CHAPTER XV 



EFFECTS OF RADIUM RAYS ON TROPISTIC RESPONSE 



It has already been shown in Chapter II that radioactivity in air 

 and soil is a factor in the normal environment of all plants, and, 

 therefore, that both roots and shoots are doubtless in a condition of 

 radiotonus. Hence it is probable, a priori, that plants possess a 

 radiotropic sensibility, enabling them to detect differences in either 

 the direction or the intensity of the rays, and to respond to a uni- 

 lateral stimulus of this kind. Moreover, since radioactivity is an 

 environmental factor, such responses would be likely to be of an 

 adaptive character. For example, the radioactivity of the soil is 

 more intense than that in the air ; therefore, within limits of inten- 

 sity such that the rays do not become injurious to the tissues and 

 produce a traumatropic response, we should expect the root to be 

 positively radioti^opic and the shoot negatively so. 



In December, 1907, I very briefly discussed the probability of 

 the existence of a true radiotropism, stating* that tropistic curva- 

 tures, being reactions to a stimulus felt unilaterally, could be called 

 forth by radium rays only with difficult}^ since the rays would 

 ordinarily pass entirely through the tissues, and thus fail to be fel^ 

 as a unilateral stimulus. In this communication the fact was over- 

 looked that the organs of a plant can detect differences in direction 

 as well as intensity of stimulus. Thus, as Pfeffer^" has pointed out, 

 gravity is felt with equal intensity throughout the diameter of a hori- 

 zontally placed root, but its direction is perceived, and responded to. 

 In like manner the root or shoot might be able to detect the direction 

 of the radium rays, however slight might be the difference in their 

 intensity on opposite sides of the organ. 



In this chapter there will be briefly treated, first, the effects of 

 the rays upon normal tropisms ; second, a brief experimental investi- 

 gation of the existence of a true radiotropism. 



I. Effects of the Rays on Normal Tropisms 

 Experiment 91 

 To ascertain the effect of the exposure of dry seeds to radium 

 rays on the geotropic response of the shoot, 5 grains of " Hickory 



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