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Seeds and the Conditions Needed for Seed germination
Each year millions of seeds are dispersed from their parents but only a few of them will germinate immediately. Some will be eaten by animals or some may become diseased. The vast majority of seeds will join the soil seed bank and remain there, waiting for a chance to germinate. These seeds are said to be dormant.
The age of seeds in the seed bank
Seeds do not live for ever. They will stay dormant for a while but if they do not germinate they will eventually die. The life spans of seeds vary enormously. The seeds of some annual grasses must germinate within a few weeks; other plants' seeds may live for hundreds of years in the soil. In general most seeds seem to be able to live for 10 to 15 years in the soil. The Arctic lupin, holds the record so far. Seeds of this plant were found in the burrows of lemmings in Alaska. They had been buried in the Arctic soil since the end of the last ice-age!
Conditions needed for germination
The opportunity to germinate may come if the soil is disturbed in some way; for example, if a fire sweeps across the land or if humans or grazing animals destroy the growing vegetation.
The conditions needed for seeds to germinate vary from species to species. In general we can say that all seeds need water and warmth to germinate.
In general, most seeds will germinate at temperatures between 10°C and 35°C. Some seeds, however, need a period of cold before they will germinate. For example, cherry seeds will germinate better if they are kept humid at 5°C for 16 weeks. If these seeds are kept at 25°C they will not germinate. This period of cold temperature "tells" the seeds that winter has passed by.
Water is important for the germination of all seeds but for plants which live in deserts the amount of water can be very important. A light fall of rain may be enough to start some plants' seeds germinating. For desert plants the seeds often need to be soaked for several hours before they will germinate. This is important in the desert because heavy rainfall means that there will be enough water for the seedling and the adult plant. This water will last for a few weeks at least. A light fall of rain will rapidly evaporate in the hot desert sun.
Most seeds will germinate quite easily in the dark. Some seeds, however, need to be exposed to the light for a certain time before they will germinate. For example, the seeds of the birch tree need 8 successive days of about 10 hours of sunlight before they will germinate. This "tells" the seeds that the days are long enough. This means that there will be enough sunlight energy for the growing seedling once it starts to feed itself by photosynthesis.
The seed coat and germination
The seed coat or testa protects the seed from damage or disease. It is often hard and waterproof (impermeable). This is a problem for the germination of the seed because the growing embryo needs oxygen and water. Some seeds will not be able to germinate until the seed coat has been damaged so that water can enter and the embryo can respire.
The seed coat may be damaged when the seed is in the soil. The soil particles rub against the seed coat and wear it away. The seed coat may be decomposed slowly by micro-organisms in the soil or it may even be damaged by fire. Fire is important for some seeds of Mediterranean plants. These germinate quickly after a forest or the maquis has been burnt down.