On the Growth of certain Fresh-water and Soil Protista. 161 



allantoin. All these substances gave negative results, and no growth of 

 Euglena could be obtained in optimal Miquel mixture to which these 

 substances were added. 



Thus, no substances other than compounds of the amido-acid type, have 

 been found suitable for stimulating the growth of the Euglena. 



If we enquire into the part played by the amido- acids in the nutrition of 

 Euglena, it may first be noted that the Euglena is obtaining the greater part 

 of its nutriment from the CO2 of the air and from the mineral substances in 

 the Miquel mixture. This was readily proved by keeping a control tube, 

 containing the optimal Miquel solution to which tyrosin had been added, in 

 the dark, in which case the growth of the Euglena was at once arrested 

 (fig- 2). 



It must also be pointed out that the amount of amido-acid present in the 

 optimal culture medium is exceedingly minute ; e.g. in the case of tyrosin 



Fig. 2. — Photograph showing the Growth of Euglena in Miquel Mixture and Tyrosin, 

 in the dark (tube C), and in the light (tube D). 



Photographed three weeks after inoculation. 



solution the amount of the salt is only 1 part in 24,000 of liquid. It is very 

 remarkable that so minute a trace of organic matter can make so great a 

 difference in the"rapidity of growth and reproduction in an organism as shown 

 in the first photograph (fig. 1). It would appear that the organic substance 

 acts more as a stimulant than as a direct source of nutriment. 



