Tree fern Cyathea rojasiana transforms dead leaves into roots

The tree fern Cyathea rojasiana, which grows in Panama, has a crown of leaves on a trunk. Sometimes a new frond sprouts, sometimes an old leaf dies. But dying is not the end for a leaf, James Dalling and colleagues discovered: the decayed leaf gets a second life. At least: the rachis.

The tree fern grows to a height of two meters, the leaves are more than two meters long. A senescent leaf will bend down, the leaf tip touching the ground. The leaf rots away, but the rachis remains. A tree can have a ‘skirt’ of twenty to thirty of those remains. They look lifeless, but when Dalling tried to remove them, they were found to be firmly stuck in the ground.

Vascular bundles run through the rachis of a green leaf, transporting water and nutrients absorbed by the roots to the leaf tissue. These vascular bundles appear to be intact in the ground-stuck leaf remains of Cyathea rojasiana. They are surrounded by a black layer that apparently protects them from rotting. And, surprisingly, at the end, i.e., in the soil, a bunch of finely branched roots has sprouted from each vascular bundle.

The conclusion is that the former leaf rachises have been transformed into roots, in which the direction of the water flow is reversed: in green leaves, it flowed from stem to leaf tip, now it is from leaf tip to stem.

Cyathea rojasiana grows in wet, extremely nutrient-poor soil. Extra roots are not so much useful to absorb water, the researchers think, but to extract nutrients from a larger area of soil. They showed that the new roots indeed absorb nitrogen and transport it upwards.

For many plant species holds: you can stick a piece of stem or leaf in the ground, and roots will grow. But converting a leaf rachis into a root is something that, as far as we know, only Cyathea rojasiana does.

Willy van Strien

Photo: Tree fern Cyathea rojasiana with decayed leaves, now functioning as roots. ©James Dalling

James Dalling tells about his discovery on YouTube

Dalling, J.W., E. Garcia, C. Espinosa, C. Pizano. A. Ferrer & J.L. Viana, 2024. Zombie leaves: novel repurposing of senescent fronds in the tree fern Cyathea rojasiana in a tropical montane forest. Ecology e4248, 18 January online. Doi: 10.1002/ecy.4248