52 J. MCA. KATER AND R. D. BURROUGHS. 



the culture which furnished the starting point for 7-4; Fig. 2b 

 was taken from ^4 two days later. Fig. 2c was taken from 2 

 on the day that F2 was seeded; 2d was taken from 7-2 two days 

 later. Fig. 2e was taken from A i on the day that Fi was seeded; 

 Fig. 2/ from Fi on the second day. A glance at these figures 

 will show that transference of these degenerate forms to a new 

 and favorable medium results in a return towards normality, 

 with the exception of the formation of starch, as Fi and F2 

 formed little of that material. 



DISCUSSION. 



The Caiises of Encystment. In the preceding section attention 

 was called to the fact that Polytomella citri never encysts except 

 when the cell body contains abundant starch. Since this is so it 

 seems as though the search for the external factors for encyst- 

 ment becomes resolved into a search for the factors which 

 encourage the formation and storage of starch. Adverse con- 

 ditions of the environing medium certainly would not be con- 

 ducive to the formation of this material, at least the above experi- 

 ments so indicate, and, consequently, we need not be surprised at 

 the conclusion drawn from the present work, namely, Polytomella 

 citri encysts only under such conditions as are favorable for 

 growth and reproduction, and adverse conditions lead to death 

 of the culture rather than encystment. This very thing, however, 

 may prolong the life of a culture, so far as active forms are 

 concerned. 



The presence of abundant starch in Polytomella as a pre- 

 requisite for encystment is quite in harmony with the obser- 

 vations of Hogue on Amoeba Umax, Carter on Amoeba proteus, 

 Kofoid and Swezy on marine dinoflagellates, Stolte on Blepha- 

 risma, Mast and Ibara on Didinium, and Hall on Oxyrrhis 

 marina. In fact, most of the recent work indicates that abundant 

 food is much more conducive to encystment than is starvation, 

 which \vas formerly supposed to be one of the principal causes of 

 encystment, and is still so given in text-books. 



In the introduction a quotation was given from Kofi man 

 (1924). From this it is seen that altering the Pa to a lethal 

 point either resulted in death or encystment, generally death. 



