The roots of a great many plants do not, on closer inspection, do their work alone. Filaments of fungi thread through soil at scales the eye cannot reach, and where root and filament meet, the two engage in a trade whose currencies — sugar from the plant, phosphorus and nitrogen from the fungus — pass each way across the membrane that joins them. The arrangement, known as mycorrhizal association, is no curiosity at the edge of botany; it is the rule among land plants, present in some form in roughly four out of five species and present, ancestrally, since plants first ventured onto land.
The picture of mutual benefit has, of late, been complicated by talk of 'wood-wide webs': networks of fungal filament said to connect distant trees, carrying sugars, nutrients, and even chemical warnings between hosts in a manner suggestive of cooperation among the trees themselves. The phrase has been useful to ecologists who want a vivid image, and useful, too, to writers who want a sentimental forest. Whether the wires of the metaphor carry the load it has been asked to carry is another matter.
Several careful studies have shown that nutrients can in fact move along common mycorrhizal networks from one tree to another, sometimes from a more sun-favoured neighbour to a shaded sapling. What is less clear is whether such transfers should be read as cooperation among the trees, as opportunistic redistribution by the fungus, or as a third thing that fits neither model. The fungus, after all, has interests of its own: it may move sugars to where they are most usefully turned into more fungus, with the apparent generosity of one tree to another a side effect rather than a goal. To call the network 'cooperative' without specifying whose cooperation is at issue is to obscure precisely the part of the picture that has yet to be settled.
This is not to dismiss the network image but to ask it to earn its keep. Talk of forests as super-organisms can, in popular writing, do real harm to the patient business of distinguishing what the data show from what the metaphor suggests. The cautious view is that mycorrhizal networks are real, ecologically important, and worth studying as they are — neither fairy-tale telegraph systems among sentient trees nor mere appendages of root, but a third thing, less tidy than either, whose particulars do not yet sit comfortably in any of the available stories.
Which one of the following best captures the main idea of the passage?