28 EFFECTS OF AKTIFICIAL SHADING ON PLANT GROWTH. 



light which in the present experiments proved to be necessary in 

 order that growth could take place. They have also shown that 

 of the incident light the greater portion is dissipated by emis- 

 sivity from the leaves and through transpiration. A limit to the 

 amount consumed in photosynthesis is therefore established by the 

 limited supply of C0 2 in the atmosphere. Although only a fraction 

 of 1 per cent of normal light is usually required for photosynthesis, 

 it does not follow that photosynthesis will continue when the light 

 intensity is reduced to this amount. 



The actual amount of solar energy received by the plants under the 

 different shades in the experiments here described necessarily varied 

 greatly at different periods of the day. The maximum daily illum- 

 ination would probably be the most important factor in fixing the 

 lower limits for growth under the shades. By comparing the data 

 given by Abbot (1911, pp. 358 and 385, fig. 72) it will be seen that 

 the rate at which solar energy is received at Baton Rouge during the 

 months of April and May during the brighter part of the day is ap- 

 proximately 150 calories per square meter per second, this value 

 including both direct sunlight and the additional radiation from the 

 sky (Abbot, 1911, p. 307). 



Since the relative shading produced by the different cloths used in 

 these experiments was measured by means of Abbot's pyrheliom- 

 eter, it is possible roughly to express the results in terms of total 

 solar energy received by the plants under different shades. In these 

 experiments growth was best when the solar energy during the 

 brighter part of the day was so reduced as to range from n/2 to n/7, 

 or to range from 75 to 21 calories per square meter per second. On 

 the other hand the lower limit of growth was somewhat below 10 

 calories per square meter per second (n/15). Previous measurements 

 by Brown (1905, p. 525) show that photosynthesis was not retarded 

 in some cases when the illumination was reduced to 7 calories per 

 square meter per second. This accords very well with the results 

 obtained in the experiments here recorded. 



SUMMARY. 



(1) When the illumination was so decreased as to range from n/2 

 to n/7 a general increase in growth resulted in potato, cotton, let- 

 tuce, and radish, which was expressed in increased green weight, 

 height, and number of nodes. 



(2) Corn made its best growth in full light. 



(3) When the light was reduced to n/15 or less none of the plants 

 tested were able to elaborate food material sufficient to produce 

 growth after the seedling stage was passed. 



279 



