Orthodox and recalcitrant seeds
The largest of these two seeds is orthodox (Acer platanoides, the Norway Maple) and the smaller one at the top (Acer pseudoplatanus, Sycamore) is intermediate.
Storable seeds are those that can dry and not die. Drying is lethal for most organisms including mature plants. However, many seeds are tolerant of drying (desiccation-tolerant). When dry, seeds age slowly, but they will eventually die with time. Dried seeds can however also withstand freezing. When placed at sub zero temperatures, the ageing process is further slowed down.
Species with desiccation-tolerant seeds are classified as ‘orthodox’. It is these species that we store in the Millennium Seed Bank (MSB).
Species bearing desiccation-intolerant seeds are termed ‘recalcitrant’. Because they cannot be dried, they cannot be frozen for long-term storage. This is because on freezing the water in the cells turns to ice which destroys the plant cells.
Dried and frozen seeds of orthodox species can live for tens or even hundreds of years. By far the majority of plant species studied are orthodox. However, the remainder are recalcitrant or intermediate between recalcitrant and orthodox in behaviour. Such species cannot be stored in the MSB. Research work is being carried out to determine why these differences exist and hopefully find alternative approaches to conventional long-term storage.