412 SHARITZ AND LUVALL 



before the initiation of the temperature study. Only fronds from 

 actively growing clones were used in the experiments. 



From each population one adult frond with a juvenile attached 

 was placed in culture solution in each of 24 4.3- by 5.1-cm 

 compEirtments in uncovered clear plastic trays. Each sample tray was 

 treated to one of five thermal regimes in controlled environment 

 chambers. This design provided 24 replicates of each temperature 

 and population treatment. Frond division and growth of the clones 

 were examined under constant temperatures of 10, 17.5, and 25° C; a 

 daily square-wave cyclic regime of 10 to 25°C; and a daily square- 

 wave acyclic regime varying within 10 to 25°C limits, with a mean 

 temperature of 17.8°C. Cyclic temperatures changed every 12 hr, 

 with the maximum (25°C) occurring in the day. Acyclic tempera- 

 tures changed every 12 hr, with the maximum occurring either in the 

 day or night so that the next temperature was unpredictable within 

 the specified range. Temperatures within 0.5° C and 12-hr light— dark 

 photoperiods with light intensities of approximately 800 ft-c 

 (cool-white fluorescent light) were maintained in all chambers. These 

 light conditions are within the range for optimal growth of the 

 Lemnaceae (Ashby and Oxley, 1935). Each tray compartment 

 contained 50 ml of half -strength Hoagland's culture medium (Hoag- 

 land and Arnon, 1950) plus 5 ppm ferric citrate. The pH was 

 adjusted to 6.8 with 300 ml O.IM KOH per 35 liters of solution. To 

 reduce algal infection, all fronds were washed initially in distilled 

 water, and the plants in each treatment were transferred to fresh 

 culture solutions weekly to remove waste products and replenish 

 nutrients. 



Growth of S. oligorrhiza clones was determined at weekly 

 intervals by adult frond count. The number of adult fronds was used 

 as a measure of population growth because high correlations have 

 been demonstrated between the rate of increase in frond number and 

 the net assimilation rate, frond area, protein content, and depth of 

 frond color in Lemna sp. (White, 1939). Frond weight was not used 

 since Hicks (1934) demonstrated that in L. minor frond weight 

 depends on the amount of stored starch and may not accurately 

 reflect population growth. The experiment was continued for 4 

 weeks until the surface area in the culture chambers was covered by 

 plants in the more rapidly growing populations. 



RESULTS 



The mean number of adult fronds of S. oligorrhiza at the end of 

 the 4-week growing period under each set of experimental conditions 



