Mar. 1. 1920 Effect of Length of Day on Plant Growth 583 



season showed that there was no appreciable difference in dates of flow- 

 ering under these varied exposures. 



Further evidence on this subject is furnished by the following ex- 

 periments in which soybeans were subjected to different degrees of 

 shading, primarily for determining the effect on oil formation in the 

 seed. Different types of shade were employed, and in some instances 

 shading was combined with regulated differences in the water supply of 

 the soil. In all these experiments the aim has been to use a type of 

 shade which would reduce to a minimum secondary effects, such as 

 modifying the air temperature and the temperature and moisture con- 

 tent of the soil. The object, in short, was to measure, as far as practi- 

 cable, only the direct action of different light intensities on the plant 

 itself, though, of course, this goal can not be fully attained. With this 

 aim in mind the triangular type of shade, shown in Plate 76, B, was used 

 in a series of tests made in 191 6. For this shade the standard cheese- 

 cloth of best grade, extensively used for surgical dressings, was employed 

 (see PI. 77, E). The opening extending around the shade near the top, 

 with loose overhanging flap, is for the purpose of facilitating ventilation. 

 The arrangement is such that the frame of the shade can be raised from 

 time to time to accommodate the growth of the plants. The width of 

 the frame was 4 inches at the base and 18 inches at the top, and it was 

 30 inches high. In these as in the later tests the Peking variety of soy- 

 bean was used. It will be recalled that this variety is quite sensitive to 

 changes in the length of the day. 



The simplest and perhaps the most satisfactory type of shade was 

 that employed in 191 7 and 191 8. A frame of iron pipe, 30 inches high, 

 40 inches wide, and of the desired length, was used to support the cloth. 

 The shades in all cases extended almost due east and west. The beans 

 in each instance were planted in a row 6 inches to the north of the center 

 line of the shade to allow for the southerly swing of the sun's course 

 through the sky. Comparatively open, loosely woven cloth, of the type 

 used for the commercial culture of cigar-wrapper tobacco in New Eng- 

 land and Florida, was used for this shade. Four different weaves of 

 cloth were used — 6 by 6, 8 by 10, 12 by 12, and 12 by 20 mesh, these 

 figures indicating the average number of threads to the linear inch. 

 These cloths are shown in natural size in Plate yy, A-D. 



In 1 91 8 tests were extended to include differences in water supply in 

 combination with three different degrees of shading (see PI. 78, A). 

 This was accomplished by planting the beans in wooden boxes 24 feet 

 long, 12 inches wide, and 14 inches deep, each dox being divided by 

 partitions into three 8-foot sections. These boxes were set in the soil 

 so as to extend about 2 inches above the surface and were filled with 

 soil up to 2 inches of the top. Under each degree of shading, three 

 different soil-moisture contents were maintained, designated as wet, 



