Optional Leaves
The Penumbra · Report · Issue 3
An Orchid Without Leaves
Whether leaves are optional for plants — and why the answer is a dial, not a switch.
Every plant makes its own food from sunlight. That is the first thing anyone learns about plants, and it is nearly true. The exceptions — a few hundred orchids among them — gave up photosynthesis and live on carbon taken from fungi in the soil. The textbook draws a line between the two kinds, and the easy way to tell which side a plant is on has always been to look at it. Green leaves, it feeds itself. No leaves and dark stems, it feeds on fungus.
Cremastra aphylla is a small orchid of cool Japanese forests, dark violet, leafless, and rare. It was placed on the fungus side of the line on the strength of three things: it has no proper leaves, it is the wrong colour for a plant that uses light, and the carbon in its tissues carries the chemical signature of fungal food rather than air. Two of those three you can judge from a footpath.
Fewer than one in five individuals ignore the rule. They put up a green leaf, about ten centimetres long, and flower anyway. For years nobody knew what those plants were. They might have been a separate, undescribed species. They might have been hybrids with Cremastra appendiculata, a leafy relative that grows alongside them. Or they might simply be the same plant, sometimes doing something different.
In a paper published this year in Annals of Botany, Kenji Suetsugu of Kobe University and colleagues settled it. They worked one population outside Tokyo, about forty plants of which ten were leafy, and read the genomes of the leafy individuals alongside leafless plants from across Japan. The two forms were genetically indistinguishable. There was no trace of hybrid ancestry. Same species, same population, same genes. The leaf is a thing the plant sometimes does. The honest limit of the result is its size: four leafy plants were sequenced, from one site.
Then they measured what the leaf does. It is a working leaf. Its chlorophyll content and its photosynthetic efficiency match the ordinary green plants growing around it. Carbon isotopes put numbers on the contribution: about forty percent of the carbon in the leaf, and about twenty percent of the carbon in the flowers, comes from sunlight. The rest still comes from a single species of wood-rotting fungus, the same one the leafless plants use. The underground parts are fed entirely by the fungus whether a leaf is present or not.
So a plant that was classified by its appearance turns out to run on a dial rather than a switch, and individual plants sit at different points on it for reasons nobody has yet identified. The paper's own summary of the field is plainer than any headline: leaf shape is not a reliable guide to how a plant feeds. The green relative makes the same point from the other direction. Cremastra appendiculata, the one with a proper thirty-centimetre leaf, was shown in 2022 to take more than eighty percent of its carbon from fungi — carbon fixed decades earlier by trees that were alive during the atmospheric nuclear tests of the 1950s and 60s, and still traceable by their fallout.
The category "plants that gave up photosynthesis" is not wrong so much as it is a position on a line a plant can move along. What changed this year was the measurement, and the orchid had been doing this the whole time.
The term to search is mycoheterotrophy, a plant taking its carbon from fungi. Suetsugu's name is on most of the recent work.
Schematic