200 Effect of TemperaUrre and Insolation upon Ch'owth 



importance of these two factors was by no means clear. For several 

 years it has been noticed that plants fail to do well in the greenhouse in 

 hot summer weather, whereas the same species flourish outdoors at the 

 same time, and it was suspected that the high temperatures reached by 

 the culture solutions had some connection with this phenomenon (4). An 

 examination into temperature conditions was therefore undertaken. 



In the last experiment described (p. 199), temperature readings of 

 the nutrient solutions taken on various occasions on hot days showed 

 very considerable differences according to the situation of the plants. 

 Two typical records were as follows: 



On hot sunny days, therefore, the spaced plants were liable to be sub- 

 jected to very high temperatures at the root, on occasion exceeding that 

 of the air. In the crowded square, however, the outer ranks received a 

 partial shelter from their neighbours and the solutions never became so 

 hot, while within the square all the temperatures were usually very even, 

 within a very few degrees, and were somewhat lower than the others. 

 Under these conditions the crowding apparently served as a measure of 

 protection either by keeping down the root temperature or by the reduc- 

 tion it effected in the amount of sunlight reaching the leaves. It is 

 obvious that beyond a certain limit the effect of high root temperatures 

 and of abundant sunlight became directly harmful and inhibited growth, 

 but the extent to which each factor was responsible was not shown by the 

 data obtained. 



Further knowledge on this point was gained from a similar experi- 

 ment carried on in the abnormally hot autumn of 1921, when readings 

 were made of the daily maximum and minimum temperatures of the 

 solutions of specified plants. No shading was applied to the greenhouse, 

 and the sun's rays struck through the sloping roof directly on to the 

 crowded square and some of the spaced plants, while the rest of the latter 

 were on a side bench under a higher roof at a different angle, from which 

 the concentration of the sun's rays seemed to be considerably less, 



