X-RAY EFFECTS ON GREEN PLANTS 981 



tions, there is still a need for careful experimentation in order that there 

 may be agreement on certain essential points. The majority of investiga- 

 tors working on physiological phases of this problem have agreed that 

 seed germination is not greatly influenced by X-rays and that medium 

 to heavy doses cause injury to roots, stems, leaves, flowers, and fruits. 

 While some believe that they can induce increased growth with light 

 doses, others are equally certain that no such stimulation occurs. 



Before the use of the dosimeter which gives an accurate measure of the 

 ray intensity in r-units, there was no certainty that one experimenter 

 could reproduce the dosage used by another. At present, however, if the 

 nimiber of r-units delivered during the exposure is given together with 

 the voltage and filtration, the same dose can be duplicated on another 

 machine. With more general use of this unit of dosage, more accurate 

 information concerning the effects of light doses should be forthcoming. 



Other factors contributing to differences in conclusions have been 

 the use of seeds or plants so few in number that individual differences 

 invalidate the results; conclusions were sometimes drawn from experi- 

 ments performed but once; statements of results in growth experiments 

 were often based on observations for very short periods of time so that 

 it was not possible to tell of effects appearing during later development. 



The question of sensitivity is important for the biologist as well as for 

 one interested in radiotherapy. Investigators have found that different 

 species as well as different individuals of the same species vary in their 

 reactions to the rays. Some are particularly ray-sensitive. It is difficult 

 to give a reason for the variation in response among different kinds of 

 cells and in the same cell at different periods for, at the present time, 

 we know too little concerning the nature of the changes which radiation 

 produces. Sensitivity is thought to vary with the metabolic rate and 

 with the amount of water present in the seed or plant tissue. Soaked 

 or germinating seeds are much more susceptible to the rays than 

 are resting cells. Respiration of rayed seedlings seems to parallel 

 growth. 



An interrupted dose has less effect than the same dose given all at 

 one time, owing to the existence of a " radiophylactic " reaction. Little 

 is known of the amount of energy which the cell absorbs. Some investi- 

 gators have found that X-ray beams of equal intensity have the same 

 biological effects irrespective of wave-length, while others believe that 

 the long wave-lengths are harmful. It is to be hoped that future studies 

 will give accurate information on this question as well as on the much 

 debated question of stimulation with light doses. 



Irregularities in shape, texture, and color of leaves developing from 

 X-rayed material are of common occurrence. In general, blossoming is 

 delayed and, if plants have been treated after buds are formed, abnormal 

 flowers and fruits result. Histological studies of stems indicate that 



