Reproduction (Figs, la and lb)
Diatoms reproduce vegetatively by binary fission, and two new individuals are formed within the parent cell frustule. Each daughter cell receives one parent cell theca as epitheca, and the cell division is terminated by the formation of a new hypotheca for each of the daughter cells. This type of division, with formation of new siliceous components inside the parent cell, leads to size reduction of the offspring. The possible size range of the diatom cells seems to be species dependent, and the specific variation may be as large as 8 to 10 times the length of the apical axis or the diameter.
The considerable size variation is often accompanied by a pronounced size dependent change in cell proportions, normally in the form of an increase in the ratio between the length of the pervalvar axis and the apical axis or diameter. In addition, size variation often causes changes in valve ornamentation, like a reduction in the number of central clustered processes in Thalassiosira spp. (E. Syvertsen, personal observations), a loss of special structures like the pili of certain species of the Cymatosiraceae (Hasle et al., 1983), and an alteration of the valve outline of morphologically bipolar species from elongate toward almost circular, e.g., Fragilaria spp. (Hustedt, 1959) and Cymatosiraceae.
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