By Robert B. McLaughlin
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Extra info for An Introduction to the Microscopical Study of Diatoms
Because the sieve membrane, when present, is very thin and its perforations are near the resolution limit of the LM, it is not easily seen. Also because of the relative thickness and height of the areolae walls, the appearance in the LM is as in Figure 18A where their outline (hexagons) and the opening in the lower pore membrane, is all the structure evident, at least on casual examination. Page 34 An Introduction to the Microscopical Study of Diatoms Figure 21. 8. Processes (Figure 21) The valve surface of diatoms is often modified by extensions and outgrowths of various sizes, lengths, and shapes.
Again, as calare - to proclaim. mentioned previously, the plasma membrane plus Middle English. band. internally, and the outer mucilage covering of the These are bands of frustule, probably in most cases, keep these complex silica that sit between structures together in the living state. This assumption the valve and the however does not offer a complete explanation of the girdle band or between way these structures are kept intact, as even when girdle bands. They cleaned vigorously, some of the most complex may or may not exist structures hold together.
The girdle width is so great as to cause the diatoms to almost never be seen in valve-view. In these latter cases the girdle is usually complex in structure, consisting of a number of bands or plates. A simple girdle, or zone, is considered to be that which is composed of only two girdle bands each of which is associated with its respective valve. Even in this simple girdle structure there is some variation, as in Hemidiscus cuneiformis the girdles are wedge-shaped, and in a number of diatoms in the genera Coscinodiscus the girdles have a groove or cut (sometimes diagonally) which leaves those ends free and able to flex.