" ...centaurs may attempt to divine by the burning of certain herbs and leaves, by the observation of fume and flame…"
-- Firenze (OP27)
Centaurs burn this, observing the fumes and flames to refine the results of their stargazing (OP27).
When Sybill Trelawney got the sack from Dolores Umbridge, Dumbledore brought in Firenze the Centaur to teach Divination. He turned his classroom into a mossy forest where the children laid back on the floor and stared at the stars. As sage and mallowsweet burned, the students were urged to "look for certain shapes and symbols in the pungent fumes" (OP27). Firenze admitted that humans were hardly ever good at that type of divination.
late 14c., from late Old English malwe, from Latin malva "mallows,"
Frustrated with and skeptical of dream interpretation in Divination class, Harry sarcastically suggested that Ron's recent dream about Quidditch meant that he would be eaten by a giant marshmallow (OP12). Funnily enough, when Firenze took over as the Divination teacher, he burned mallowsweet (from which marshmallows used to be made) and had his students look for signs in the smoke - although admitting that he did not really expect them to have much success (OP27). -BB
"Mallowsweet" is not the name of any known Muggle plant, however it is quite similar to swamp mallow, Althaea officinalis, otherwise known as the common marshmallow plant.
The name of the family, Malvaceae, is derived from the Greek "μαλακός" (soft; Latin "mollis"), from the special qualities of the mallows in softening and healing. ...Most of the mallows have been used as food, and are mentioned by early classic writers with this connection. Mallow was an edible vegetable among the Romans; a dish of marsh mallow was one of their delicacies. Prosper Alpinus stated in 1592 that a plant of the mallow kind was eaten by the Egyptians.