This type of electron microscopy gives an apparent "three-dimensional image with only one of the surfaces revealed. This provides the possibility to differentiate between the various valve processes, provided the internal surface of cleaned single valves are examined. SEM also shows the respective positions of areola vela and foramina. On the other hand, the construction of the cell wall cannot be studied unless broken parts are available. The resolution is often poor. However, the use of high-resolution low-voltage SEM may be an improvement since this instrument allows high resolution without beam damage to the specimens (Navarro, 1993).
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