734 BIOLOGICAL EFFECTS OF RADIATION 



levulose and dextrose. Two per cent sugar was less favorable than 

 1 per cent. 



Etiolated leaves were found to be lower in manganese content than 

 green leaves (10). This held true not only for leaves of dandelion cul- 

 tivated in darkness, as contrasted with those of the same species grown 

 in light, but also for the interior leaves of head lettuce, cabbage, chicory, 

 and celery, as contrasted with the exterior leaves. They also found the 

 white portions of the variegated leaves of Aucuha japonica to have a lower 

 manganese content than the green portions of the same leaves. Man- 

 ganese content seemed to be correlated with chlorophyll content. 



Eisenmenger (30) placed one-month-old tobacco plants in darkness, 

 and after 11 days compared their chemical composition with similar 

 plants left in light. Total nitrogen, nitrate nitrogen, and amino nitrogen 

 were higher in the plants in darkness. The plants in darkness did not 

 live long enough to exhaust the original nitrogen content to any appreci- 

 able extent. 



Cannon (18) studied the effect of light and darkness on the rate of 

 oxygen absorption by roots. Helianthus annuus and rooted cuttings of 

 Salix laevigata were grown in Knop's solution, then transferred to dis- 

 tilled water for testing. The plants were given from 2 to 4 tests in one 

 day, one or more in darkness, and the others in full sunlight, or sunlight 

 passing through a bell jar. The tests were of from 1^^ to 3 hr. duration. 

 Of the 53 tests run, 60 per cent showed less oxygen absorption when the 

 shoot was in the light than when in darkness. When his results are 

 analyzed statistically,^ the oxygen absorption of sunflower in light proves 

 to be less than that in darkness by odds of over 20:1. For willow there 

 is no significant difference. Since oxygen absorption by roots increases 

 with temperature and transpiration, and since both of these were higher 

 in the light, he concluded that exposure of tops to light per se tends to 

 decrease the rate of oxygen absorption by the roots. It is possible that 

 oxygen liberated in the photosynthetic process may tend to depress the 

 rate of oxygen absorption by roots. 



THE INFLUENCE OF LIGHT ON ETIOLATED PLANTS 



When etiolated plants are brought into the light, they begin to 

 develop the characteristics of normal green plants. The leaves unfold 

 and expand rapidly, they develop chlorophyll, and differentiation — if 

 not too long arrested — takes place (65, 89). Schonfeld (89) ger- 

 minated and grew plants in darkness, samples of which were removed 

 after definite intervals and placed in the greenhouse. Measurements 

 of the length of the petiole, length of leaf blade, and width of blade, were 

 made on all plants each time a sample was removed. Plants contin- 



' Statistical analysis was made by Shirley. 



