Typically, a seed’s number one job is to have patience. Before it grows into a new clover or pumpkin vine or oak tree or hydrangea, it has to wait. Only when conditions are just right will the seed sprout, which gives it the best chance of survival.
Yet for a few tree species, the seed’s job is different. It doesn’t wait. It starts growing right away, while still attached to its parent plant, and only separates later. Trees that do this are called viviparous, or live-bearing. It’s the same name scientists give to animals, such as humans, that birth live babies instead of laying eggs.
Despite the unexpectedness of this trait, researchers studying the genetics of viviparous trees recently showed that the pathway to their evolution might have been surprisingly simple.
While viviparity is rare among trees in general, it’s common among the mangroves, roughly 80 species that live on warm coastlines around the world. These trees are already unusual, as they absorb water that’s up to 100 times saltier than what most plants can tolerate.
A live-birthed baby mangrove doesn’t look like a chubby infant, or like a miniature adult mangrove. Instead, it’s like a string bean with a bulbous cap, topped by a little crown of roots. The babies hang from their parent tree in clusters, and when they reach a certain stage of development they drop straight down into the mud or sand below, says Yingjia Shen, a researcher at China’s Xiamen University.
If the tide is out when the baby mangroves fall, their roots grow rapidly, Shen says, with the plants starting to take hold within a few hours of hitting the ground. In other cases, though, the young plants may take a journey. Baby mangroves are buoyant, and “those that fail to root in the mud can drift in the ocean currents for several months,” Shen says, “potentially reaching coastlines thousands of kilometers away and taking root there.”
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