As early as 252 million years ago, some plant leaves can be bent at night to encourage “sleep”.
Two leaves now-extinct Gigantonoclea The species’ nocturnal, or circadian rhythmic, nocturnal rhythmic patterns, researchers report, date back to February 15 in Current Biology. That makes these specimens the first known fossil examples of this curious plant behavior, the team says.
Two leaf fossils were found in a layer of stone in southwest China, dating back to between 259 million and 252 million years ago. In both species the leaves were broad, with serrated edges. But more curiously, they bear variously symmetrical holes.
The insects made those burrows by feeding on leaves while they were folded, say paleontologist Zhuo Feng of Yunnan University in Kunming, China, and colleagues. Similar symmetrical patterns of insect damage on fossil leaves can be used to distinguish folding behavior from leaves that, like dead plants, are called stacks.
Modern plants, including many in the legume family such as the orchid tree, develop their folds and leaves using special cells called cushion cells, which act as a sort of muscleSN: 2/3/23). As the water moves from one side of the leaf to another, the cells can swell or deflate, the leaves can fold or curl.
These cells would be at the base of the leaves, which were not preserved in the fossils, so it is not possible to say whether these ancient plants also had cushion cells, the team says. Although it is also difficult to prove that this was the behavior of the night, the leaves were also folded long enough to make the insects to chew on them. But the finding suggests that such leaf folds arose independently in different plant lineages: Almost all modern plants that do this are angiosperms or flowering plants. But Gigantonoclea the plants were gymnosperms, seed-producing plants such as conifers and ginkgos.
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