Plutarch Quaestiones Convivales 4.2.2

[2] Then I began: Agemachus himself helps us exceedingly toward this discovery; for nothing at the present seems more probable than that, together with the thunder, oftentimes generative waters fall, which receive that quality from the heat mixed with them. For the piercing pure parts of the fire break away in lightning; but the grosser flatulent part, being wrapped up in the cloud, changes its nature, taking away the coldness and rendering the moisture mild and gentle, and altering and being altered with it, warms it so that it is made fit to enter the pores of plants, and is easily assimilated to them. Besides, such rain gives those things which it waters a peculiar temperature and difference of juice. Thus dew makes the grass sweeter to the sheep, and the clouds from which a rainbow is reflected make those trees on which they fall fragrant. And our priests, distinguishing it by this, call the wood of those trees rainbow-struck, imagining that Iris, or the rainbow, hath rested on them. Now it is probable that when these thunder and lightning showers with a great deal of warmth and spirit descend forcibly into the caverns of the earth, the ground is moved thereby, and knobs and tumors are formed like those produced by heat and noxious humors in our bodies, which we call wens or kernels. For a mushroom is not like a plant, neither is it produced without rain; it hath no root nor sprouts, it depends on nothing, but is a being by itself, having the consistence only of the earth, which hath been a little changed and altered. If this discourse seems frivolous, I assure you that such are most of the effects of thunder and lightning which we see; and upon that account men think them to be immediately directed by Heaven, and not depending on natural causes.