268 The Ohio Journal of Science [Vol. XXI, No. 8, 



edition of his text-book (1916) all either definitely state or 

 imply that a high degree of resistance to organic acids is char- 

 acteristic of Euglense (or even of Euglenidas) in general. 



The present writer became interested in this matter when, 

 having successfully cultivated Euglena gracilis in strongly acid 

 media, he attempted to use similar methods with Euglena deses. 

 These attempts met with failure in every case, and even rel- 

 atively weak solutions of citric acid proved fatal to this species. 



How was this to be interpreted? Is the case of Euglena deses 

 merely a striking exception to the general rule for the group? 

 Or is Euglena gracilis the exceptional form? Or, lastly, may it 

 be that no general rule for the group is justified? 



Good cultures of several species of Euglena and of one species 

 of the closely related genus Phacus were at hand at the time, 

 and it occurred to the writer that a comparatively simple 

 series of experiments w4th them might throw light on these 

 questions. Such a series of experiments was actually planned 

 and carried out, as described in the following paragraphs. 



A quantity of clear surface water was procured from a 

 near-by pond (known to be a habitat of several species of 

 Euglena), and filtered. Tests showed it to be decidedly alkaline 

 in reaction. By careful titration, using neutral red as an indi- 

 cator, it was found that 1 c. c. of 1% HCl (chemically pure) 

 was needed to bring 190 c. c. of this water to the point of 

 neutrality corresponding to that of distilled water. Accordingly, 

 the original quantity of filtered water was now divided into 

 two parts, one of which was left unchanged, while the other 

 was made neutral by the addition of the appropriate quantity 

 of 1^ HCl. With this neutralized pond water the following 

 solutions of citric acid were then made up (chemically pure 

 citric acid being employed): .025%, .05%, .1%, .25%, .5%, 

 1%, 2%, 3%, 4%, 5%.* 



As seven different species were available for the experiment, 

 seven series of these solutions were now put into Syracuse watch 



*It may be noted that the hydrogen ion concentration of these solutions is by 

 no means proportional to the concentration of the acid, since the degree of ioniza- 

 tion decreases as the concentration of the acid becomes greater. But the work of 

 Collett (Jour. Exp. Zool., 1919) and others has shown that other factors besides 

 hydrogen ion concentration are involved in the toxicity of organic acids; hence, 

 for the purposes of these experiments, known concentrations of the acid are pref- 

 erable to known concentrations of hydrogen ions. Per cent solutions were used 

 rather than molar solutions in order to make the results directly comparable to 

 those of Zumstein. 



