RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 



549 



perature falls. On the other hand, the magnitudes of the effects are 

 different. When the changes induced by the combinations of 30° C. and 

 300 foot-candles and 20° C. and 1,200 foot-candles are compared, the 

 leaf weight goes up by a factor of 11.4 but the corresponding ratio 

 for area is only 3.2. Between 1,200 and 1,800 foot-candles there is some 

 suggestion that at 20 and 25° C. the highest intensity is superoptimal. 

 It has been possible to interpret the responses still further, for by 

 the macerating techniques evolved by Brown and Broadbent ( 1951 ) 

 the total number of cells per leaf has been counted directly. Thus for 

 the first time, it is believed, it has become possible to express the modi- 

 fications from a "shade" to a "sun" leaf precisely in terms of cell num- 

 bers. From the trends for leaf area and weight, it is not unexpected that 

 the number of cells should be smallest when the highest temperature is 

 coupled with the lowest intensity (Figure 13). There is also a marked 

 interaction between the factors: at 30° C, above 600 foot-candles, the 

 change in cell number is small, but at 20° C, between 600 and 1,200 

 foot-candles, there is a considerable rise. When the ratio of cell num- 

 ber at 30° C. and 300 foot-candles and 20° C. and 1,200 foot candles is 

 calculated, it is found that this figure (3.1) is nearer the corresponding 



'%. '' 



Figure 13. The interacting effects of light and temperature on the total 

 number of cells per leaf of Salvinia nutans. 



