With the aid of osmium tetroxide vapour, dry pollen and pollen at various stages of hydration has been fixed anhydrously for examination with the transmission electron microscope (TEM). In addition to establishing features characteristic of grains at different states of hydration, this technique has enabled the detection of a superficial layer investing both the exine and the pollen coating. This layer, some 10 nm in depth, binds both lanthanum and Alcian Blue and is shown to be the first component of the pollen grain to make contact with the stigmatic pellicle. The use of vapour fixation has also rendered it possible to chart the passage of water into the pollen grains with great accuracy, for each level of hydration displays a strikingly different cytoplasmic organization. For example, dry pollen is characterized by the presence of unusual structures at the protoplast surface and large numbers of spherical fibrillar bodies, whilst the protoplast of hydrating pollen is conspicuously stratified and contains a peripheral layer of membranous cisternae, subjacent to which is a fibrillar matrix derived from the spherical bodies found in the dry grains. Vapour-fixed, fully hydrated pollen resembles conventionally fixed grains. The pollen coating appears electron-translucent after anhydrous fixation and contains discrete, slightly rounded bodies some 50 nm in diameter. The uptake of water by grains on the stigma is accompanied by conspicuous structural changes in this layer for, after a short period in contact with the papillar surface, the spherical bodies rapidly disappear and the coat becomes electron-opaque. Close examination of this ‘converted’ coating reveals the presence of membranous vesicles and other structural components.

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