What Is a Seed? A Package That Changed What Plants Could Do

Plants reproducing by spores face two constraints. The spore is a single cell with minimal reserves, so it must germinate where conditions are immediately suitable and has no capacity to wait. And fertilisation in those groups requires sperm to swim to an egg, which ties reproduction to the presence of water, however briefly, and confines such plants to damp conditions during that stage. A seed addresses both. The embryo is already a multicellular plant with a supply of stored food, so it can establish itself in conditions a spore could not survive. It can remain dormant, sometimes for years, until conditions are right. And fertilisation happens by pollen delivered through the air or by an animal, which removes the requirement for water entirely and allowed plants to colonise dry ground and to reproduce in seasons when nothing was wet.

Three parts do distinct jobs and each is a separate innovation:

• •The embryo, a young plant with the beginnings of root, shoot and leaves already formed
• •Stored food, either in dedicated tissue or in the embryo's own leaves, which fuels establishment before photosynthesis begins
• •The coat, derived from the parent plant, which protects against drying, damage and digestion
• •Dormancy mechanisms, which prevent germination until specific conditions are met
• •Structures for dispersal in many species, including wings, hooks and fleshy coverings
• •Chemical defences in many, since a package of stored food is attractive to anything that can eat it

Seed plants appear in the Devonian and expand through the Carboniferous and Permian, and the shift in the fossil record is one of the clearest large-scale vegetation changes visible. The coal forest flora, dominated by spore-producing lycopods and their relatives, was tied to permanently wet lowlands and collapsed when the climate dried. Seed plants, which were present but subordinate, were not so tied and expanded into the drier ground the earlier groups could not occupy and then into the lowlands the drying had opened. Conifers and their relatives dominated the Mesozoic. Flowering plants, which added the enclosed ovary and later the fruit, appeared in the Cretaceous and expanded rapidly to become the overwhelming majority of plant species. Each step added capability to the same basic package rather than replacing it.

Flowering plants added two further components to the package and the additions account for their dominance. An enclosed ovary protects the developing seeds and, crucially, permits the ovary wall to develop into a fruit after fertilisation, which buys animal dispersal with edible tissue. The flower itself concentrates reproductive structures into a unit that can be advertised to pollinators, which makes animal pollination efficient and targeted in a way that wind pollination cannot be. Double fertilisation, a peculiarity of the group, produces the nutritive tissue only after fertilisation has occurred, which avoids investing food reserves in seeds that will not develop. Those innovations together allowed faster reproduction, more reliable pollination and better dispersal, and the group expanded from nothing to the overwhelming majority of plant species within a comparatively short geological interval.

The ability to postpone germination is the feature with the most surprising consequences. Dormancy mechanisms include coats too hard for water to penetrate until abraded or heated, chemical inhibitors that must be leached out by rain, and requirements for a period of cold, for a period of warmth, for fire, for light or for a specific temperature alternation, all of which prevent germination until the conditions the species needs are actually present. Seeds accumulate in soil as a bank that can persist for decades and that regenerates vegetation after disturbance, which is why a cleared or burned site produces plants nobody planted. Documented germination of very old seeds includes specimens recovered from archaeological sites and from permafrost, with several cases of viable germination after centuries and one contested claim running to considerably longer.

An embryo with stored food and a protective coat can wait for conditions rather than needing them immediately, and pollen removes the requirement for water at fertilisation. That freed plants from damp ground and from wet seasons. Seed plants expanded as the coal forests collapsed with drying climate. Dormancy mechanisms hold seeds in soil for decades and regenerate vegetation after disturbance.