962 BIOLOGICAL EFFECTS OF RADIATION 



rays were filtered or not. Whenever the dosage is not given in r-units, 

 the set-up of the machine should be given. 



The discussion which follows will present a critical review of the 

 studies which have been made on the effects of X-rays upon green plants. 



PHYSIOLOGICAL EFFECTS OF X-RADIATION 



Many of the earlier studies of the effects of X-rays upon green plants 

 center around the question of their influence upon the processes of 

 germination and seedling growth. Often conclusions have been based 

 on a few experiments involving a limited number of seeds without 

 the utilization of enough check plants grown under similar environmental 

 conditions. Other conclusions were drawn from short-time experiments 

 in which the investigators did not allow the plants to grow to maturity, 

 hence they have not been able to judge how irradiation affects the entire 

 life cycle of the plant. There is agreement that in general the effect 

 of X-rays upon living material is destructive. Medium to heavy doses 

 cause a depressing action on growth. Contradictory results, however, 

 are reported by those using light doses. Many report a stimulative 

 action while others believe that, if there is an apparent hastened growth, 

 it is but an acceleration following a retardation which occurs immediately 

 after exposure. 



Some of the conflicting results published by those who have worked 

 with light doses may be due to different interpretations of the term 

 "stimulation." It is assumed in this paper that the investigators have 

 used the term in its more popular sense to indicate a positive reaction 

 unless a definite statement is made to the contrary. A report that 

 light doses "stimulate" growth is interpreted to mean that there is 

 increased growth, which results in greater dry weight. A more technical 

 meaning of the term indicates merely the response to a stimulus, whether 

 positive or negative. In this sense, any response of the plant shown by 

 either increased or decreased growth would be considered a stimulation. 

 The use of the word to indicate quickened growth is much more general 

 than the use in the latter sense. 



The writer (19) has found that, when medium doses of X-radiation 

 are used, a depressing effect proportional to the doses, within certain 

 limits, is evident during the first few weeks of growth. Later the growth 

 inhibition disappears and for a time the irradiated plants may actually 

 show faster growth, until at maturity there is little difference in total 

 height between controls and treated plants. Some of the earlier workers 

 noted the growth-checking tendency an^ regarded the later acceleration 

 as a natural consequence similar to that which occurs in plants after 

 mild injury. Others, however, using a small number of plants and 

 taking measurements during the time of growth acceleration, have 

 recorded the results as stimulation due to the action of the rays. 



