[00:00.00]Our study of the life cycle of a plant usually starts with the germination of a seed.[00:05.86] The seed contains the hereditary information the plant needs to grow.[00:10.66] It also contains enough food to nourish itself during germination.
[00:14.86]Heredity—the genes inherited from the parent plant— decides several things about the life of the plant. [00:21.56]Heredity determines what the plant will look like, how it will grow, and what kind of seeds it will bear.[00:28.46] Inside the seed is all the genetic information the plant needs to grow to maturity. [00:33.96]An individual plant’s survival depends partly on its hereditary characteristics. [00:39.46]But a plant depends on more than just heredity. [00:42.46]It needs the outside world to supply its daily needs: sunlight, carbon dioxide, nutrients, and water.
[00:49.66]Some seeds germinate as soon as they’re in a suitable environment. [00:54.36]Other seeds remain dormant and won’t germinate until a specific environmental cue causes them to “wake up” and come out of dormancy. [01:03.16]Seeds of desert plants germinate only after a heavy rainfall.[01:07.66] If they germinated after a slight rain, the soil might soon be too dry to support the seedlings.[01:13.86] Seed dormancy increases the chances that germination will take place at a time and place that are best for the seedling.
[01:22.16]Germination begins when water penetrates the seed’s outer coating and reaches inside to the live embryo. [01:30.16]Water is a basic requirement, and no seed will germinate unless it absorbs water. [01:35.66]This is because the embryo inside the seed is dehydrated, and cells need water for active metabolism. [01:43.26]Water is the substance in which most of the chemical reactions of the plant take place.
[01:48.96]For a seed to sprout and grow, it needs a great deal of energy.[01:53.16] Seeds get this energy by converting the energy of their stored fuel molecules, and this requires oxygen. [02:00.16]Germination also requires a certain combination of temperature and light, which, in North America, is triggered by the start of spring. [02:08.86]Some seeds respond early in the season, others much later.[02:13.36]Each plant species has an ideal temperature at which the largest number of seeds will germinate.
[02:20.16]When the embryo takes in water, it often swells to several times its original size.[02:26.16] It gets this first burst of energy by tapping its own food stores. [02:30.86]Eventually, the seed expands enough to rupture its coating and burst through the seed case.
[02:37.76]The first organ to emerge from the germinating seed is the embryonic root. [02:43.42]The root pushes outward into the soil and anchors the seedling. [02:47.56]Now the seedling begins to gather up water and nutrients through thousands of tiny hair-like roots. [02:54.66]After a certain period, the seedling sprouts upward.[02:58.16]The tip of the shoot has to break through the soil surface.[03:02.16]Light is the environmental cue that tells the seedling when it’s broken ground. [03:06.66]Only when the seedling senses light will it straighten up and begin to grow taller. [03:12.16]As it straightens up and grows, it unfolds its solar collectors, its first leaves. [03:18.16]Once the seedling has its main leaves, it doesn’t need its embryonic food stoics anymore because now it draws most of its growth energy directly from the sun.
[03:29.16]The germination of a seed is a critical stage in the plant’s life cycle. [03:34.66]The tough seed becomes a fragile seedling, and only a small fraction of seedlings survive long enough to become parent plants. [03:43.36]Because the chances of a seedling’s survival arc so low, plants make up for this by producing enormous numbers of seeds.
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