1 66 ARTHUR WHITE GREELEV. 



has kept cultures of various Infusoria for long periods of time, 

 and has found that some of the carnivorous forms, notably Oxy- 

 trichia, form cysts after they have been deprived of food. Hert- 

 wig 1 has observed that the same fact holds good for Actino- 

 splucrhiiii. In the same series of experiments he found, however, 

 that the consumption of an excess of food may cause encystment 

 as well as starvation. Other cases are on record also in which 

 various carnivorous Infusoria have been seen to encyst after en- 

 gulfing a large amount of food. Klebs,- in a large number of ex- 

 periments on fresh water Algae, has observed that in ] r auc/icria 

 zoospores are formed when the filaments are transferred from 

 the light to the dark. Klebs reared Vaiichcria in the following 

 solution, used at concentrations of from o. I to 0.4 per cent.: 

 Ca(NO 3 ) 2 , 4 parts; MgSO 4 , I part; KNO 3 , I part; K 3 HPO 4 , i 

 part ; and found that when the filaments were transferred from 

 this solution to distilled water, irrespective of light or dark, 

 zoospores were formed. He also succeeded in producing par- 

 thenogenetic spores in Spirogyra by plasmolyzing the cells with 

 a sugar solution. Klebs makes the general statement that the 

 formation of zoospores in }^aitclicria is aided by lowering the 

 temperature, but made no experiments to show that a lowering 

 of the temperature itself will actually cause the plant to form 

 spores. He apparently had in mind only the limits of tempera- 

 ture at which spore formation may take place, when the process 

 is initiated by other means. Beyond these observations, which 

 are rather inconclusive as far as the Protozoa are concerned, 

 nothing is known about the conditions which determine encyst- 

 ment or spore formation. 



Dr. Loeb suggested that I take up the problem of asexual re- 

 production from an experimental point of view. Many authors 

 had noticed that the mode of reproduction changes in certain 

 aquatic animals or Protozoa when the pond in which they live 

 begins to dry out. But the question was, how can the lack of 

 water in a pond interfere with the mode of reproduction? Dr. 

 Loeb's idea was that the real physical factor at work in this case 



1 Hertwig, Sitzungberichtc tier Gesellschafl fur Morphologie unJ Physiologic in 

 Alii >i f hen, 1899. 



2 Klebs, Die Bedingungen der Fortpflanzung bei eini^en Algen itnd Pilzen, Jena, 

 1896. 



