Figure 5.

TEM images indicate that ungerminated pollen from sec8-m3 heterozygotes responds to signals to germinate. A, D, G, and H, Wild-type pollen grains in vivo, illustrating (respectively) a likely progression of intracellular morphologies during germination. D and G, Vacuolar space generated adjacent to the pollen cell wall (black arrow). H, An apparently emptied coat. Larger, apparently coalescing, vacuoles (examples marked by asterisks) are also characteristic of germinating grains and are not present in mature pollen (I). B, Detail of A, adjacent to the pollen cell wall. Mitochondria (white arrowhead) and small membrane-bound structures of unknown identity (black arrows) are apparent. E, Detail of a pollen grain similar to D, showing vacuoles (asterisks) apparently fusing with the coat-associated vacuolar space and a few of the apparent small membrane-bound structures seen in B. C and F, Representative ungerminated grains (i.e. no separation of the cytoplasm from the coat) from a sec8-m3 heterozygote of class I and II, respectively (see text). Class I and II grains show certain characteristics similar to A and D, respectively. Both contain larger vacuoles (examples marked by asterisks); class I grains (C) contain small membrane-bound structures, whereas class II grains do not. Bar = 2 μm. White arrows, Sperm cells (A and I); black arrowhead, vegetative nucleus (I).