Some plants may have folded up their leaves for a snug “sleep” as early as 252 million years ago.
Researchers report in Current Biology on February 15 that fossilized leaves of two now-extinct Gigantonoclea species show traces of nyctinasty or circadian rhythmic folding at night. According to the researchers, these are the first fossilized examples of this unusual plant activity.
The two leaf fossils were discovered in rock strata in southern China that date to between 259 million and 252 million years old. The leaves of both species were wide, with serrated edges. But, most intriguingly, they have bizarrely symmetrical holes.
According to paleontologist Zhuo Feng of Yunnan University in Kunming, China, and colleagues, insects formed the holes while feasting on the folded leaves. According to the researchers, similar symmetrical patterns of insect damage in leaf fossils may be utilized to identify folding behavior from leaves that may have shriveled as the plant perished.
Modern plants, including those in the legume family such as the orchid tree, that fold and unfold their leaves require specialized cells called pulvinus cells, which operate much like muscles (SN: 2/3/23). The cells may bloat or deflate by moving water from one section of the leaf to another, allowing the leaves to fold or curl.
Because these cells would be near the base of the leaves, which were not preserved in the fossils, the scientists cannot determine if these ancient plants also contained pulvinus cells. Although it’s also impossible to show this was nocturnal behavior, the leaves would also have had to be folded long enough for insects to conduct their chewing. The discovery, however, suggests that such leaf folding evolved separately in various plant lineages: nearly all extant plants that accomplish this are angiosperms, or blooming plants. Gigantonoclea plants, on the other hand, were gymnosperms, or seed-producing plants like conifers and ginkgos.
Reference: Carolyn Gramling@sciencenews.org
