RESPONSES TO ENVIBONMENT BY PLANTS IN THE VEGETATIVE PHASE 545 



paper, and a calculating machine v^'ill suffice; in the field, admittedly, 

 the equipment required for recording the environment may be both 

 elaborate and costly. Nevertheless, the cost is small compared to the 

 price of a panoply of growth chambers which rarely duplicate the 

 high light intensities found in nature. This acerbity does not carry the 

 implication that growth chambers are not valuable for more detailed 

 analysis, but field studies should come first. 



Now I turn to laboratory experiments in which the levels of light 

 and temperature have been controlled. 



Twenty-five years ago, at Imperial College, Ashby and Oxley 

 (1935) demonstrated the many advantages possessed by the floating 

 aquatic plant Lemna minor for studying the effects of light and tem- 

 perature on vegetative growth. Clonal material can be readily multi- 

 plied in containers placed in a water bath, and the level of light re- 

 ceived can be controlled within fine limits, since the plane of growth 

 is horizontal. 



The scale of the experiments undertaken by Ashby and Oxley was 

 impressive. The plants were grown under a range of five temperatures 

 (10 to 29° C); they received continuojLis light at eight intensities (80 

 to 1,600 foot-candles); and the changes in the frond weight, frond 

 area, and net assimilation rate were examined under the 40 combina- 

 tions of light and temperature. It was demonstrated that the weight per 

 frond reached a maximum value under a combination of the lowest 

 temperature (10° C.) and a light range of 500 to 1,600 foot-candles; 

 conversely, a minimum weight was attained when the highest tempera- 

 ture (29° C.) was combined with the lowest light intensities (80 to 

 150 foot-candles ) . Irrespective of temperature, the area per frond rose 

 until a level of 500 foot-candles was reached, and thereafter the area 

 remained the same as the intensity was further increased. The areas 

 of fronds grown at 10 and at 18° C. were equal and greater than those 

 of plants subjected to 21° C. Between 21 and 29° C. there was little 

 further effect of temperature. 



Over all light levels, raising the temperature from 10 to 18° C. 

 more than doubled the net assimilation rate, but increasing the tem- 

 perature beyond 18° C. had no further marked effect. Excluding the 

 data for 10° C, over the whole range of light intensities the net as- 

 similation rate rose in a logarithmic manner; that is, the rate was still 

 being restricted by the highest fight treatment. 



Though we are still wedded at Oxford to L. minor for many re- 

 search purposes, the tropical aquatic fern Salvinia natans, which can be 

 cultured in the same manner, has a number of advantages for growth 

 studies. Current research is concerned with the factors that determine 

 the development of its floating leaves. A series of multifactorial experi- 

 ments is being undertaken by R. C. Achurch on the effects of three 



