15 



Indeed, that this was the case was pretty effectually shown by several 

 repetitions of the experiments with most of the solutions. It is also 

 indicated by tlie general regularity with which toxic effect is shown 

 to increase with every increase in concentration of the solution of 

 each salt. By several times repeating experiments with solutions 

 of ai^i^roximately the critical strength the above-mentioned source of 

 error due to fluctuations in temperature, etc., was likewise reduced to 

 a minimum. 



DETERMINATION OF THE LIMIT OF ENDURANCE. 



In ascertaining the degree of concentration of a given salt solution 

 which will just i)ermit the root tips to retain their vitality' during the 

 period of experiment, one must of course be able to determine also 

 the point at which death definitely occurs. The death point is evi- 

 dently to be sought far below the degi-ee of concentration which per- 

 mits no elongation whatever to occur during the period of experi- 

 ment, for often radicles, of which the marked zone had increased in 

 length several millimeters (even 6) at some time during the experi- 

 ment, were indubitabh' dead at the end of twenty-four hours. ^ The 

 mere fact of elongation, irrespective of the time in which it has taken 

 place, does not therefore determine the concentration of a salt solu- 

 tion in which roots will survive, although sometimes useful in ascer- 

 taining whether the root is absolutely dead at the end of a given 

 period. It is to the general condition of the apical portion of the 

 root that we must look for a criterion. While it is sometimes difficult 

 to describe those symptoms which denote the death of the root tip, 

 it is comparatively easy to recognize them after one has acquired 

 sufficient experience with the behavior of plants grown in toxic 

 solutions. 



One of the most easily detected of the phenomena accompanying 

 death in i^lants is final loss of turgor due to excessive plasmolysis. 

 In other words, the tissues lose their water, and are unable to make 

 good the loss, even when restored to normal conditions. This is due 

 primarily to a change in the osmotic equilibrium of the plant cells. 

 Ordinarily, through the controlling activity of the protoplasm, a suffi- 

 cient osmotic pressure is maintained in the sap cavity of the cell to 



^ Experiments were made with solutions of a strength known to be fatal, yet 

 permitting some elongation during twenty-four hours. Sodium sulphate (0.05 

 normal), sodium carbonate (0.02 normal), and magnesium chloride (0.05 normal) 

 were selected, and in every case it was found that elongation ceased entirely after 

 three to five hours. In a water control, on the other hand, growth was still pro- 

 gressing at the end of six hours, and an examination at the end of twenty-four 

 hours showed that it had been pretty equally distributed throughout the entire 

 period. These results as to toxic action correspond with Sachs's statement [Land- 

 wirthsch. Versuchsst, 1, 219 (1859;]; Gesammelte Abhandl., 1, 430 (1892)] that 

 "roots appear to lose more and more the power of absorbing water containing 

 salt the longer they are in contact with it." 



