2l8 



EFFECTS ON TROPISTIC RESPONSE 



Experiment 92 



In order to ascertain the effect on geotropic response of growth 

 in an atmosphere containing the radium emanation, 7 germinated 

 pea seeds [Piswn sativum), were placed horizontally in conditions 

 suitable for further growth, under a glass bell-jar containing the 

 radium emanation as described in Experiment 45 (p. 152). In the 

 control jar (no radium emanation) were similarly placed 8 seeds. 



Fig. 62. Experiment 91. Absence of Geotropic Response (Pot 27) in Shoots of 

 Zea Mays following Exposure of the Grains, before Planting, to Radium Rays: 45 

 exposed to Radium of 1,500,000 X ; 27 and 24 to 1,800,000 X- Cf. figure 14. 



After 24 hours 5 of the 7 exposed seeds were found to have curved 

 geotropically, and 7 of the 8 control seedlings. 



The experiment was repeated, using 6 germinated pea seeds 

 under each bell-jar. After 24 hours all of the seeds showed posi- 

 tively geotropic curvatures in both cultures (except one seedling in 

 the control jar which became injured), but the radicles exposed to 

 the emanation have grown slightly more than those of the control. 



Exposure to radioactivity as described appears to have no influence 

 on geotropic sensibility, so far as can be detected by observing the 

 growth curvature. The response, however, being a function of 

 growth, may be varied according as the rate of growth is modified 

 by the rays. 



To further test the effect of the rays on geotropic response, 5 un- 

 soaked grains of " Hickory King" corn {Zea Mays) were exposed 

 in contact with the sealed glass tube of 10 mg. of radium bromide 

 of 1,800,000 activity for 12 hours. Eight days after the exposed 

 seeds had been planted in soil, they showed less than one half the 

 growth in length of control seedlings, and two of them had failed 

 to grow erect, and lay horizontally over the surface of the soil. 

 They were perfectly turgid and hence failure to keep erect could 



