SUMMARY. 29 



(4) From Abbot's measurements, the total maximum rate at which 

 solar energy was received during any considerable period while the 

 experiment lasted was probably approximately 150 calories per 

 square meter per second. In terms of solar energy, growth was 

 best when the energy received varied from 21 to 75 calories per 

 square meter per second. Photosynthesis and consequently growth 

 practically ceased when the energy was reduced to 10 calories per 

 square meter per second or less. 



(5) The apparent tolerance of shade exhibited by the younger 

 plants was probably the result of the food supply still remaining in 

 the seed and not of any special ability of seedlings to carry on pho- 

 tosynthesis in weak light. 



(6) The effects of variations in temperature and humidity incident 

 to shading were so slight in this experiment that they could not be 

 detected by a comparison of the plants in the bed in which these 

 conditions were equalized by the use of an electric fan with the plants 

 in the bed in which no attempt was made to equalize conditions by 

 this means. Differences in shade produced such marked effects on 

 plant growth that the effects due to shading were entirely pre- 

 dominant, and compared with these the resulting effects of change in 

 humidity and temperature were practically negligible. 



279 



