Spring has arrived and new growth is starting to peek out of the ground. Some of that fresh green arises from perennials, bulbs and enduring roots, and some is the immediate product of seeds that have been waiting for spring to germinate and grow.
A plant seed holds an embryonic plant, of course, but it also usually has stored food to help jump start the embryo when it starts growing, and it has a coat to protect the combination. Frequently the seed coat has features that help disperse the seed from parent to new ground, distributing the species.
There’s a fascinating variety in the forms of seeds, seed coats, and other structures that aid in seed dispersal: feathery bits and wings on some seeds catch the wind, fluffy fibers and air pockets allow other seeds to float on water, hooks grab passing animals to give some seeds a ride, and delicious coatings and flesh tempt animals to eat some seeds that later pass through to the ground -- often encased in, um, fertilizer. Some animals collect and store seeds for later dining, but may forget or abandon their cache, leaving the seeds to germinate later. And in some plants, the seed case is spring-loaded to fling mature seeds far away from the parent.
Having offspring that don’t grow right away gives additional time for that offspring to disperse far from the parent before growing, distributing the species and decreasing crowding among siblings. Neat little packages, most seeds are dormant until conditions are ripe for germination and growth.
A seed can be dormant for days or decades -- or longer. While nearly all seeds are viable for one to two years, a 2,000-year-old date palm seed found in Herod the Great’s palace in Israel germinated in 2005. And the current, documented record-keeper for seed dormancy is a trio of Arctic wildflower seeds found inside ground squirrel burrows deep in the Siberian tundra permafrost. Those seeds were germinated in the laboratory after being dormant for some 32,000 years and grew into full-fledged, flowering plants.
Of course, once a seed is dispersed it needs to settle in a location that will prompt germination and allow successful growth of the seedling to a mature, reproducing plant.
What signals seeds to germinate?
First, water seeps into the seed, infusing it -- swelling it -- and stimulating enzymes that begin growth of the embryo. The root grows first, driving down seeking more water. The shoot, the first leaves and the stem, follows shortly, reaching up to gather sunlight’s energy.
You have free articles remaining.
Plants need some oxygen, too, and germinating plants that are using up the stored food in the seed before able to make their own food are especially dependent on oxygen.
The proper temperature is important, as well, but different plant seeds need different temperatures for successful germination. Further, some seeds need very cold temperatures followed by warmer temperatures to germinate, which postpones germination until spring, with its guarantee of summer to follow.
A few seeds need sunlight to germinate, and at least some sunlight becomes crucial for the seedling to sprout and begin to make its own food. (Sending the root down and the shoot up is a response to gravity, not light.)
In addition, the seeds of some plant species need extra nudging to germinate. That nudging might come from the chemicals in animal digestive systems that break down or thin the coating, letting water into the seed, or that otherwise aid in growth.
For some seeds, fire or other extreme heat is required to free the seed from the parent plant or to weaken the seed coat and allow germination. And some seeds are encouraged to germinate by chemicals in smoke.
As you’d expect, the details of these prompts required for germination can vary quite a bit from species to species.
If this spring’s freshly-germinated plants miss being eaten by animals, survive our capricious spring weather, and escape being too-hot, too-cold, too-dry, too-wet long enough to flower, they’ll set their own seed. In doing so, they’ll send their genes off to start a fresh generation next spring.