RESUMPTION OF MITOTIC-ACTIVITY IN ALLIUM-CEPA L ROOT-TIPS DURING TREATMENT WITH LEAD SALTS

Allium cepa L. root meristem cells were observed for 30 hr during trea tment with 2.5 mug ml-1 Pb2+ from PbCl2. Root growth and mitotic activ ity were gradually inhibited during the initial hours of incubation an d were accompanied by a rise in the incidence of abnormal mitoses (c-m itoses) to about 40%. The most pronounced effects of lead were found b etween 7.5 and 12 hr of incubation. Metabolic activity resumed in the roots from 13.5 to 30 hr, as manifested by intensification of root gro wth, mitotic activity comparable to control levels and a normal course of cell division. The toxicity of lead to meristematic cells decrease d as incubation progressed. This result correlates well with the obser ved lack of symptoms of lead poisoning in plants growing under natural conditions.