CRYPTOBIOTIC STAGES IN BIOLOGY 9 



the burls of redwood trees when these are placed in a dish with 

 moisture. 



The trigger mechanism involved here must certainly be bio- 

 chemical, and an understanding of this would not only be very 

 interesting but also very useful. 



CYSTS 



Among the protozoa, the cysts are perhaps the best example 

 of a cryptobiotic state. In many respects these are similar to the 

 seeds of plants and the spores of bacteria in that, while they 

 remain as cysts, there is virtually no metaboHsm going on. In 

 many cases these cysts remain as such only when they are stored 

 at a low temperature or when they are dehydrated, but in other 

 cases the cysts can remain stable for long periods of time in 

 media where one would normally expect them to undergo 

 excystment. With these, as in the case of some plant seeds, 

 external factors other than moisture are needed to trigger the 

 excystment process^'. Although a large number of environ- 

 mental factors have been implicated by various workers, there 

 is so much conflicting evidence that one cannot be certain which 

 factors are important. In a few instances carefully controlled 

 experiments tend to show that the principal factors are catalytic 

 amounts of certain organic compounds, such as the potassium 

 and sodium salts of 1-malic, citric, acetic, fumaric, oxalic, and 

 lactic acids, with some carbohydrates acting as co-factors. These 

 acids were even more effective when supplemented by low 

 molecular weight organic compounds isolated from hay infusion. 

 These unknown compounds were effective in concentrations as 

 low as 10-^. In the absence of these acids and co-factors, the 

 cysts can remain dormant for long periods in a medium that 

 will support the growth of the normal adult and at temperatures 

 that would favor this growth^^. In some instances these organic 

 substances may be supplied by bacteria that are growing in the 

 medium. 



In addition to the cysts, vegetative forms of protozoa of 



References p. 13 



