The frequency of nucleus and cell division in prothallia ofEquisetum arvense L. grown under aseptic, defined conditions shows a considerable stabillity in the course of long-term experiments. This stability can be impaired if germinating spores are afflicted in the period 50 ±5 h after sowing by the just occurring Forbush effect in the period of decreasing intensity of cosmic radiation but not in the period of its repeated increase.