18 



only was necessarily involved.^ In the more dilute solutions which 

 are still considered toxic, because destroying the root tip, the proxi- 

 mal portion of the root and the upper part of the plant are often not 

 conspicuously injured by twentj^-four or forty-eight hours' exposure. 

 After a certain lapse of time lateral roots are sometimes put forth 

 and grow vigorously in a solution (especially of calcium chloride) 

 which had killed the apical portion of the primary root. 



This power of gradual accommodation on the part of the plant to a 

 solution Avhich at first checked its growth and even destroyed the 

 sensitive tissues of the root tip has often been remarked. It is but a 

 step from this to the well-known fact that by gradually increasing the 

 strength of a salt solution in which plants are cultivated they can be 

 made to endure a degree of concentration which would soon be fatal 

 if administered directly.^ It follows that the limits of endurance here 

 recorded for Lupinus albus are merely those of its root tip, selected 

 as being the most sensitive indicator, and are in some cases lower than 

 the limits which would denote death of the plant as a whole. Further- 

 more, the limit of endurance for the entire plant could undoubtedly be 

 still further elevated by gradually increasing the strength of solution 

 in which the plants are cultivated. 



But our present investigation aims merely at a comparison of the 

 relative toxicity of tlie various "alkali" salts, to attain which the 

 simplest and readiest means are to be preferred. A standard for 

 further comparisons, rather than a thorough investigation of the 

 problem in all its ramifications, is the end of the present paper. 



^ This was likewise the objective of the experiments of Kahlenberg and True 

 [Bot. Gazette, 22,88, (1896)]. In order to obtain results closely comparable with 

 theirs, especially as bearing upon the hypothesis of electrolytic dissociation, their 

 mode of procedure has been closely followed in this as in other details. In advocacy 

 of this method of determining toxic action, Professor True writes: -'Repeated 

 experiments for years have convinced me that the method used gives the most deli- 

 cate and easily managed test that I know of for bulky objects like Lupinus roots." 



Coapin [Rev. Gen. de Botanique, 10, 177 (1898)] criticises the work of Kahlenberg 

 and True, previously ( noted, to the effect that it is impossible to accurately deter- 

 m ne the to-ic limit of a solution in the short period of experiment (twenty-four 

 hours) allowed by those authors. However, as Professor True observes, it was not 

 the point at which the who'.e plant succumbs, but that which marks the death of 

 the zone of growth in the primary root, which formed the objective of his experi- 

 ments. Coupin's method was to grow his plants for several days in the solutions 

 to be tested, taking the strongest solution in which the plant as a whole continued 

 to grow after the first few hours as marking the limit of endurance ("equivalent 

 toxique"). It is obvious, therefore, that no direct comparison is possible between 

 the results obtained by Coupin on the one hand and by Kahlenberg and True, as 

 well as those here recorded, on the other, Coupin's limits of endurance being 

 necessarily much higher. 



'^ Thus Stange [Bot. Zeitung, 50, 292 (1892)] fonnd that root ti-pa oi Lupinus albus 

 and Phaseolus vulgaris soon died if exposed directly to a 0.5 per cent solution of 

 potassium nitrate, but by gradually increasing the concentration they could be 

 made to endure nearly 1 per cent without death of the protoplasm. 



