In the rainforests of western Panama, plant biologist Jim Dalling stumbled upon some tree ferns with 1.8-meter-long leaves that leaned toward the ground as they died, surrounding the plant like a skirt.
“I was trying to get these things out of the way and then in the process I realized they were rooted in the ground,” Dalling said.
A professor and forest ecologist at the University of Illinois, he had been searching for a
completely unrelated plant.
The remains of the fern leaves were brown and withered, dead to all appearances (although still attached at their base to the trunk of the tree fern).
Dissection of the roots of Cyathea rojaiana, a tree fern from the Panamanian rainforest.
The vascular bundles develop rootlets at the base of the stem. Photo Ames W. Dalling
He asked himself: How could lifeless plant matter be given roots?
"I really couldn't believe what I was seeing," Dalling said.
A more detailed study revealed that this tree fern, known as
Cyathea rojaiana
, transforms the inner part of its dead or dying leaves.
The remains of the xylem and phloem (tubules that transport water, sugars and nutrients through living leaves) somehow
become a root.
From the tips of these leaves, nicknamed “
zombie leaves
,” new fine roots then sprout and penetrate the soil, said Dalling, co-author of a study describing the findings published in January in the journal
Ecology.
In the process of transmutation of the central vein of the leaf, the plant undergoes a proliferation of
new vascular tissue
and prevents it from rotting while the rest of the leaf withers.
"This is completely unknown in any other plant in the world," he said.
"Normally, vascular tissue is deposited on the leaf and that's it," said Robbin Moran, a fern expert and curator emeritus at the New York Botanical Garden in the Bronx.
But in this species, after the rest of the leaf dies, “it begins to differentiate, proliferate.
“I don’t know how that happens.”
"I've never seen anything like it," added Moran, who was not involved in the paper.
This tree fern is found in a soggy area of the
Fortuna Forest Reserve
, which receives more than 6 meters of rain a year.
Rain leaches sandy, volcanic soil,
draining nitrogen and phosphorus
.
Researchers believe the fern's adaptation allows it to take advantage of pockets of nutrients in the nearby topsoil that it otherwise couldn't access.
Many plants are known for their almost unlimited capacity to adapt, and other species can produce new roots from living leaves.
Notable examples include the walking fern (Asplenium rhizophyllum), native to the United States, which is propagated on moss-covered rocks using this technique.
But the “zombie leaves,” or leaf roots, of this tree fern represent the first known example of reusing dying tissue, said Eddie Watkins, a professor and fern expert at Colgate University who was not involved in the study.
By converting existing leaf material into roots, the plant will most likely save energy, Watkins suggested.
That could help you in the “
battle for nutrients”
in your part of the rainforest.
Heavy rainfall and poor soils provide a unique set of plants, including a conifer known as Podocarpus, which is Greek for "footed fruit" (a reference to the stem-like appearance of another species in the genus).
This evergreen tree sends strange bulbous roots everywhere, even into the trunks of surrounding trees.
The conifer was what led Dalling to rummage through the undergrowth in the first place.
"It's pure chance that we were digging at the base of these tree ferns," he said.
Dalling inspected dozens of these ferns and discovered that each one of them was growing roots from its "zombie leaves."
But his work was interrupted by the onset of the coronavirus pandemic, forcing him to leave Panama with his wife and dog and return to Illinois in February 2020.
Two years later he returned.
With his colleagues, Dalling dug up roots from the leaves of three individual plants, placed them in pots and added a chemically labeled nitrogen fertilizer.
A month later, they examined new leaves at the top of the fern and found that nitrogen was indeed being incorporated into the plant, confirming that these roots were
actively transporting water
and nutrients.
There is still much left to know, including basically all the biochemical and developmental secrets of how this particular tree fern accomplishes such a transformative feat.
The discovery speaks to the importance of taking the time to study and appreciate the natural world, Watkins said, adding that this type of exploration has become
less frequent and difficult to fund.
"If you stop and look at the organism, there are some really new and interesting things out there," Watkins said.
"There are stories to tell that we haven't discovered."
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