Three short lines give us one small, strange picture: a bright red rose sitting in a glass of water, hour after hour, while it "sweats." The title, "Transpiration," is the word for how plants release moisture through their leaves and petals. So the poem is quietly describing something real, a cut flower slowly giving off water as it sits there overnight.
But "sweating" is a human word, and that changes everything. The rose starts to feel like a person lying awake, feverish or anxious, unable to rest. "Fiery" adds heat and intensity, so the flower seems almost to be burning in its own glass of water. The words "all night long" stretch the moment out, making it feel slow and restless.
The speaker doesn't say how they feel, and the poem suggests they are simply watching, perhaps awake too. The mood is hushed, intimate, and a little uneasy: beauty under strain, quietly struggling in the dark.