c^A Journal of Agricultural Research vo1.xviii.no.ii 



medium, and dry. Rainfall was largely excluded by laying boards over 

 the boxes on each side of the plants, the boards having sufficient pitch 

 outward to turn the flow of the water. In addition, control plantings 

 were made in the field, a portion without shade and the remainder 

 covered with the shade cloths; and these received no water except the 

 rainfall. Only those features of the test which relate to shading will be 

 considered here, details of the differences in water supply and their 

 effects pertaining more properly to the next section of the paper. To 

 ascertain whether the simplified form of shade exerted any decided 

 indirect effect through the soil, soil thermographs were installed in the 

 soil at a depth of 3 inches under the 12 by 20 cloth shade, in a position 

 near the plants and in a similar position on the field row receiving no 

 special treatment. No significant differences in the temperature records 

 were obtained. 



A matter of special importance, of course, is the degree of shading 

 produced by the different types of shade and different weaves of cloth 

 used. For several reasons only approximations can be had as to the 

 intensity of the light received by the plants under the shades. The 

 positions of different plants and different parts of the same plant with 

 respect to the light necessarily vary, and the shape of the shade involves 

 a constantly changing transmission rate by the shade cloth. The normal 

 daily range in light intensity is magnified by the shade, since the coefficient 

 of transmission of the cloth is greatest at midday and decreases toward 

 sunrise and sunset. In the 191 6 type of shade there is a relatively small 

 coefficient of light transmission furnished by the sloping side walls 

 covered with cheesecloth. In the simplified type of shade only the 

 transmission through the top comes into consideration, since there are 

 no side walls. The southward extension of the top is such, however, that 

 only diffuse light reaches the plants from the side, with the exception 

 of their extreme lower portions, which are exposed to the direct sunlight 

 in the early morning and late afternoon. In the open type of shade, 

 diffuse light naturally becomes a larger factor. Observations made by 

 Prof. H. H. Kimball, of the United States Weather Bureau, by means of 

 the pyrheliometer gave transmission coefficients of 0.441, 0.292, 0.452, 

 0.613, Q-nd 0.727, respectively, for cheesecloth 12 by 20, 12 by 12, and 8 

 by 10, and for 6 by 6 mesh netting when exposed normally to the sun's 

 rays. Formulas also were developed by Prof. Kimball which make it 

 possible to compute the shading effect at any hour of the day and for any 

 date. Since the sun's rays never strike the shade cloth at normal inci- 

 dence, the maximum intensity of the transmitted light, which is attained 

 at midday, is slightly less than indicated by the above values. The 

 computed shading effect produced by each type of netting at various 

 hours of the day on June i, July i, and August i is shown in Table V. 

 It is seen that for horizontal exposures the shading effect is almost 

 constant from 10 a. m. to 2 p. m. but increases considerably from 10 



