Orchids and their fungi

Pick up a handful of soil under an orchid in flower in the Mount Lofty Ranges and you will not see, or even suspect, the thing that made the flower possible. It is not in the flower. It is not even in the plant. It is a thread of fungus, invisibly fine, wound through the soil and into the orchid’s roots, without which the plant now swaying in the spring breeze would never have existed at all.

Start with the seed, because that is where the deal is struck. Most seeds travel with a packed lunch — a store of starch and oil the botanists call endosperm, enough food to get a seedling through its first stretch of independent life. Orchid seeds have almost none of it, being little more than a coat wrapped around an embryo, dust-fine and dust-light, built to be flung about on the wind in their millions rather than provisioned one by one (Bonnardeaux 2007). That is a fine trick for scattering far and wide, and a hopeless one for growing — unless the seed lands where a particular fungus is waiting to infect it. Only then, fed by threads it did not grow itself, can the orchid begin to build the tissue that will one day photosynthesise on its own account. No fungus, no seedling: about as complete a dependency as biology offers.

Much of what is known about which fungus goes with which orchid was worked out here, at the Waite Agricultural Research Institute, by a mycologist named J.H. Warcup, through the 1970s and into the 1980s. Working through germination trials orchid by orchid, Warcup found the pairings were not casual but fussy, in a fairly consistent, genus-level way. The greenhoods, the drooping green-and-brown Pterostylis that nod along shaded tracks, germinated only with a fungus called Ceratobasidium. The donkey orchids, the yellow-and-purple Diuris of open woodland, held out instead for one called Tulasnella (Warcup 1971, 1981). Each orchid keeps to its own named partner, and will not substitute.

It is tempting to conclude that a fussy orchid must be a rare one, and sometimes that is right — but not always; a plant can be extremely picky about its fungus and still turn out to be common wherever that fungus lives. What the specificity guarantees is difficulty for us rather than for the orchid. You cannot dig up a ground orchid and pot it on, or scatter its seed on promising-looking dirt, and expect either to work, because the seed or the transplant is only ever half the plant. The other half is underground, unseen, and it has to already be there.