What Are Lithophytes?

A gray rock covered in bright green moss and pale gray lichens for article on lithophytesAsk most people what plants need to grow in, and they’ll say “soil” or “dirt”, right? And for the majority of terrestrial plants that’s the case. But given the sheer scale of biodiversity and the ability of species to make use of any niche–no matter how small–left unoccupied, there are of course exceptions. Take epiphytes, for example, that cling to the bark of trees and other plants. Rather than drawing nutrients and water from soil, they instead absorb what they need from the air. Psammophytes also get what they need from the air, but instead sink their root system into shifting sand dunes.

I am especially fascinated by lithophytes. “Litho-” means “stone”, and so a lithophyte is simply a plant that grows on stone. There are two main types of lithophyte. Epilithic lithophytes grow on a stone’s surface, and a crevice in the stone may be populated by endolithic lithophytes. Some of these plants can only grow on stone, so they’re described as obligate lithophytes, but their facultative lithophyte neighbors are those that are able to colonize both stone and soil or another substrate at the same time–some lithophytes can even live as tree-dwelling epiphytes instead!

Like epiphytes, a lithophyte may have some ability to absorb water and nutrients from the air. But they also capitalize on anything that ends up washed into their roots by rain. Endoliths may find that over time debris accumulating in their crevice offers a much-needed resource boost. As part or all of a lithophyte dies, the surrounding plants extract nutrients from the decaying matter–nothing goes to waste in nature, after all. They do not, as a general rule, have a negative effect on the rocks themselves; while some rock-dwelling lichens may chemically weather the stone beneath them, lithophytic plants simply use the rock as a convenient surface to take root.

A black and white microscope photo of a thin slice of plant root with dark spots created by fungi embedded within the cellular matrix for article on lithophytes
Arbuscular mycorrhizae within a root as seen under a microscope

What I find really cool is that lithophytes can be mycorrhizal! Their roots are pierced by colonies of various arbuscular mycorrhizal fungi that draw up nutrients from the soil and share them with the plants. While this is a very common relationship in nature–four out of every five vascular plant species uses arbuscular mycorrhizal networks–lithophytes seem to have cultivated a greater concentration of these helpful fungi.

A moss-covered rock is often someone’s first encounter with lithophytes. Lacking proper roots, mosses hang onto the stone with tiny rhizomes. Over time they might cover its entire surface, and if said surface is relatively flat and protected from weather and other erosive forces, their decaying remains could be the very beginning of a new patch of soil.

But it’s not just the little bryophytes like mosses that can eke out a living on a rock. More complex vascular plants may also take root on or within stone. One of my favorite ferns, the licorice fern (Polypodium glycyrrhiza) commonly grows as an epiphyte on trees in the Pacific Northwest, but given the right opportunity it will colonize a suitable crevice in a cliff. Orchids may have a reputation for being difficult to care for in captivity, but in the wild there are a lot of lithophytic species. Many, like Dendrobium teretifolium or many Phalaenopsis species, can also live quite well as epiphytes on a tree or other plant. And the wallflower, Erysimum cheiri, got its common name for its tendency to grow out of cracks in rocky slopes.

A close-up of a pitcher plant, looking like a light green tube with a wide mouth and a small cap above the opening for article on lithophytes
Nepenthes campanulata

Unsurprisingly, some carnivorous plants make their homes on rocks, and their carnivory allows them access to much-needed nutrients in an otherwise limited setting. The pitcher plant Nepenthes campanulata often grows in colonies on cliff faces. Heliamphora exappendiculata, another pitcher plant, will happily grow both in wetlands and on constantly damp rocks. Sanderson’s bladderwort (Utricularia sandersonii) doesn’t eat insects, but instead sucks up microscopic organisms using bladders the plant buries under nearby soil or sediment.

One more thing: are the plants you see growing in gravel also lithophytes? Not necessarily. There may be soil beneath the gravel that the plant is exploiting. Or the gravel itself may be part of a mineral soil–one that has a lot of stone and not much organic material. A true lithophyte is going to be attached to a rock or rooted in its crevice, though it’s possible to find lithophytes growing on stones that, through weathering, may be feeding fragments into a nearby mineral soil over time.

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