Effect of C/ieifiirrils o)i Grmvtli 533 



which time half the growth in length has occurred), is due al- 

 most solely to the imbibition of water. Up to this time growth 

 occurs in much the same way whether there are food substances 

 present in the water or not; as we have seen, it takes place when 

 all the solid substances are removed from the fluid by filtering, 

 or even in pure distilled water containing a little sodium chloride. 

 After about ninety minutes the presence or absence of bacterial 

 food in the medium has a noticeable efi^ect on the growth. If 

 plenty of bacterial food is present the animals are thicker, but 

 do not grow so long as with little food; this is owing to the fact 

 that where food is abundant, fission takes place more frequently. 

 The animals may, however, reach the normal length in a medium 

 containing almost no food; we have seen that this occurred in 

 distilled water containing a Httle NaCl. But in such cases fission 

 does not occur; this growth by mere imbibition of fluid can mani- 

 festly not continue for more than one generation. 



5. In investigating the different resistance of young and adult 

 Paramecia toward the stronger solutions of the chemicals, it 

 was found that adult Paramecia were much more resistant than 

 very young animals toward sodium chloride, strychnine, and 

 alcohol, while the reverse was true with nicotine, the young being 

 more resistant than the adult. It would be naturally expected 

 from the work done on higher animals that the adult would be 

 more resistant. The reversal of the effects found in nicotine are 

 probably due to some specific effects of the nicotine which are 

 not present m the other chemicals. 



6. Acchmatization of Paramecium to strengths of sodium 

 chloride, which would kill the animals if introduced directly, 

 was found to be easy, but this acclimatization was not permanent 

 and the animals finally died. In acclimatizing Paramecia to 

 a Ji NaCl solution the ratios of the different salts within the cell 

 are probably changed to such a degree that it takes a long time 

 for the cell to regam its equilibrium so as to continue its normal 

 metabolic processes. Acclimatization of Paramecia to the other 

 chemicals was not attempted. 



