X-RAY EFFECTS ON GREEN PLANTS 969 



with the other growing parts, the relative effect on the leaf is 6, primary 

 root 16, and lateral roots 18. The lateral roots are then, according to 

 Cattell, 18 times as ray-sensitive as the coleoptile. 



Fibrous roots developed from treated seedlings of Thunhergia alata, 

 maize, and castor-bean plant were found by Johnson (22a) to show 

 markedly less growth in length of main roots, and particularly less growth 

 in number and length of lateral roots. The main root of maize showed 

 48 per cent less growth in length of root eight days after receiving a dose 

 of 2000 r-units. Main roots of castor bean, Ricinus communis, proved 

 equally ray-susceptible. In both species there was entire suppression of 

 lateral roots for several days after irradiation. Tap roots and roots 

 developing from bulbs also showed serious injury from treatment with 

 medium doses. Tap roots of radish irradiated in the seedling stage were 

 shorter than in the control; in the former there was also a noticeable 

 relative decrease in number and length of lateral roots. Raying the 

 root end of narcissus bulbs with 3500 r-units caused a necrosis and marked 

 stunting of roots. Three weeks after treatment, the experimental 

 plants showed 52 per cent relative decrease in number of roots. 



The age of the seedling and the hour of the day when the root tips 

 are most sensitive have been the subject of some study. Seedlings of 

 V. Faha 48 to 72 hr. old appeared to be the most sensitive, according to 

 Patten and Wigoder (47). Ancel (2c) found that the sensitivity of root 

 seedlings to X-rays continues to increase from the appearance of the 

 embryo until the root has attained a length of about 1 cm., and there- 

 after it decreases. 



Reinhardt and Tucker (51, 51a) have reported that roentgen irradia- 

 tion during the night is more harmful to the growing seedlings of V. Faha 

 than day treatment. Different groups of seedlings were treated each 

 hour for 24 hr. These, with an equal number of controls, were placed 

 under the same conditions of temperature, moisture, and light. Measure- 

 ments at the end of two weeks showed that there had been the greatest 

 retardation of growth and the greatest reduction in number of side roots 

 in those which were irradiated from 9 to 10 a.m. and from 10 to 11 p.m. 

 There is general agreement that cells in the process of division are 

 affected more than resting cells by the same dose of radiation. In view 

 of the fact that Kellicott (25) has reported that the periods of the greatest 

 cell division in onion tips are at 1 p.m. and 11 p.m., it seems reasonable 

 that irradiation given the growing points of seedlings just before these 

 periods would cause an interference with the normal mitoses. Jiinghng 

 and Langendorff (24), however, have reported that the maximum number 

 of mitoses in unrayed root tips occurs during the day. Investigators of 

 the cytological effects of X-radiation might well investigate in other 

 species the problem of the time of day when the growing points of seed- 

 lings are most sensitive to radiation. 



