14 



light when the observations were made for a period of six months 

 or one year. This is shown in all the experiments, whether the 

 records were taken for three hours before and following noon, or 

 whether from sunrise to sunset. These records were all obtained 

 the same year, the percentage of difference being based on monthly 

 averages in all cases. 



There are, of course, other methods of determining the differ- 

 ence between morning and afternoon light than the chemical one 

 we used. For instance, plants free from starch might be exposed 

 to light during corresponding periods in the forenoon and after- 

 noon, and the amount of starch formed determined by chemical 

 analysis; or by bleaching and treating the leaves with iodine so- 

 lutions, naked eye determinations could be made as to the relative 

 amount present. The same determinations might also be made 

 by weighing identical parts of the leaves exposed at different 

 periods. The crop yield in a house running north and south would 

 afford some idea of the influence of morning and afternoon light, 

 as would also measurements of the radii of tree stumps. All 

 these methods have been used by us. For a few months we ex- 

 perimented with young cucumber plants which had been growing 

 in the dark for 24 hours and whose leaves, therefore, were devoid 

 of starch. One set of these plants was exposed in the morning for 

 a certain length of time, and the other in the afternoon, and all 

 were revolved on a clinostat, an instrument provided with a disk 

 moving in a horizontal plane and making a revolution every 

 twenty minutes. By this means each leaf was exposed regularly 

 to similar light conditions. The leaves were removed and dried 

 and chemical analyses made. Comparisons were drawn, but the 

 results proved to be of little value from faulty chemical analyses 

 or from other causes not determined. The amount of starch form- 

 ed in leaves under different light intensities is easily determined 



FIG. 2 — Showing cross section of a typical even span, double glazed 

 cucumber house, 20 x 106 ft.^ running north and south. The yield was much 

 superior on the east than on the west side. The middle rows are so shaded 

 in winter that they are of little value. 



by the naked eye when the leaves are bleached and treated with 

 iodine solutions, and we have made many such observations with 

 fairly satisfactory results. The leaves containing the most starch 



