MEMBRANE COATINGS ON THE GENERATIVE CELL-SURFACE OF FREEZE-SUBSTITUTED MONOCOTYLEDON POLLEN

High-pressure freezing/freeze-substitution/TEM were employed for study ing plasma membrane coatings in mature pollen grains. Data are present ed on 6 species belonging to 4 not closely related orders. Two sorts o f coatings were observed at the cytoplasmic face of the vegetative pla sma membrane bordering the generative cell: (i) A dense, uniform layer with an average thickness of 10 nm (''fluffy coating''). (ii) ''Strip -shaped projections'' protruding rectangularly into the vegetative cyt oplasm. They are 25 to 35 nm in height and set at a constant distance of about 30 nm from each other. Their maximum length, estimated from g razing sections, is about 400 nm. This is the first description of per iodically arranged plasma membrane protrusions in freeze-substituted p lant cells. Both kinds of membrane coatings are species- and age-depen dent features. Especially ''strip-shaped projections'' are frequently intimately associated with polymorphic endoplasmic reticulum surroundi ng the generative cell, a hitherto unreported pattern. It is presumed that ''strip-shaped projections'' are similar to, or even identical wi th, ''ordered ridges'' observed in freeze-fractured Phoenix dactylifer a pollen.