The Era of the Electron Microscope

As pointed out by KNOX (1984, p. 204): "The terminology applied to the pollen wall is daunting, especially as it has been developed from early light microscopy work, and then transposed to the images seen in the transmission and scanning electron microscopes".

Electron Microscopy with its two most important types, TEM and SEM, facilitated the major breakthrough in palynology: the ultrastructure of developing and mature pollen and the stunning visualization of pollen morphological characters.

During the 1950s and early 1960s considerable progress in TEM preparation methods (from fixation to microtomy and staining) took place. The resolving power of the TEM was the basis for new information on pollen grain ultrastructure and pollen development. Nevertheless, EM-based information on ornamentation details of pollen grains was rare up to the mid-1960s. Only TEM-based casts or replica methods were available, all of them with limited resolution and depth of focus (e.g., the single-stage carbon replica technique; ROWLEY and FLYNN 1966, FLYNN and ROWLEY 1967). The time-consuming and laborious TEM replica procedures were an obstacle to extensive surveys of pollen morphology and have now been successfully replaced by SEM (HARLEY and FERGUSON 1990).

Today barely conceivable, the introduction of SEM in palynology in the second half of the 1970s was a key innovation in the study of the fine relief of pollen surfaces. Advantages of SEM include the relatively simple and rapid preparation methods, the unsurpassed depth of focus revealing an overwhelming vividness and power. SEM was accepted in the very first moment as the quantum leap in EM (HAY and SANDBERG 1967). The first SEMs of pollen grains were published by THORNHILL et al. (1965) and ERDTMAN and DUNBAR (1966).

Since then palynologists have been provided with a plethora of beautiful micrographs. "The scanning electron microscope has provided a greater impetus to palynology than any other technical development during the history of the subject." (BLACKMORE 1992).

Nowadays the LM (with basic and advanced equipment) and the two main types of EM form an expedient combination of imaging techniques. The LM remains the workhorse method (TRAVERSE 2007; see the compendia by REILLE 1992, 1995 and 1998) but is limiting insofar as morphological and structural features at species level, not observable by LM but of diagnostic value, are routinely determinable only by SEM. The role of SEM as an essential part in illustrating exine sculpture and ornamentation cannot be overrated (HARLEY and FERGUSON 1990).

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