THE CAUSE AND NATURE OK 1*.\( YSTMKNT. 45 



would think that the two days which the cultures lasted at 35 C. 

 would have been sufficient for the organisms to encyst rather 

 than die. It is also noteworthy that the six cultures at 30 did 

 not encyst until after their controls. It seems entirely justifiable 

 to conclude that extremely low or high temperatures prevent 

 encystment and that medium temperatures are not only con- 

 ducive to rapid growth and division but also to encystment. 



Hydrogen-ion Concentration. It was noted in the above ac- 

 count that room temperature was the best one employed for the 

 growth and division and encystment of Polytomella citri. Con- 

 sequently, all of the remaining experiments were carried out at 

 room temperature. 



The procedure for the work on hydrogen-ion concentration has 

 largely been given. Unaltered hay infusion (Ph 5.4) was used 

 for the controls and, of course, these were not altered for the 

 duration of the experiment. For the remaining cultures the Ph 

 was restored to that at which the cultures were started on the 

 third, ninth, and sixteenth days. Forty-eight cultures were 

 seeded and the hydrogen-ion concentration was changed to the 

 following P h values (six cultures in each series): 3.5 (series Ci), 

 4.5 (series 2), 5.4 (series 3), 6.2 (series 4), 7.5 (series 5), and 

 8.5 (series C6). Twelve cultures were kept as controls (series C'j). 



By the third day the cultures of series Ci were in fair condition 

 and so they remained for the first week. After that the numbers 

 decreased to such an extent that all could truly be classed as poor 

 cultures. On the twenty-second day they were still poor and 

 no cysts had been formed. Series 2, 3, 4, and 5, were 

 practically identical throughout the first twenty two days. This 

 correspondence was so great that it is unnecessary to give separate 

 results for each, which would be merely repetition. They were in 

 good condition on the third day and excellent on the fifth. A 

 very few cysts appeared on the fifth day. On the twenty-second 

 day, the cultures were still in good condition and some cysts 

 were present. Series C6 remained rather poor throughout the 

 experiment and only a very few cysts were found on the twenty- 

 second day. 



The controls (C'j) behaved quite differently from the other 

 cultures. On the fifth day all twelve cultures of C'j were in 



4 



