in Thoreau’s Journal:

I observed some mulleins growing on the western slope of the sandy railroad embankment, in as warm a place as can easily be found, where the heat was reflected from the sand oppressively at 3 o’clock p. m. this hot day; yet the green and living leaves felt rather cool than otherwise to the hand, but the dead ones at the root were quite warm. The living plant thus preserves a cool temperature in the hottest exposure, as if it kept a cellar below, from which cooling liquors were drawn up.

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