208 Effect of Temperature and Insolation upon Growth 



insolation rather than to the high root temperatures, as the tank maxima 

 were much lower than the table maxima of the spring experiment and so 

 were under the limits at which growth is adversely affected. On the 

 other hand, the mean minima ranged several degrees (4-8-5° C.) higher 

 than in the earlier test. Previous experiments with peas (3) have shown 

 that with high maximum temperatures a rise in minima is disadvan- 

 tageous and checks growth considerably. Temperatures of 13-15-5° C. are 

 distinctly harmful when associated with 26-5-35° C. as maxima. In the 

 present case the mean minima were higher and ranged from 15-5-20° C, 

 being above 18-5° C. for most of the time, and may have exercised a 

 harmful effect even though the associated mean maxima only reached 

 20-24° C. The total growth in the tank in summer may therefore have 

 been depressed by the high minimum temperatures as well as by the 

 excessive insolation, but the influence of these two factors cannot yet be 

 dissociated. 



Summary. 



1. Under ordinary environmental conditions of temperature and sun- 

 light the growth of peas, as of barley, is seriously hindered by over- 

 crowding, even when each plant receives a similar supply of food and 

 water. Not only is less dry weight produced, but the pods become thin 

 and distorted and fail to develop their seeds properly. 



2. Growth tends to be depressed in hot sunny weather when no pro- 

 tection is afforded. The chief detrimental factors concerned appear to 

 be high temperatures at the roots associated with strong and prolonged 

 sunshine, though the two factors acting individually are much less potent 

 for harm. Under these conditions crowding shelters tlie roots from over- 

 heating and the leaves from too much sunlight, and up to a certain point 

 crowded plants make better growth than those spaced well apart. Over- 

 crowding, however, still depresses growth, probably because the light 

 and root temperature reductions are too great. 



3. Provided insolation is not excessive the amount of daily fluctu- 

 ation of root temperature over a total range of about 22° C. (6-7-28-9° C.) 

 has comparatively little influence upon growth; high maxima and low 

 minima give similar results to low maxima and relatively high minima, 

 provided the average mean temperatures are not too dissimilar. 



4. With high root temperatures a difference in the degree of insola- 

 tion or in the angle of incidence of the sun's rays may have a considerable 

 influence on growth, a slight easing off of the solar conditions enabling 

 much better growth to be made. 



