From Seed to Seed:
Seeds, and the embryos they contain, have evolved the ability to remain in a dormant state until conditions are favorable for growth. Plants vary considerably in how long their seeds will remain viable. Some types of seeds are capable of germinating for only a few weeks after dispersal, while other types will germinate after hundreds of years if the conditions are right.
Germination, the process during which a seed begins to develop, is controlled by both internal and external factors. The external factors include water, temperature, oxygen, and sometimes light.
The presence of water is the most important factor. Without water, cells cannot carry out their necessary activities and the seed will not germinate. When a seed absorbs water, it is called imbibition. Water enters the seed either through a tiny opening in the seed called the micropyle or through the seed coat. The micropyle is also the site at which the pollen tube enters the ovule during fertilization.
Other seeds require cold temperatures before they can
germinate. This is primarily a requirement of seeds in temperate regions,
where winter provides this condition naturally. When these seeds are removed
from their natural habitats, they must undergo stratification
in order to germinate. If we break this word down, we see that the root
is stratify, meaning to layer. Historically, seeds were stratified by
layering them in damp vermiculite, peat moss, sawdust or sand. Botanists
have now discovered that they achieve the same results if the seeds are
layered in damp paper towels within a plastic bag and placed in the refrigerator
for two to three months.
Temperature is also an important factor for the process of germination itself. For most plants, the optimal germination temperature is 25 to 30oC (77 to 86oF).
Some seeds require light in order to germinate. Since light can reach only those seeds that are positioned at or near the surface of the soil, these seeds-those that require light-will not germinate unless they are located in this portion of the soil. Often these types of seeds do not have a large enough food supply to allow them to grow through a great distance before they reach the surface.
Because exposure to the wrong growing conditions can kill a seed, they also have special internal blocking agents (commonly called germination inhibitors) that prevent them from germinating until the next growing season. These inhibitors do not allow water to enter the seed, thus preventing germination from taking place. A period of drying destroys the blocking agents.
So why is it that seeds don't germinate inside the fruit? Maybe they do. Your students can explore this idea by planting a fleshy fruit and seeing if the seeds inside germinate. They won't, because the pulp of most fruits contains blocking agents that prevent germination. Once the pulp has been removed and the seed is allowed a drying period, the blocking agents inside the seed are destroyed, allowing the seed to germinate. This enables seeds that are embedded in fruits to hold off on germinating until they are dispersed (usually by animals).
Why don't I have to do any of these things to get my store-bought seeds to germinate? Commercial seeds are artificially selected to germinate once exposed to favorable conditions.
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Copyright 2001, National Gardening
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