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epithallus
The epithallium or epithallus is the outer layer of a crustose coralline alga, which in some species is periodically shed to prevent organisms from attaching to and overgrowing the alga.
== Structure ==
It is defined as the cells above the intercalary meristem; these are not involved in photosynthesis.〔 In ''Phymatolithon'', the epithallium is usually one cell thick,〔 whereas in other genera, such as ''Pseudolithophyllum'', multiple cells exist, with the thickness determined by the difference between their rate of production at the intercalary meristem, and the rate of shedding at the surface; thicknesses of 16 cells or more, spanning 100 µm, have been measured in a representative coralline (''Clathromorphum''). The thickness is variable within species; in Lithothamnion, a single cell thickness is the norm, but three- or four-cell thick regions are also common. The epithallus sometimes overlies the roof of conceptacles, which are exposed only when the overlying epithallus is eventually shed.〔
The epithallium is less strongly calcified than the underlying cells, facilitating its removal. The meristem itself is the least calcified portion; sometimes there is no mineralization at all, which makes it a plane of weakness where breaking often occurs.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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