Why Will Some Seeds Not Germinate? The Shell Is Refusing

The coat is formed from the tissue of the parent plant that surrounded the developing ovule, and it performs several jobs at once. It excludes water and gas, which keeps the embryo dry and dormant and allows a seed to survive for years or in some cases centuries. It resists physical damage, fungal attack and being eaten. It carries chemical defences in many species. And in a great many plants it enforces a delay, refusing to admit water until the coat itself has been broken down by weathering, by passage through an animal, by fire or by prolonged cold, so that germination happens under conditions likely to favour a seedling.

Each mechanism corresponds to a situation the plant is waiting for:

• •Abrasion by soil and gravel, which takes time and indicates burial
• •Passage through a digestive tract, which indicates dispersal by an animal
• •Fire, which cracks coats in many species of fire-prone habitats
• •Prolonged cold and damp, which indicates that winter has passed
• •Alternate freezing and thawing, which splits coats mechanically
• •Microbial decay in the soil, which takes a season or more

Growers imitate these processes deliberately and the techniques are standard. Scarification abrades or nicks the coat with sandpaper, a file or a knife, taking care to avoid the point where the embryo attaches, which is sufficient for many hard-coated species including sweet peas and many legumes. Soaking in hot water swells and softens the coat for others. Stratification stores seed damp and cold for weeks or months, imitating winter, which many temperate trees and perennials require. Smoke treatments work for species from fire-prone regions and are available commercially. Applying the wrong treatment achieves nothing, and knowing which a species needs is the main practical knowledge involved in raising plants from seed.

Understanding what the layer is protecting explains why it matters so much. A seed contains an embryo, which is a complete miniature plant with a root, a shoot and one or two seed leaves already formed. It contains a food store, held either in a separate tissue or in the seed leaves themselves, which supplies the seedling until it can photosynthesise, and that store is what makes seeds valuable as food for everything else. The embryo is dried to a very low water content, which is what allows it to survive dormancy without its cells being damaged, since metabolism essentially stops. Germination begins with water entering through the coat, which restarts that metabolism and allows the root to emerge first.

Seeds surviving for very long periods have consequences that reach well beyond gardening. A soil seed bank holds viable seed accumulated over years, which allows vegetation to return after disturbance and which is why clearing ground produces a flush of plants nobody planted. Agricultural weeds persist in soil for decades, which is why a single season of seeding causes problems for a generation. Seed banks for conservation exploit the same property, storing dried seed at low temperature for centuries, with the Svalbard facility holding duplicates of crop collections worldwide. And record germinations of very old seed, including date palm seed around two thousand years old and a narrow-leafed campion from Siberian permafrost, demonstrate how durable the arrangement can be.

The coat excludes water and gas, resists damage and attack, and in many species refuses to admit water until abrasion, digestion, fire or prolonged cold breaks it down, which times germination to favourable conditions. Gardeners imitate those processes by scarification, soaking, cold storage and smoke treatment. Long dormancy fills soil seed banks, sustains agricultural weeds for decades and underpins conservation storage.