Corroded exines from Havinga's leaf mold experiment. SEM

Abstract

Abstract Representative types of exine corrosion, seen on scanning electron micrographs, are illustrated from a leaf mold sample from the corrosion experiment of Havinga (1964). This work complements earlier transmission electron microscopy of the same sample (Rowley et al., 1990). Corroded areas are among the most dramatic indicators and products of exine destruction in samples of pollen and spores from Havinga's leaf mold material. Severely corroded regions, as seen using the secondary emission mode of the SEM, were most often found on exines with a continuous tectum (Myrica, Betulaceae, Corylaceae, Ulmus), but they also occurred to some degree in Fagus and on spores of Lycopodium. In taxa having pollen with a more or less continuous tectum the surface of the tectum was not destroyed but instead became depressed to the level of the floor of corroded areas. This “sunken tectum”; aspect of corrosion results from tunneling within the tectum without destruction of the surface. Muri of reticulated exines showed little indication of corrosion. Muri of Lycopodium, considered by Havinga as the most resistant exine in his experiment, were, however, often seen to be somewhat corroded and in some spores severely so. Fractures or tears, common to exines of many of the 19 taxa in Havinga's material, are illustrated for Fraxinus, Fagus, Quercus, Tilia, Pinus, and Lycopodium. Isolated sectors fractured from some taxa, e.g., Pinus and Tilia, are identifiable, but others are difficult to identify. In Pinus, where a portion of a sacculus was broken off, the surface of the foot layer showed hemispheroidal unit‐structures. Examples of folding and collapse were especially prominent in Taxus, Juniperus, Salix and Acer. Exines of these taxa were extremely difficult to recognize as pollen grain exines under oil immersion or with moderate SEM magnifications, although at higher magnifications exine structural details were seen to be intact. Small pit‐like circular holes were frequent on exines of Myrica and other betuloid grains, Fagus, Acer, Polypodium, and Lycopodium. These small holes may be of special interest since in Lycopodium they have been reported following abiological experiments.