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Histological studies of roots of Vicia faba and of Zea mays seedlings grown from seeds given a medium dose of soft x-rays, indicate considerable distortion and irregularity about the apical meristem and surrounding tissues after the first week of germination (7, 8). These are not apparent during the early part of germination when the radicles first appear; the meristem is relatively normal in arrangement as compared with that in nontreated control seedlings (8). In view of this effect, it was considered of interest to determine the ability of root tips of seedlings germinated from x-rayed seeds to continue growth in vitro; the results of which might indicate whether the abnormalities found at the apical meristem are localized effects of the x-ray radiation, or effects influenced more directly by functional irregularities occurring elsewhere in the root or in the seedling as a whole. Unsoaked beef steak tomato seeds were irradiated 8 cm. from the focal spot of a copper target gas x-ray tube operated at 30 peak kv., and 10 ma. for 30 minutes. It was observed by separate tests that lateral roots do not appear during later germination of tomato seedlings from seeds given this x-ray treatment. The culture methods employed followed procedures listed by BONNER (4), BONNER and ADDICOTT (5), ADDICOTT (1), ADDICOTT and BONNER (2), ADDICOTT and DEVIRIAN (3), and ROBBINs and BARTLEY (6). When root lengths of approximately 1 cm. were attained in aseptic seedlings from both irradiated and non-irradiated seeds, 5to 10-mm. root tips (50 seedlings of each group) were cut and transferred to apreviously sterilized nutrient medium containing: 236 mg. Ca (NO3)2 4H2O; 81 mg. KNO3; 36 mg. MgSO4 7H20; 65 mg. KC1; 20 mg. KH2PO4; 1.5 mg. Fe-tartrate; 20 gm. sucrose, per liter of Pyrex redistilled water. Nicotinic acid (0.5 mg.), vitamin B1 (0.1 mg.) and vitamin B6 (0.1 mg.) were added per liter of nutrient. The root tips of both seedling groups appeared to be alike in growth rate and size at the time of isolation. In view of the uniformity of the data, one repetition of the original results seemed adequate to confirm the observations made. 321

FAILURE OF ROOT TIPS OF TOMATO SEEDLINGS GERMINATED FROM X-RAYED SEEDS TO GROW IN VITRO