Micronuclei and Radiosensitiviy in the Root Meristem of Vicia faba

Percentages of cells with micronuclei in four regions of theroot meristems of Vicia faba are used as measures of sensitivityto acute X-irradiation. There are two peaks in these percentages,occurring at about four and eight days after 360 rads and twoand six days after 180 rads. Two peaks exist, probably becausethe radiation delays cells that were in G₁ much more than cellsthat were in G₂ in reaching the first post-irradiation mitosisand consequently in displaying micronuclei in the followinginterphase. The relative heights of the two peaks thereforereflect the relative numbers of cells in G₁ and G₂ as well asthe relative sensitivity of the two phases to chromosomal damage. The cells of the quiescent centre are injured least by the radiationas they are mostly held at Gt. The meristem thus obeys the lawof Bergonié and Tribondeau, but differs from that ofZea in that the meristematic cells of the cap initials and steleimmediately adjacent to the quiescent centre resemble the quiescentcentre much more closely than the stele 250 µ away inthe numbers of micronuclei produced. This is consistent withthe differences already known between the two species concerningrates of division in the different regions of the meristem andthe behaviour of the meristem after severe radiation injury.