EFFECTS ON THE SYNTHESIS OF CARBOHYDRATES 



189 



The control leaflet stained blue evenly throughout. The leaflet 

 exposed to the rod of 10,000 activity showed no effect from the ex- 

 posure, staining evenly throughout, but the leaflet exposed to the rod, 

 of 25,000 activity, showed only slight starch reaction in the portion that 

 was under the rod, but stained deeply in all other parts. Even though 

 the rod of 10,000 activity showed no effect, it is possible that the 

 effect produced by the more active rod was due either wholly or in 

 part to the shading of the leaf by the rod. This source of error was 

 eliminated by placing the coated rod under the leaf, as in the fol- 

 lowing experiment. 



Experiment 68 

 May 29, 10 A. M. 



A nasturtium [Tropaeolum) plant was removed to the light after 

 being for 18 hours in the dark-room. Under one of the leaves was 

 placed a Lieber's radium-coated rod 

 of undetermined activity (probably 

 25,000 x). 



May 30, 10 A. M. 



After 24 hours' exposure the leaf 

 was tested for starch. ' Abundant 

 starch was found in the part of the 

 leaf farthest from the rod, but very 

 slight traces only in the remainder 

 of the leaf, particularly in the part 

 that was directly over the coated rod. 



Experiment 69 

 May 28, 5 P. M. 



Under a healthy green leaf on a 

 nasturtium {Tropaeohim) plant that 

 had been in the dark-room for 7 

 hours, was placed 10 mg. of RaBr2 

 (1,800,000 X ), in a sealed box, por- 

 tected from the leaf by only a thin mica window. 



After 19 hours' exposure in the dark-room to the radium rays, 

 the radium was removed and the plant placed in the sunlight. 



After an exposure of 30 minutes to sunlight, the leaf exposed to 

 the radium rays, and a control leaf were tested for starch, with 



Fig. 48. Experiment 68. Retard- 

 ation of Photosynthesis by Radium 

 Rays. The print was made by placing 

 the leaf itself in contact with velox 

 paper in sunlight. Thus the darker 

 portions of the print indicate the more 

 translucent portions of the leaf-blade, 

 due to absence of starch. The darker 

 region was directly over the radium- 

 coated rod. 



