Winifred E. Hhknchley and Kharak Singh 207 



The plants in which the roots were protected from excessively high 

 temperatures made therefore about half as much growth again as the 

 unprotected survivors on the table. The increase was chiefly due to shoot 

 growth as the roots weighed much the same in both cases, thus suggesting 

 that the injurious action of combined strong insolation and high tem- 

 perature is more marked on the assimilatory tissues than on the roots, 

 the organs of absorption, the ratio of shoot to root being thus reduced. 

 This is in contrast to what happens when growth is adversely affected by 

 overcrowding, in which case the shoot/ root ratio increases (3), probably 

 owing to an attempt on the part of the plant to increase its assimilatory 

 surface in view of the decreased illumination. 



Throughout the period the mean temperatures in the solutions were 

 from 4-5-5° C. higher than during the earher test, all being above the 

 highest means previously registered, but the differences between the 

 table and tank maxima and minima were very much the same in both 

 cases. The table maxima, however, ruled very high, ranging from 28-6- 

 33-3° C, i.e. at temperatures above the initial optimum, which w'ould 

 cause a depression in the rate of growth during their period of operation. 

 Added to this, there was a great deal of strong sunshine during the first 

 and third weeks, and this association of excessive insolation with high 

 root temperatures wrought havoc among the plants on the table, and 

 gave them a very bad start. During the last three weeks there was a 

 great drop in the amount of sunshine, but the temperatures remained 

 high, so that at the end of the period the temperature effect was the 

 more marked. The same amount of sunshine, however, had far less 

 detrimental effect when the roots were kept cooler, and not only did 

 nearly all the plants in the tanks survive, but they made much greater 

 individual growth. 



Nevertheless, a comparison of the dry weights shows that the con- 

 ditions in the later test were less favourable even in the tank, though 

 the depreciation was not nearly so great as on the table. 



Total dry weights. 



April — June July — ^Aug. 

 gm. gm. 



Tank 4-561 2066 



Table 5169 1-371 



Growth in the second experiment was practically finished in six wrecks 

 instead of in eight, but with the speeding up less than half as much dry 

 weight was produced. This may possibly be attributed to the excessive 



