Diploid

FIGURE 1 (a) Sexual reproduction of a centric diatom (oogamy) and (b) of a pennate diatom (morphological isogamy, physiological anisogamy). #, Zygote; nucleus; O, pycnotic nucleus.

FIGURE 1 (a) Sexual reproduction of a centric diatom (oogamy) and (b) of a pennate diatom (morphological isogamy, physiological anisogamy). #, Zygote; nucleus; O, pycnotic nucleus.

The decrease in the average cell size of a diatom population during vegetative growth implies a need for a means of restoring the cell size. This is made possible by auxospore formation, in which a cell sheds its siliceous theca, thereafter forming a large sphere surrounded by an organic membrane. Within this sphere, a new diatom frustule of maximal size is formed, and the cycle starts anew. The first cell formed inside the auxospore, the initial cell, may have a morphology deviating in girdle structure, valve outline, and process pattern from that of a "normal" vegetative cell (vide, Thalassiosira decipiens, Hasle, 1979, Fig. 41; Cymatosira lorettziana, Hasle et al., 1983, Fig. 19).

Auxospore formation is size dependent and normally takes place when the cell has reached about one-third of its maximal size (Drebes, 1977). Below

FIGURE 1 (Continued).

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