202 Effect of Temperature and Insolation upon Growth 



exposure, and that with prolonged exposure to the initial optimum the 

 rate of growth falls off rapidly. It may therefore be concluded that the 

 higher temperatures near the optimum for short exposures exercise an 

 adverse influence when they continue to act throughout the life of the 

 plant. Balls (2) attributed the decrease and ultimate cessation of growth 

 at high temperatures to the accumulation of katabolic products in the 

 cells, prolonged exposures to submaximal temperatures favouring the 

 rapid production of these substances. 



In the experiments under consideration the initial optimum of 30° C. 

 for peas was exceeded on nine occasions, most within a single week, the 

 highest maximum reaching 34° C. These air temperatures were only 

 maintained for a short time, at the hottest time of the day, the period 

 of exposure being thus very short and rare in occurrence. The average 

 maximum temperature ruled several degrees lower, except for the one 

 week. Furthermore the diurnal fall to the minimum temperature was 

 considerable, 10-15° C. or more, and, as Askenasy(i) and Leitch(7) have 

 both demonstrated that the rate of growth follows immediately and 

 accurately any considerable change of temperature, the slowing off of 

 growth would permit of the reduction of accumulated katabolic products 

 and mitigate the effects of exposure to high temperatures. It would seem, 

 therefore, unlikely that the temperatures, j)er se, were high enough to be 

 harmful to growth, as almost the whole of the air temperature curve fell 

 below 31° C, the initial optimum for short period exposures, especially 

 as the root temperatures during the same period were on the whole 

 rather lower, though they followed the air temperatures fairly closely. 

 The adverse factor is to be sought in the intensity of the sun's rays 

 — much depression of growth occurring where they were focassed on the 

 leaves under the sloping roof. The different angle of incidence of the rays 

 on to the side bench prevented such undue concentration on the leaves, 

 and growth was correspondingly better under similar temperature con- 

 ditions. This is further corroborated by comparison with the May-June 

 (1920) experiment. In both cases the mean weekly temperatures were 

 very similar, as the higher summer maxima were almost compensated 

 for by higher autumn minima. The May-June plants received far more 

 sunshine — 411 hours against 262 hours, but showed less signs of distress 

 throughout their growth, and produced 1-913 gm. dry matter as against 

 1-055 gm.^ In the summer, however, the greenhouse was shaded and the 



^ For fair comparison only those plants growing in the same situation under the 

 sloping roof are here taken into consideration though it happens that for May-June the 

 mean of these is the same as that of the whole series (p. 199). 



