Summary
In two species of the diatom genusMelosira the inner profile of the silicalemma fuses with the plasmalemma covering the “older” part of the cell at, or slightly before, maturity of the new siliceous cell wall component. Subsequently, the outer profile of the silicalemma and the remainder of the plasmalemma are cut off. Though there are indications that the valves may continue to add silica after this time the wall component now lies to the outside of a membrane which must,de facto, be considered the plasmalemma. When cingula move apart as development continues the membrane fragments are allowed to disperse and it is thought unlikely that they contribute to the formation of an organic investment of the siliceous components of the frustule.
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Crawford, R.M. Valve formation in diatoms and the fate of the silicalemma and plasmalemma. Protoplasma 106, 157–166 (1981). https://doi.org/10.1007/BF02115969
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DOI: https://doi.org/10.1007/BF02115969