Winifred E. Brenchlev and Kharak Singh 203 



leaves did not receive the full force of the sun's rays, the harmful action 

 of excessive insolation being thus mitigated, enabling the plants to make 

 better growth than when they were exposed to the full power of the sun, 

 although acting over a much shorter total period in the latter case. It 

 would appear, therefore, that a high degree of insolation (excessive 

 power of the sun's rays) is a more potent factor for harm than either 

 high temperature or the actual total duration of sunshine. 



Further experiments were undertaken to ascertain whether the harm- 

 ful effects of excessive insolation could be reduced by alteration in tem- 

 perature conditions. As has been already indicated, the difference 

 between the day and night temperatures of water culture solutions is 

 often considerable, especially in hot weather, when it may be 22-5° C. 

 on occasion. This is considerably greater than the fluctuation occurring 

 under soil conditions in the open, where the minimum soil temperature 

 remains considerably above the air minimum, especially in the summer (5), 

 and the maximum does not rise so high as in the water culture solution 

 under glass. In dull weather the maximum and minimum temperatures 

 approximate more closely, as there is less heating up during the day and 

 a less marked fall in the temperature of the glasshouse at night. A method 

 was therefore devised whereby the plants were subjected to a more even 

 temperature at the roots, in order to ascertain whether this affected 

 growth to any appreciable extent at different seasons of the year. The 

 whole of the practical work in connection with this experiment was 

 carried through by Professor Kharak Singh, of Lyallpur, India. 



Two 100 gallon tanks were set up, with an outlet pipe from below 

 the rim running down inside to within an inch of the bottom of the tank. 

 Water was admitted from above at the other end of the tank and kept 

 running day and night, so that a continuous slow circulation was main- 

 tained. A platform weighted with bricks to carry the water culture 

 bottles was so arranged as to bring the necks to the rim of the tank, just 

 above the constant level of the water. To exclude the light from the 

 roots black cotton covers were fastened round each bottle, as the ordinary 

 paper coats are useless when submerged, and the necks were painted 

 with black enamel in addition. A platform of similar height and size 

 was placed close by to carry a set of bottles in which the variation of 

 temperature was not controlled by a water jacket, both tanks and table 

 being under the sloping roof of the glasshouse. Under these conditions 

 the shoots of the peas were subjected to similar insolation and air tem- 

 perature, but the temperature at the roots varied with the situation. 

 Twenty-four plants were grown in each case, spaced far enough apart 



