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Our study of the life cycle of a plant usually starts with the germination of a seed. The seed contains the hereditary information the plant needs to grow. It also contains enough food to nourish itself during germination.
Heredity—the genes inherited from the parent plant— decides several things about the life of the plant. Heredity determines what the plant will look like, how it will grow, and what kind of seeds it will bear. Inside the seed is all the genetic information the plant needs to grow to maturity. An individual plant’s survival depends partly on its hereditary characteristics. But a plant depends on more than just heredity. It needs the outside world to supply its daily needs: sunlight, carbon dioxide, nutrients, and water.
Some seeds germinate as soon as they’re in a suitable environment. Other seeds remain dormant and won’t germinate until a specific environmental cue causes them to "wake up" and come out of dormancy. Seeds of desert plants germinate only after a heavy rainfall. If they germinated after a slight rain, the soil might soon be too dry to support the seedlings. Seed dormancy increases the chances that germination will take place at a time and place that are best for the seedling.
Germination begins when water penetrates the seed’s outer coating and reaches inside to the live embryo. Water is a basic requirement, and no seed will germinate unless it absorbs water. This is because the embryo inside the seed is dehydrated, and cells need water for active metabolism. Water is the substance in which most of the chemical reactions of the plant take place.
For a seed to sprout and grow, it needs a great deal of energy. Seeds get this energy by converting the energy of their stored fuel molecules, and this requires oxygen. Germination also requires a certain combination of temperature and light, which, in North America, is triggered by the start of spring. Some seeds respond early in the season, others much later. Each plant species has an ideal temperature at which the largest number of seeds will germinate.
When the embryo takes in water, it often swells to several times its original size. It gets this first burst of energy by tapping its own food stores. Eventually, the seed expands enough to rupture its coating and burst through the seed case.
The first organ to emerge from the germinating seed is the embryonic root. The root pushes outward into the soil and anchors the seedling. Now the seedling begins to gather up water and nutrients through thousands of tiny hair-like roots. After a certain period, the seedling sprouts upward. The tip of the shoot has to break through the soil surface. Light is the environmental cue that tells the seedling when it’s broken ground. Only when the seedling senses light will it straighten up and begin to grow taller. As it straightens up and grows, it unfolds its solar collectors, its first leaves. Once the seedling has its main leaves, it doesn’t need its embryonic food stores anymore because now it draws most of its growth energy directly from the sun.
The germination of a seed is a critical stage in the plant’s life cycle. The tough seed becomes a fragile seedling, and only a small fraction of seedlings survive long enough to become parent plants. Because the chances of a seedling’s survival are so low, plants make up for this by producing enormous numbers of seeds.