14 



place since the first examination, it was regarded as reasonably cer- 

 tain that the root tips had perished, and a less concentrated solution 

 was tried. To obviate the possibility of mistaking a temporary condi- 

 tion of plasmolysis for final loss of vitality the roots were in earlier 

 experiments transferred, after twenty-four hours, from the salt solu- 

 tion to distilled water; but this precaution soon proved to be need- 

 less. In all the experiments a control culture in distilled water was 

 maintained under conditions of temperature and illumination iden- 

 tical with those of the salt cultures. As a matter of course, the growth 

 of the roots is by no means as rapid in distilled water as in ordinary 

 river water or in a prepared culture solution. 



It was sought to keep the external conditions as nearly as possible 

 uniform during the entire series of experiments and a temperature of 

 19° to 21° C. was maintained in the laboratory.^ 



The rate of growth during the period of experiment was ascertained 

 bj^ marking the radicle with India ink just before placing it in the solu- 

 tion. The mark, which was made as fine as was compatible with per- 

 manency, was placed at a distance of 15 mm. from the root tip in the case 

 of the lupines and 10 mm. in the case of alfalfa (Medicago) so as safely 

 to include the entire zone of active growth in the primary root.^ This 

 method of measuring the growth of roots was employed hj Sachs in 

 his classical studies upon the growth of primary and lateral roots, ^ and 

 has been widely adopted by plant physiologists.^ By comparison of 

 the marked root with a ruled surface the amount of growth during 

 any given period can be determined with all the accuracy necessary 

 in experiments of the kind here described.^ 



By using a considerable number of individual plants in each experi- 

 ment with each solution (usually five in case of Lupinus and ten or 

 twelve in case of Medicago) it is believed that the variant due to 

 individual differences in vigor has been practically eliminated.^ 



^In this connection Klemm [Jahrb. f. wiss. Botanik, 28, 659 (1895)] calls atten- 

 tion to the great variability exhibited by plants as to their limit of endurance in 

 solutions of acids of definite concentration if other external conditions be varied. 

 Askenasy [Ber. deutsch. hot. Gesellsch., 8, 61 (1890)] describes the effect upon the 

 growth of roots produced by different temperatures or by a variation of temper- 

 ature during a limited period of time. 



" Sachs determined the length of the growing portion, in the case of roots of 

 other Leguminosae, to be 8 to 10 mm. for Viciafaba and 3.5 to 6.5 mm. in Pisum 

 sativum. [Arb. d. hot. Inst. Wiirzburg, 1, 413 to 419 (1873) ; Gesammelte Abhandl, , 

 2, 803 (1893)] 



=^ Gesammelte Abhandl., 2, 778. 



-Tor example, Kahlenberg and True use this method in all their experiments 

 with plants in solutions of toxic substances. [See Bot. Gazette, 22; 88 (1896)] 



^Askenasy [Ber. d. deutsch. bot. Gesellsch., 8,64, (1890)] shows that this method of 

 marking causes a retardation of growth during the first hour thereafter, but that 

 this is overcome after two hours. Consequently the method could be used with- 

 out hesitation in these experiments, although it is sometimes attended by disad- 

 vantages when the phenomena of growth itself are studied. 



•^More than 2,500 seedlings of Lupinus albus and 700 of Medicago sativa were 

 employed in the whole series of experiments. 



