GROWTH OF DUCKWEED 417 



(1963) referred to a number of experimental studies demonstrating 

 that organisms generally have higher thermal tolerance when tem- 

 perature exposure is cyclic rather than continuous. His work 

 demonstrated that certain species of fish respond to the upper 

 extremes of the thermoperiod and have maximum thermal tolerance 

 under a 24-hr temperature cycle. Van Winkle's (1969) study of 

 Modiolus demissus showed that the net effect of a cyclic temperature 

 regime (26 to 34° C) on byssal thread formation was similar to that 

 of a constant 30° C exposure. Likewise, survival of the mud crab, 

 Rhithropanopeus harrisii, exposed to 10°C cycles is similar to that at 

 constant exposure to the mean temperature of the cycle (Costlow 

 and Bookhout, 1971). Insects have frequently been shown to 

 develop more rapidly under fluctuating temperatures than under 

 constant temperature exposures (Hagstrum and Hagstrum, 1970). In 

 a study paralleling this one. Thorp and Wineriter (1978) observed 

 higher mortality in juvenile crayfish, Procambarus acutus acutus, 

 under the cyclic regime (10 to 25°C) than under acyclic or constant 

 17.5°C exposures. This organism apparently suffered negative effects 

 at 25° C, even for short periods of exposure. 



Within the 10 to 25° C temperature range studied, frond 

 multiplication of S. oligorrhiza appears to respond to the total 

 temperature exposure. Cyclic and randomly fluctuating temperatures 

 yielded the same growth response as constant-temperature treat- 

 ments at the same mean temperature (17.5°C). The adaptability of 

 duckweed species to a wide range of environmental conditions has 

 been noted by other investigators (Hicks, 1934; Ashby and Oxley, 

 1935; Hodgson, 1970; Hillman, 1976). One explanation for duck- 

 weed's adaptability to a wide thermal range, even in natural 

 populations, is that surface-dwelling aquatic species are more likely 

 to be selected for tolerance to extreme temperatures than are 

 bottom-dwelling forms, such as crayfish, which inhabit sites where 

 temperatures normally remain more constant. This explanation is 

 also applicable to the apparent selection for high thermal tolerance in 

 bluegill, Lepomis macrochirus, living in a heated reservoir (Holland et 

 al., 1974); whereas populations of the surface-dwelling mosquitofish, 

 Gambusia af finis, from thermal and natural areas showed no 

 differences in thermal tolerance (Smith, 1978). It is likely that this 

 species, like the duckweeds, has a high thermal tolerance in the 

 conditions of its natural habitat (Smoak, 1959). 



The ability of Spirodela to develop turions (specially resistant 

 vegetative fronds produced under adverse environmental conditions) 

 also contributes to its wide distribution and its effectiveness as a 

 colonizing species in disturbed aquatic habitats. Since many man- 



