Preparation of modern palynomorphs for scanning electron microscopy by the critical point drying method

Abstract

Complete details are given of a method for preparing small numbers of modern or fossil pollens and spores by the critical point drying method of Cohen, Marlow, and Garner (1968) for examination with the scanning electron microscope (SEM). Samples from seven modern genera were prepared and scanning micrographs illustrating results from five of these are presented. Results show that genera react variously to drying by evaporation. By comparing evaporative dried examples with critical point dried examples of the same genus, the extent of artifact drying can be noted. The morphologies of some are found not to be affected by the surface tension effects of evaporation; others show distortions that may prove, in much more extensive work, to be a general index of their structure. Distortions were classified into three categories: (1) changes in the overall size (i.e., volume) of the grain; (2) changes in the relationships and losses of surface ornamentation; and (3) changes in the relationships and losses of unique structures of the grain type.

Freeze drying for the purposes of this work was not included, as other work (Wickham and Worthen, 1972) indicated that critical point drying was an improvement over freeze drying in tissues, and previous experiences of the authors included several serious technical problems specific to freezing. The two main values of this method are: (1) Small quantities of rare modern or fossil pollens and spores can be prepared for SEM without specimen loss, and drying distortions can be minimized. (2) Such specimens can be compared to evaporative dried modern examples with a view to possible correlations in the fossil record.