CHAPTER II 

 RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 



Previous to the present decade it was possible to classify the 

 known inorganic factors of the plant's environment as either molar, 

 molecular, or undulatory. The discovery of radioactivit}'^ and other 

 revelations closely related to it, as briefly outlined in the preceding 

 chapter, together with the investigations referred to below, lead to 

 the recognition, not onlv of another element of environment, but to 

 an entirely new kind of environmental factor, viz., radioactivity. 



It is a matter of considerable interest to ascertain the effects of 

 this new kind of energ}^ on the life-processes of plants and animals, 

 but when we realize that it forms a part of the natural surroundings 

 of all living things, and must be reckoned with as a possible factor 

 in all their vital activities, this interest greatly deepens. 



The life-processes of plants are regarded as reactions to stimuli. 

 Permanently remove all stimuli and all processes (/'. ^., life itself) 

 cease. In last analysis these different stimuli are all different mani- 

 festations of energy. We are familiar with some of the effects of 

 the more ordinary ones, such as water, air, heat, sunlight, and 

 gravity, with the corresponding tropisms (hydrotropism, geotropism, 

 etc.), and with the condition of adjustment to the normal for each 

 factor, tonus (phototonus, thermotonus, etc.). The discoveries in 

 radioactivity show us that the effects of these long-recognized 

 factors cannot be completely interpreted unless we take into account 

 the newl}^ discovered facts. 



Probably all plants are in a state of radtotonus, or adjustment to 

 the radioactive forces of their normal environment, and evidence is 

 at hand that we shall be able to add radiotropic response to the other 

 and well-known tropisms. Whether we consider water, sunlight, 

 air, or soil, radioactivity is a factor involved, and the following para- 

 graphs will briefly outline the investigations which compel to this 

 conclusion. 



Radioactivity in Water : Thomson ^^®' ^'^' ^^^ was the first to an- 

 nounce that air bubbled through Cambridge (England) tap-water 



