 

 majority of the plants to survive. In other words, individual varia- 

 tion plays such an important part that the strength of a solution 

 which would permit no root tips to survive would be far above that 

 representing the limit of endurance for the variety as a whole. 

 Attention is thus once more directed to the fact that results obtained 

 from a few individuals are as a rule very inaccurate and unreliable. 

 The characters of a variety (and resistance to toxic effects is one of its 

 characters) are the mean of those of the whole number of individuals 

 composing it. Of course all the individuals of a variety can not be 

 examined, Init the number of seedlings experimented with should 

 be large enough to overcome the effect of marked individual varia- 

 tion. It was this consideration that urged the writer to make such 

 a large number of tests. On the other hand, a concentration Avhich 

 would just permit all root tips to survive would not represent the 

 general limit for the variety because of those few individuals Avhich 

 are far inferior to the average in their ability to resist toxic salt 

 solutions. The limit of endurance for the mean of the largest pos- 

 sible number of individuals is the end sought. 



After consideration, it seemed that the most perfect idea of the 

 limit for each variety could be obtained by taking the concentration 

 in which about half the seeds survived and about half died. For 

 instance, if 60 seeds were tested in a 0.01 normal solution of magne- 

 sium sulphate and 30 survived and 30 died, the toxic limit would be 

 represented by that concentration. Of course it was seldom possible; 

 to secure such an equal division, but a slight excess one w^ay or the 

 other would not materialh' alter the results. In practice it was, 

 furthermore, often found expedient to take the mean of the concen- 

 trations actually tested as representing the toxic limit. 



To illustrate: The roots were often found dead in a 0.01 normal 

 solution of magnesium chlorid, and alive in a 0.007;') normal. The 

 aj)proximate toxic limit was fixed at the concentration intermediate 

 between these two, although no solutions intei-mediate in concentra- 

 tiou between 0.01 and 0.0075 of a normal solution were actually 

 made up. 



