In Etidorhpa, part one on the site, The Man and his Guide encounter a fungus forest in the inner shell ( See pic above ):
But toadstools are foul elements of low organization. They are neither animals nor true vegetables, but occupy a station below that of plants proper, I said.
You are acquainted with this order of vegetation under the most unfavorable conditions; out of their native elements these plants degenerate and become then abnormal , often evolving into the poisonous earth fungi known to your woods and fields. Here they grow to perfection. This is their chosen habitat. They absorb from a pure atmosphere the combined foods of plants and animals, and during their existence meet no scorching sunrise. They flourish in a region of perfect tranquility, and without a tremor, without experiencing the change of a fraction of a degree of temperature, exist for ages. Many of these specimens are probably thousands of years old, and are still growing; why should they ever die? They have never been disturbed by a breath of moving air, and, balanced exactly on their succulent, pedestal-like stems, surrounded by an atmosphere of dead nitrogen, vapor, and other gases, with their roots imbedded in carbonates and minerals, they have food at command, nutrition inexhaustible.
Still, I do not see why they grow to such mammoth proportions.
Plants adapt themselves to surrounding conditions, he remarked. The oak tree in its proper latitude is tall and stately; trace it toward the Arctic circle, and it becomes knotted, gnarled, rheumatic, and dwindles to a shrub. The castor plant in the tropics is twenty or thirty feet in height, in the temperate zone it is a herbaceous plant, farther North it has no existence. Indian corn in Kentucky is luxurient, tall, and graceful, and each stalk is supplied with roots to the second and third joint, while in the northland it scarcely reaches to the shoulder of a man, and, in order to escape the early northern frost, arrives at maturity before the more southern variety begins to tassel. The common jimson weed ( datura stramonium ) planted in early spring, in rich soil, grows luxuriently, covers a broad expanse and bears an abundance of fruit; planted in midsummer it blossoms when but a few inches in height, and between two terminal leaves hastens to produce a single capsule on the apex of the short stem, in order to ripen its seed before the frost appears. These and other familiar examples might be cited concerning the difference some species of vegetation of your former land undergo under climatic conditions less marked than between those that govern the growth of fungi here and on surface earth. Such specimens of fungi as grow in your former home have escaped from these underground regions, and are as much out of place as are the tropical plants transplanted to the edge of eternal snow. Indeed, more so, for on the earth the ordinary fungus, as a rule, germinates afte sunset, and often dies when the sun rises, while here they may grow in peace eternally. These meandering caverns comprise thousands of miles of surface covered by these growths which may yet fulfill a grand purpose in the ceremony of nature, for they are destined to feed tramping multitudes when the day appears in which the nations of men will desert the surface of the earth and pass as a single people through these caverns on their way to the immaculate existence to be found in the inner sphere.