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frustule
A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on diatoms as pectin, a fiber most commonly found in cell walls of plants.〔(Access to articles : Nature Nanotechnology )〕 This layer is actually composed of several types of polysaccharides.〔Progress in Phycological Research: v. 7 (1991) by F.E. Round (Volume editor), David J. Chapman (Volume editor)〕
The frustule's structure is usually composed of two overlapping sections known as thecae (or less formally as valves). The join between the two thecae is supported by bands of silica (girdle bands) that hold them together. This overlapping allows for some internal expansion room and is essential during the reproduction process. The frustule also contains many pores and slits that provide the diatom access to the external environment for processes such as waste removal and mucilage secretion.
==Thecae==
A frustule is usually composed of two identically shaped but slightly differently sized thecae. The theca which is a bit smaller has an edge which fits slightly inside the corresponding edge of the larger theca. This overlapping region is reinforced with silica girdle bands, and constitutes a natural "expansion joint." The larger theca is usually thought of as "upper," and is thus termed the epitheca. The smaller theca is usually thought of as "lower," and is thus called the hypotheca.〔 Diatoms: More on Morphology 〕 As the diatom divides, each daughter retains one theca of the original frustule and produces one new theca. Interestingly, this means that one daughter cell is the same size as the parent (epitheca and new hypotheca) while in the other daughter the old hypotheca becomes the epitheca which together with a new and slightly smaller hypotheca comprises a smaller cell.
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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