I think it's a true slime mold, Physarum polycephalum. Physarum is fairly common on rotting logs, mulch, or leaves, and sometimes you can see it on the soil surface, but this is the first time I've ever seen it "growing" on a sidewalk!
The slime molds aren't true fungi. They appear gelatinous (slimy) and can grow quite large. Physarum is bright yellow and netlike in appearance. We buy Physarum cultured on media in Petri dishes, for biology lab. Within each strand of the protoplasmic network, you can see the cytoplasm rapidly stream and change direction. The plasmodium actually migrates!
The true slime molds are thought to be a single cell, kind of like a bag of cytoplasm containing thousands of nuclei. Slime molds feed on micro-organisms, and when the food supply runs out, the "supercell" forms fruiting bodies (sporangia) which release spores. The spores hatch into single celled amoebae, which can mate and form zygotes. The zygotes then grow into plasmodia "supercells." It's a really weird life cycle!
Since the fall rains started in mid-September, it feels like we've been living in a monsoon season. I just wish the rains wouldn't last all weekend. I've already got cabin fever. Now even the slime mold seems to be trying to escape from the wet leaves!
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