Observations on the structure of the protonema of Adiantum capillus-veneris L. undergoing cell division following white-light irradiation.

The structure and distribution of cell organelles were observed in the gametophytes of Adiantum capillus-veneris L. undergoing the first cell division induced by white-light irradiation. Under continuous red light, the nucleus is spindle-shaped, encased in a sheath of chloroplasts, and located some distance back from the tip. The apical pole of the nucleus has an invagination which contains microtubules. Two types of chloroplasts, both of which have starch and prolamellar bodies, may be distinguished. One is cigar-shaped and may be "anchored" at the cell membrane. The other is smaller, and more pleiomorphic. Two populations of microtubules may be distinguished within the cortical cytoplasm, one set aligned parallel to the long axis of the cell, the other set aligned circumferentially around the cell at the region of the dome except for the extreme apex. Some of these microtubules do not run for long distances. If cell division is induced by irradiation of the gametophytes with white light the vacuolation and swelling of the tip region begin at least 2 h after the induction and continue till 8-10 h, by which time the nucleus has reached the neck of the protonema. Very elongated and rather abnormal forms of mitochondria were noted during this period. During cell division, mitochondria aggregated at the poles of the spindle and at the equatorial plane of the spindle margin. After cytokinesis the vacuole system becomes fragmented and is redistributed uniformly throughout the tip cell, the daughter nucleus rounds up and becomes surrounded by oil droplets.