Mutually beneficial relationships (a type of symbiosis, which just means “living together”) are common under the aegis of Mother Nature. Some obvious ones are bees and hummingbirds getting nectar from flowers while moving pollen from plant to plant, and robins and bears eating fruits and excreting, and thus dispersing, viable seeds. There are many other mutualisms too, but there is a not-obvious one that is and has been fundamentally important to most of the plants on earth.

Many kinds of fungi form close, mutually beneficial, relationships with plant roots, obtaining some carbohydrates from the plant while transferring phosphorus and other nutrients from the soil to the plants. These are the mycorrhizal (“fungus-root”) relationships (between the roots of plants and the underground filaments of the fungus) that have often been mentioned in the essays in this space. Many local species of fungus (including, for example, the Amanita mushroom) participate in such relationships. Experiments have shown the benefits of these relationships to the plant partners; the benefit to the fungi is not always clear, but many of these fungi cannot live without their plant partners. It has been estimated that at least 80 percent of all plants are involved with mycorrhizal partners.

Beyond the critical ecological importance of present-day mycorrhizal mutualisms, however, lies an important historical and evolutionary perspective: Researchers have good evidence that the first green plants to colonize the land — a few hundred million years ago — had some kind of close fungal associate. This relationship was not technically mycorrhizal, because the first land plants (derived from freshwater green algae) did not have proper roots, but it was, nevertheless, a close physiological association of fungus with plant. Fungi invaded land well before the green plants did, and must have been adept at foraging for nutrients in the poor soils that then prevailed. So they were there, perhaps availing themselves first of exudates from the photosynthetic plants and eventually drawing nutrition directly from the plants themselves and providing soil nutrients in exchange. If those researchers are right, then all the great diversity of plants we see around us was made possible in the beginning by a mutualism.

All of this raises a big question in my head: If fungal partners have been so important historically and so many kinds of current-day plants have such fungal partners, then why are there a number of plants that do not have these partners? For example, mosses are reported to lack fungal partners, although their cousins, the liverworts and hornworts often have them. Examples of non-mycorrhizal plants in our local flora include starworts, paintbrushes, louseworts, sundews, lupines, sweetgale, mistletoes, many members of the mustard family, and numerous others. A look at that abbreviated list tells us that one way to thrive without a fungal partner is to have another means of supplementing nutrition: Many of the non-mycorrhizal plants are insectivorous (sundews), or at least partly parasitic (paintbrush, louseworts, mistletoe), or harbor nitrogen-fixing bacteria (lupines, sweetgale). In addition, some plants-without-fungal-partners have extraordinary roots (the so-called cluster-roots of some lupines, for instance) that give the plant a huge amount of root surface for taking up nutrients. All those ways of supplementing plant nutrition may account for some of the species that lack a fungal partner, but not all, so the question is only partly answered.

By definition, mutualisms involve some reciprocity between participants. But it is not uncommon for the balance to favor one partner more than the other. In fact, there are hundreds of examples of relationships in which one participant simply exploits the other, giving nothing back — what was probably once mutualistic has become exploitative. For instance, orchids produce seeds that require a mycorrhiza for germination, but because the seed contains no food stores for the embryo, the fungi get little or nothing; the orchids apparently control the relationship by chemically attracting the fungal filaments. Some orchids (such as the coralroots) lack green leaves altogether and are totally dependent on their fungi, without reciprocating: The plant gets both nutrients and carbohydrates from the fungus, which draws such sustenance from the soil or other plants. The two-sided arrangement has become one-sided. Undoubtedly there are instances in which the fungus gets one-sided benefits too, and this may have been the case very early in the evolution of the land plants.

In some cases, the relationships are still more complex. Some plants require “companion plants” to support their mycorrhizae, so there are three participants in the relationship. For example, certain Australian Lobelias require the presence of mycorrhizas associated with broombush (Melaleuca uncinata) in order to germinate. Some species in the gentian family (such as centaury, Centaurium) develop full mycorrhizal associations only when certain other plants (such as clover, Trifolium) are present, too.

Some plants have very specific requirements as to what fungus makes a good partner. Many orchids are fussy this way. And when plants are introduced to new countries, as the pines have been to the southern hemisphere and eucalypts have been to the northern hemisphere, they may not grow well unless their own particular mycorrhizae are moved with them. The local varieties of fungus won’t do.

In contrast, garlic mustard is a useful herb in the Old World that grows without harm to other plants, with which it coevolved. But when introduced to North America, it became a highly invasive weed, in large part because it kills or damages the plant/fungus partnerships that prevail here, so the native species fail to thrive.

• Mary F. Willson is a retired professor of ecology.