1980). In a growing protonema, microtubule and microfilament strands connect thenucleus to the cortex of the apical and basal parts of the cell (Kadota and Wada, 1995), although how these cytoskeletal strands control nuclear migration is not yet known. The growth rate varies with species and also with environmental conditions in the same species. In the case of A. capillus-veneris under continuous red light (0.5 W m-2s-1)at25 .Ctheprotonemata grew at an average rate of about 200 µm/day (Wada, 1988a).
In some species (including those of Anemia, Osmunda, and Lygodium), even under red light conditions, gametophytes germinate as two-dimensional prothallia, and no protonemal stage is observed (Raghavan, 1989). In Ceratopteris, when spore germination was induced by white light irradiation for 1 day after imbibition and then the spores were kept in the dark, gametophytes germinated as a two-dimensional, strap-shaped prothallium in four cell-columns. A cell mass proliferated at the apical part of the gametophyte and each cell at the basal part of the cell mass grew in the dark parallel to the cell polarity (Murata et al., 1997). In this species, cells can grow in the dark, similar to protonemal cells of A. capillus-veneris grownunder red light, but the cells are not protonemata.
The cell diameter under red light is reasonably constant. How do cells know the diameter and how do they maintain it? At the basal part of the apical dome of protonemata, a circular array of microtubules and microfilaments is observed (Murata et al., 1987;Kadota and Wada, 1992b). Because this will be discussed in detail in Section 1.6, it is sufficient to note that these cytoskeletal structures play a key role in maintaining a constant diameter, as has been well established in higher plant cells (Shibaoka, 1994).