Shadow Algae: I had an idea for a species of algae that would grow on the surface of rocks as a film one cell thick. It is eaten by animals that scrape/lick it off the rock. Because of this it has a remarkable defense mechanism. Whenever an algal cell detects a sharp decrease in light levels, as when an animal's shadow covers it (but not when a cloud's shadow covers it, or a nearby rock or tree casts a moving shadow due to the movement of the sun across the sky), it automatically chemically disassembles its photosynthetic pigments (chlorophyll?) at high speed and instantly turns transparent. Thus, it no longer stands out as a colored blotch on the rock and just blends in. This strategy doesn't always work, but it is better than nothing. Just in case, it has a second defense mechanism: It flattens itself onto the rock, holding tighter. It does not always hold this position, because it prefers a certain gas-exchange rate and the greater surface-area-to-volume ratio increases this rate.
Popcorn Slime Mold: These slime molds move slowly as blobs along the forest floor, absorbing and digesting leaf litter. However, when they "smell" nearby carrion in large amounts (or even blood from a recent predator's kill) they grow small bulbs on their backs in a matter of minutes or seconds. The outer layer of cells of these bulbs become rigid and airtight, while the inner cells unleash a chemical reaction that rapidly raises the internal temperature to the boiling point. Soon, the bulbs explode and spray spores in all directions, sometimes for several meters, hoping that some of them will get to the carrion before other competing slime molds do.
Self-Righting Plants: Each cell in these plants has a tiny vacuole that holds a tiny bead. Gravity pulls the bead to the lowest side of the vacuole, allowing the cell to sense the direction of gravity. When the plant is pushed over by wind (or anything else) the cells on one side of the plant contract and the cells on the other side vertically stretch. This happens so quickly and responsively that the plant can catch its balance before it falls over. This way, the roots don't have to go as deep. This would be an advantage for plants growing in shallow soil or directly on stone (or on other plants (epiphytes)).