So it turns out, not only do some birds fly through the Stratosphere when migrating over tall mountains, but several types of bacteria and micro-fungi have been found on dust floating in the Stratosphere, making the Stratosphere officially part of the Biosphere.
That being said, the Stratosphere is thought to contain only 1% of Earth’s Water Vapor, though, as I understand it, the majority of that Water Vapor is thought to enter into it over the Tropical zone, and in theory there might be concentrated enough for floating plants to live that high specifically over that area. The Mesosphere, on the other hand, is believed to have no Water Vapor at all, and there are no accounts of life outside of man-made aircraft even briefly reaching the Mesosphere.
Additionally, some algae produces hydrogen instead of oxygen when they are photosynthesizing, and some of those (though I believe not all) can then consume their own hydrogen to make back some of their expent energy. Theoretically, plants that used this method could evolve a balloon like shape, use their own hydrogen to float up, and if they got to high to find water vapor, they could consume their own hydrogen to both sustain themselves until they found more, and to sink back down to where there was more of it. Floating plants in the Lower Stratosphere, and certainly in the Troposphere, are not only possible, but they could certainly lead to some interesting new species of flying animals adapted to live off of them. A very interesting and under utilized Biome.