The origin of seed plants: from spores to seeds

Seeds changed how plants reproduced on land, but the innovation did not appear all at once. Fossils preserve several stages between spore-producing plants and the earliest recognised seed plants. Ovules retained and protected the female reproductive structure, while pollen carried the male contribution without requiring sperm to swim through a film of open water. These changes made reproduction more resilient in many terrestrial settings, though the first seed plants still lived within the ecological limits of their time.

Before seeds: spores and dependence on water

Many early land plants reproduced with spores. A spore can disperse widely, but it contains limited stored resources and is exposed to drying. In groups where fertilisation required motile sperm, water was also needed for the sperm to reach the egg. These conditions did not prevent plants from colonising land, but they shaped when and where reproduction could succeed.

Some vascular plants evolved heterospory: they produced small microspores and larger megaspores rather than one spore type. The larger spore could support a female gametophyte with more stored resources. This division of reproductive roles was one step toward the seed condition, not a seed by itself. The evolutionary path involved changes to the spore-bearing structures as well as to the spores.

Precursors and the first seed plants

The Middle Devonian plant Runcaria, known from Belgium, had a branching, cup-like structure around a central megasporangium and a long extension that may have helped capture pollen. It is often described as a seed-plant precursor because it combines several relevant features but lacks a fully enclosed integument and the complete pollen-receiving arrangement of later ovules. It shows that components of the seed system were evolving before the earliest widely accepted seed plants.

Late Devonian fossils provide clearer evidence of ovules. Elkinsia polymorpha from West Virginia had cupulate ovules on branching structures. A cupule surrounded the ovule, while an integument formed protective tissue around the megasporangium and left an opening through which pollen could reach it. These remains are fragmentary, so researchers reconstruct the whole plant by combining different specimens rather than studying one complete fossil.

What a seed changed

A seed packages a developing plant embryo with a food reserve and a protective coat. It can remain dormant until conditions improve, and it can disperse in ways that differ from a naked spore. Pollen transports male reproductive cells through air or by other means, reducing dependence on free water for fertilisation. The seed system did not make plants independent of moisture: embryos still need water to grow, and pollen transfer also has biological limits.

The transition reshaped the life cycle. In seed plants, the female gametophyte develops within the ovule, and the young sporophyte is protected as an embryo. The resulting seed can survive a period of unfavourable conditions. This reproductive package offered new possibilities for colonising varied habitats, but the earliest seed plants remained part of diverse plant communities rather than instantly replacing spore-producing groups.

From Devonian ovules to later forests

During the Carboniferous and Permian, seed plants diversified into forms often called seed ferns, along with lineages related to later gymnosperms. The name “seed fern” refers to fern-like leaves combined with seeds; it does not mean these plants were true ferns. Conifers and other gymnosperm groups later became important elements of many Mesozoic forests. Flowering plants appeared much later, and their rise transformed terrestrial ecosystems during the Cretaceous.

Plants and dinosaurs therefore belong to a long ecological history that began before dinosaurs existed. Mesozoic herbivores fed within landscapes shaped by older plant innovations, and later flowering plants added new forms of vegetation. The origins of seeds are not a single moment but a series of anatomical changes documented unevenly in fossils.

A gradual innovation in an incomplete record

The fossil record places recognisable seed plants in the Late Devonian and preserves earlier precursors. It does not give us every intermediate generation or a complete picture of the first seed-bearing plant. Researchers infer relationships from reproductive structures, anatomy and the geological age of each specimen. New fossils can refine the sequence, but the broad lesson is secure: seeds arose through gradual modification of spore-producing systems and opened new ways for plants to reproduce and disperse on land.