CORVALLIS, Ore. — Oregon State University research has discovered the first fossil evidence of a rare botanical condition known as early sprouting in which seeds sprout before leaving the fruit.
In an article published in Historical Biology, George Poinar Jr. of the Oregon State College of Science describes a pinecone, about 40 million years old, encased in Baltic amber from which several embryonic stems.
“Essential to the development of all plants, seed germination usually occurs in the soil after a seed has fallen,” said Poinar, a international specialist to use plant and animal life forms preserved in amber to learn about the biology and ecology of the distant past. “We tend to associate viviparity – embryonic development while remaining inside the parent – with animals and forget that it sometimes occurs in plants.”
By far most often these events involve angiosperms, Poinar said. Angiosperms, which directly or indirectly provide most of the food consumed by people, have flowers and produce seeds enclosed in fruits.
“Seed germination in fruit is quite common in plants that don’t have seed dormancy, such as tomatoes, peppers and grapefruit, and it happens for a variety of reasons,” he said. “But it’s rare in gymnosperms.”
Gymnosperms such as conifers produce “naked”, or unenclosed, seeds. Early sprouting in pinecones is so rare that only one natural example of the condition, dating from 1965, has been described in the scientific literature, Poinar said.
“That’s part of what makes this discovery so intriguing, even beyond that, it’s the first fossil record of plant viviparity involving seed germination,” he said. “I find it fascinating that the seeds of this little pine cone can start to sprout inside the cone and that the sprouts can grow so far before they perish in the resin.”
At the tips of the shoots are clusters of needles, some in bundles of five, linking the fossil to the extinct pine species Pinus cembrifolia, which was previously described in Baltic amber, Poinar said.
Pine cones in Baltic amber are not common, he added. Those that do appear are prized by collectors and because the scales of the cones are hard they are usually very well preserved and appear realistic.
Viviparity in plants typically manifests in two ways, Poinar said. Early germination is the more common of the two, the other being vegetative viviparity, as when a bulblet emerges directly from the flower head of a parent plant.
“In the case of seed viviparity in this fossil, the seeds produced embryonic stems which are quite evident in the amber,” he said. “It is unclear whether these rods, called hypocotyls, arose before the cone was encased in amber. However, based on their position, it appears that some, if not most, growth occurred after the fall of the pine cone in the resin.
“Often some activity occurs after creatures are entombed in resin, like trapped insects laying eggs,” Poinar said. “Furthermore, parasitic insects sometimes flee their hosts into the resin after the hosts become entrapped. In the case of the pinecone, the cuticle covering the exposed parts of the shoots could have protected them from the rapid entry of the resin’s natural fixatives.
Research on viviparity in extant gymnosperms suggests that the condition may be related to winter frosts. Light frosts would have been possible if the Baltic Amber Forest had had a humid, warm temperate environment as has been postulated, Poinar said.
“This is the earliest fossil record of seed viviparity in plants, but this condition likely occurred somewhat earlier than this Eocene record,” he said. “There’s no reason vegetative viviparity couldn’t have occurred hundreds of millions of years ago in ancient spore-bearing plants like ferns and club mosses.”