(Photograph: Cactus-seeds, Alfredo Eloisa)
Introduction
Soil seed banks are made by many plants. The seeds are naturally spread after flowering and often survive dozens of years in the soil. Many desert plants make seed banks that only germinate in rainy years. Thus, the seeds are not wasted by germinating when there is a bit of water but not enough to guarantee reproduction. Seeds from many generations of flowers can collect together in the soil.
The Details
Some plants have seeds that only germinate when they are exposed to sunlight and water. Other seeds only germinate when there is warmth and water. Still other seeds have an internal dormancy (an internal clock) that keeps them from germinating until a certain period of time has passed.
F. wislizeni seeds can accumulate in the soil because they are small and once in the soil need light, water, and warmth to germinate. This combination ensures that the tiny seedlings have optimal conditions for growing. F. wislizeni seeds can last years in the soil before some disturbance brings them into the sunlight at the right time.
Aloe marlothii makes seed banks that can germinate after a fire has killed off the parent plants. This ability provides insurance against fire.
Harrisia fragrans seeds can survive years in the soil, perhaps protecting the species against rare freezes.
Over 100 succulents have been reported to form seed banks in parts of the Karoo Desert in South Africa. Perhaps this helps the plants survive very dry times (De Villiers AJ, 2000, PhD thesis, University of Pretoria, Pretoria).
Additional Reading: Ferocactus wislizeni seed banks
Additional Reading: The Importance of seed banks
