Jennifer Schultz Nelson
Extension Educator, Horticulture
Gardeners get very anxious this time of year. It is time to start seeds for their gardens. Many have already started the process, eager to see tender young shoots that hold within them the promise of sunny days and warm weather. Stop and consider what a truly amazing structure a seed is. It is for all practical purposes nature's own suspended animation system.
Most seeds have some sort of dormancy requirement, meaning that they will not grow immediately upon maturity. Considering that most plants produce mature seed at the end of the growing season, seeds with a dormancy requirement have an advantage. They will not grow immediately. If they did, chances are the environment would not remain favorable for very long. In central Illinois for example, if every seed shed in the late summer and early fall germinated, they would soon be frozen to death as winter set in. In other climates other factors may play a role, as in places where there is a wet and dry season.
Plants have evolved dormancy requirements that reflect their native environment. For instance, seeds of plants that grow in regions with seasonal temperature variations often will not germinate without a period of cold temperatures. Some seeds need more extreme treatments to be coaxed into growing, such as fire, or traveling through the digestive system of an animal. There is a chemical and/or physical explanation for these phenomena.
There are many physical and chemical components in a seed that all contribute to keeping the next plant generation safe and sound until the next growing season. Just as we put on a coat to protect ourselves from the environment, seeds are covered with a coat, the "seed coat". Typically, this is a tough layer of tissue designed to protect the tiny plant embryo inside.
In all cases, a seed needs to take up water to initiate germination. Sometimes the seed coat will not let water through, so the seed coat must be worn away physically or chemically to allow water to be taken up by the seed. Some seed coats may let water enter the seed, but the seed coat itself produces chemicals that inhibit the growth of the plant embryo. These chemicals must be neutralized by environmental factors such as temperature, light, or dilution with water before germination can begin.
Within the seed coat is the plant embryo, containing all the genetic instructions to build a full-size adult plant. Seeds also contain storage tissue filled with energy to keep the plant embryo alive. The seed itself is very dry, at only five to twenty percent water, and the metabolic needs of the embryo are few. Energy in the storage tissues is mobilized during germination to help the embryo grow and develop.
Many seeds can remain viable for years after they develop. An extreme example is a date palm seed germinated in Israel nicknamed "Methuselah". Carbon-dating confirmed the date palm seed was over two-thousand years old, making it the oldest seed ever successfully germinated.
Seeds are also the means by which plants disperse throughout the land. In the coming weeks we'll explore some of the more strange and unusual ways that some plants travel via seeds.
** Remember: the 2006 Potpourri of Gardening is on Saturday, March 25, 2006. There are seven different classes to choose from, and cost is only $10 when pre-registering. Call 217-877-6042 for more information.