Episode 135 – Seeds

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Every plant starts somewhere, and often that somewhere is a small, secure, sturdy container capable of protecting, nourishing, and carrying a plant embryo until it’s time to sprout. This episode, we discuss the biology, evolution, and incredible paleontological importance of Seeds.

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Embryonic Plant Cocoons

Seeds are the structures plants use to develop and spread their offspring, produced after pollen fertilizes a plant egg cell. Specifically, seeds are found in angiosperms (flowering plants), gymnosperms (including conifers and cycads, for example) and some extinct groups like seed ferns. A seed usually consists of an embryo, the developing plant within; a seed coat, the outer protective surface; and often a packet of nutrition called endosperm. So, seeds are surprisingly similar in form and function to animal eggs.

Unsurprisingly, seeds come in a massive variety of shapes, sizes, and behaviors.
Image by Alexander Klepnev, CC BY-SA 4.0. Details here.

In addition to nourishing and protecting plant embryos, seeds have an amazing capacity to carry these embryos to new places and hold them until the right time. Seeds are excellent at dispersal – that is, moving from one place to another – some have wing-like structures to help them glide on wind currents, some are buoyant to allow them to float along streams, some have barbs that stick them onto passing animals, and some are surrounded by fruit to entice animals to swallow them in one place and deposit them elsewhere. Many seeds are also capable of remaining dormant, sometimes for surprisingly long times, until conditions are right for the new plant to develop.

Left: A seed of Quercus macrocarpa (bur oak), held by our guest, Aly!
Right: A matchbox containing a seed from every plant native to the Republic of Ireland, seen here at the National Botanical Garden of Ireland in Dublin.
Both images taken and shared by Aly Baumgartner.

Seeds in Deep Time

The earliest stages of seed evolution are known in seed ferns from the Devonian Period, including the presence of “pre-ovules” (seed-like but not-quite-seeds) in the Middle Devonian and the oldest known true seeds in the Late Devonian, around 370 million years ago.

The early evolution of seeds has a lot in common with the evolution of amniotic eggs in vertebrates. A protective seed coat, an internal food source, and new methods of delivering sperm to egg (in this case, various forms of pollen propagation) were critical developments that allowed land plants to diversify into the astonishing diversity we see today.

Left: Fossil hackberry seeds, Celtis willistoni, dating to the Miocene or Pliocene. Photo by Aly Baumgartner.
Right: Fossil heaven seed (Ailanthus lesquereuxi) alongside a fossil fly, from the Green River Formation (Eocene, ~52 million years old) in Wyoming. Photo from the National Park Service.

Most seeds are tough and resilient – that’s part of their job – so it’s no surprise that they make for great fossils. Fossil seeds are well known from across the geologic record, and they can be especially useful for characterizing ancient biomes and especially for identifying plants. Seeds, with their various unique features and great fossilization potential, are often used for identifying fossil species. For example, in the last few years, seeds from our favorite fossil site – the Gray Fossil Site – were used to identify new species of winter hazel and passion flower.

More to Explore

Seed Plant Fossil Record

The Seed Biology Place

The Evolution of Seeds, 2010 (technical)

If you enjoyed this topic and want more like it, check out these related episodes:

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