A leaf (plural leaves or foliage) was a key organ in the anatomy of plants and trees; specialized for photosynthesis. Leaves were consumed as a food, such as by herbivores, and also used as the primary ingredient for salad.
It was determined by T'Pol, that as of 2151, the soil and foliage in the region of Terra Nova, where the Novans lived, was going to remain irradiated for a minimum of decade as a result of the fallout from an asteroid impact in 2083. (ENT: "Terra Nova")
The leaves of the singing plant vibrated in the near constant winds on the planet's surface, the singing plant emitted a beautiful, wailing chime, which was very pleasing to the ear. Holding the plant's leaf still in one's hands would cause the song to cease, but it would resume again once let go, as was discovered by Spock, during his visit to the planet Talos IV in 2254. (TOS: "The Cage", "The Menagerie, Part I")
While speaking of her "spells" used to "keep" Tyree in 2268, Nona showed him a leaf, asking him if he remembered it. He recalled the "night of madness" that the leaf caused and how it "brought up evil beasts from my soul." She then proceeded to rub the leaf on his arm, scratching him with it, leaving residual pieces of it stuck to his arm. (TOS: "A Private Little War")
In 2373, Jadzia Dax compared the Jem'Hadar's tracking of the cloaked USS Defiant through the Badlands by the ionization of its warp engines to the passage of a Ligorian mastodon through the forest: it could not be seen, but it made the leaves rustle. (DS9: "Blaze of Glory")
One of the song's that The Doctor sang with Harry Kim and the Kimtones for the Qomar in 2375 was a song called "That Old Black Magic". One of the song's lyrics contained a reference to the feeling the singer had: "Round and round I go, Like a leaf that's caught in the tide." (VOY: "Virtuoso")