PHYSICAL CONDITIONS. 9 



was a complete gradation in light intensity from one section to 

 another. At the east end of bed A a pointed extension of the board 

 frame was built and an electric fan was placed in the small opening 

 at the point (fig. 1). In this way outside air was constantly supplied 

 during the day, and the effects of overheating, rise in atmospheric 

 humidity, and change in water content of the soil in the deeper 

 shades were reduced to a minimum. The other bed was covered in 

 exactly the same way but without the extension, since no fan was 

 used. At the back or north side of each frame a door was provided 

 for taking observations under each different shade. The experiment 

 was continued until May 23, or for a period of 51 days. 



PHYSICAL CONDITIONS. 



LIGHT. 



At New Orleans, the nearest point at which light conditions are 

 recorded by the United States Weather Bureau, the following is the 

 record for April and May, 1908, the period of experimentation: 



Table I. — Record of light conditions at New Orleans, La., for April and May, 1908. 



Month. 



Cloudy 

 days. 



Days 

 partly 

 cloudy. 



Clear da vs. 



Percentage 

 of possible 

 sunshine. 



ApriL. 

 May.. 



These measurements should indicate fairly well the conditions at 

 Baton Rouge, which is only 70 miles distant. 



In the measurement of the light reduction a photographic ' 'printing- 

 out" paper was used. The time required for this paper to reach a 

 definite shade was recorded. The chief objection to be made to this 

 method, as weU as to the Wiesner (1907) method is that it measures 

 only the chemical rays. But since the light which reached the plants 

 passed through cloth, the amount of reduction is probably nearly the 

 same for the heat rays as for the chemical rays. Moreover, the recent 

 work of Kneip and Minder (1909) indicates that too great a distinction 

 has been made in the effect of different wave lengths on photosyn- 

 thesis. Consequently, where only relative intensities are to be meas- 

 ured, a "printing-out" photographic paper offers a ready means of 

 comparing the degree of shade under different cloths. In the cooper- 

 ative experiments above referred to Dr. L. J. Briggs and the writer 

 have developed a method for measuring by means of the? pyrheli- 

 ometer the relative transmission of energy by screens when placed 

 normal to the sun's rays. The reduction of light by the cloth covers 

 used in the Louisiana experiment has since been measured in this 

 way by means of the pyrheliometer devised by Abbot (1910), and 

 71574°— Bull. 279—13 2 



