Seed banks hold wild plant material in cold storage for decades, and the method works because a dry seed is barely alive. The limits of that trick decide which species can be banked and which cannot.
Why seeds are stored instead of plants
A living collection of wild plants requires land, water and constant attention, and a single disease outbreak or drought can remove years of work in a season.
Seeds compress the same genetic material into something the size of a grain. Thousands of individuals from a single population fit inside one sealed container.
That density is what makes broad coverage affordable. A bank can hold material from many populations of one species, which matters because genetic variation is spread unevenly across a range.
What drying and freezing actually do
Seeds are dried until their internal moisture falls very low, then held at deep-freeze temperatures. Both steps slow the chemical reactions that gradually degrade cell membranes and genetic material.
Water is the problem. It carries the reactions that break down stored fats and proteins, and it forms damaging ice crystals if a moist seed is frozen.
Remove most of the water first and the freezing becomes safe rather than destructive. Under those conditions many species stay viable for decades, and some far longer.
The seeds that refuse to be banked
A substantial group of species, concentrated in wet tropical forests, cannot tolerate drying. Their seeds are shed with high moisture content and germinate quickly rather than waiting out a dry season.
Drying kills them outright, so conventional banking is unavailable. Many large-seeded rainforest trees and several mangrove species fall into this group.
Alternatives exist, including freezing isolated embryos in liquid nitrogen, but the techniques are laborious and have to be developed species by species.
Why stored seed has to be retested
Viability declines slowly even in ideal storage, and the rate differs between species and between collections of the same species gathered in different years.
Banks therefore germinate small samples on a schedule and record what proportion sprouts. A falling result signals that the accession needs regrowing before it is lost.
Regeneration means planting out the remaining seed, raising mature plants and harvesting a fresh batch. It is the expensive part of running a bank, and it never stops.
What a bank cannot hold
Seeds preserve genetics, not ecology. The pollinators, fungal partners and soil conditions a plant depends on are not stored alongside it.
A species returned to the wild after decades in a freezer meets a site that has changed, and often lacks the associates it evolved with.
Banking therefore buys time rather than providing a solution. It keeps options open while habitat work proceeds, which is a narrower claim than it is often given credit for.