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Experiments by Willcock I on the effect of radium radiations on Hydra viridis and Hydra fusca show that the green form is far more resistant to rays than the brown. The green hydras are not killed by an exposure of 41⁄2 hours to 50 rag. of radium bromide; the brown hydras, on the other hand, die after "comparatively short exposures." I have repeated this experiment several times and have obtained the same results. Willcock also found that protozoa containing chlorophyll are uniformly more resistant than those which have none. Since these experiments were carried on in daylight, it seemed possible that the difference in the reaction between the two classes of animals experimented upon might be due to the activity of the chlorophyll. If this assumption is correct, we should expect that plant cells radiated in complete darkness, when the chlorophyll is not active, would be more sensitive to the rays than in the light. To test this point I have made a number of experiments on Spirogyra and Volvox. The radium (20.4 rag. of element, enclosed in a thin-walled silver tube) was placed in a watch-glass filled with water, and the Spirogyra filaments were laid across it. In the light, the cells nearest the tube began to show a typical phenomenon of disintegration in 2 to 3 hours. This disintegration consists in the following. The chlorophyll bands first contract somewhat, losing their spiral arrangement. A few minutes later they resolve into an irregular heap of chlorophyll in one end of the cell. In the light, this phenomenon began to be noticed after an exposure of from 21⁄2 to 3 hours. The

DIFFERENCE IN THE ACTION OF RADIUM ON GREEN PLANTS IN THE PRESENCE AND ABSENCE OF LIGHT