More botanical weirdness

In biology, it is often the exceptions to the rules we teach that are the most interesting organisms. For example, every child knows that the sky is blue and the grass is green. With a few leading questions you can get a child to generalize that all plants are green. We all know this, right? Plants are green because they have chlorophyll, which allows them to perform the magic of photosynthesis. And yes, it really is magic. Harvesting the power of the sun to build complex molecules out of CO₂ and H₂O? Hell yeah, photoautotrophs are freakin’ amazing!

But what about the plants that aren’t green? How do they make a living?

I’ve already written about dodder, a parasitic plant that is commonly seen growing on pickleweed at Elkhorn Slough. A few weeks ago when I was at Lake Tahoe I encountered another plant that has a parasitic lifestyle: snow plant.

Snow plant (Sarcodes sanguinea) is a non-photosynthetic plant that has zero chlorophyll and thus zero green color, and is instead a rich blood-red color hinted at by its species epithet. It lives on the forest floor in close proximity to coniferous trees. The blood-red inflorescences shoot up from the ground, apparently out of nothing; the rest of the plant lives underground. If you break an angiosperm into its basic anatomical components you have: leaves, stems, roots, and flowers. Snow plant isn’t photosynthetic, so it doesn’t need or have leaves. And since stems are essentially support structures to hold leaves up to the light it doesn’t have those, either. The roots and vegetative parts (rhizomes?) of snow plant are underground and for most of the year there’s no indication that it’s there at all, until it sends up an inflorescence in the late spring as the winter snow is melting.

Since snow plant isn’t autotrophic and doesn’t fix its own carbon, it has to obtain fixed carbon from elsewhere. Snow plant lives under conifers, but is not a parasite on the trees the way that dodder is a parasite on pickleweed. The relationship is much more complex and involves a third player. And all of the action happens underground.

Enter the third player, a mycorrhizal fungus. This fungus’s mycelium spreads through the roots of the conifers with which it has a mutualistic relationship. The tree shares photosynthate (i.e., fixed carbon) to the fungus, which in turn provides minerals to and enhances water uptake for the tree. These mycorrhizal symbioses are very common in Nature, but most often go unnoticed because they occur in the soil.

Sarcodes sanguinea, the third partner in this unusual plant-plant-fungus ménage à trois, takes advantage of the intimacy between the conifer and the fungus. Instead of parasitizing the tree it targets the fungus, siphoning off part of the fungus’s share of photosynthate. I suppose this makes snow plant an indirect parasite of the tree. The tree is doing all the work, as it is the only autotrophic member of the trio. It shares photosynthate with the fungus and gets something vital in return. Snow plant, on the other hand, doesn’t contribute anything to either the fungus or the tree. Rather, it takes directly from the fungus and only secondarily from the tree.

It would be interesting to investigate the energetics of this three-way relationship. How do the fungus and tree react to parasitism by snow plant? On which of the mycorrhizal partners does snow plant have the strongest effect? The fungus, because its share of fixed carbon is being drained directly? Or the tree, which suffers because feeding the snow plant via the fungal intermediary means less photosynthate available to support its own metabolic activities? Does the tree have any way to stop the flow of fixed carbon to an area of the fungal mycelium that is being parasitized by the snow plant?

One last note. Many of the snow plants that we saw on the trail out of Carson Pass to Big Meadow had been surrounded by stones. We never saw any signs so aren’t sure why, but I think hikers want to keep the snow plants from getting trampled. The species isn’t endangered or threatened, although it is restricted to higher altitudes in California’s mountain ranges.

I think the stone rings were put there both to point out and protect the S. sanguinea inflorescences, although it would be hard to miss them. Nothing else is that bloody shade of red, and it really does stand out. Even small plants are very conspicuous.

What a bizarre plant. It challenges our preconceived notions of what plants are all about. Ain’t Nature grand, and weird?