Seeds are tiny capsules with an embryonic plant and stored food, surrounded by a hard shell or husk. They are the result of sexual reproduction in gymnosperm and angiosperm plants that involves pollen from one flower mixing with an ovule in another flower.
The best seeds for your garden are the ones that germinate quickly. Read the packets carefully to learn what conditions are required for each type of seed to sprout.
What is a seed?
Seeds contain genetic information that, in the right conditions, could give rise to a new plant. They are also the source of many important medicines including castor oil, tea tree oil and the quack cancer drug laetrile.
Seeds come in a wide variety of shapes and sizes. They can be tiny such as a poppy seed or enormous like a coco de mer coconut. They can be disc-like, globose, ellipsoid or lenticular and they can be striped with parallel lines or ridges.
The three basic parts of a seed are the embryo, food reserve and the protective coat. A seed contains food that it needs to grow and it is stored within the embryo until a trigger event stimulates growth. This can be a disturbance such as rodents chewing on the seed, water currents, freezing and thawing or passing through an animal’s digestive system. Seeds are the distinguishing feature of flowering plants that are classified as gymnosperms (naked-seeded) or angiosperms (with seeds enclosed in ovaries). All other plants are known as pteridophytes, which include ferns and mosses.
How do seeds grow?
Seeds contain a miniature plant embryo and stored food in a hard shell, plus DNA (a set of instructions for how an organism should look, work, and grow). Seed plants have an advantage over other plants that reproduce with spores because seeds provide them with a much bigger head start.
The hard seed coat protects the embryo and stored food until the right conditions arrive. Then a process called germination begins.
As water seeps through a small opening in the seed coat, it hydrates enzymes and activates cells inside the seed. The seed then absorbs its food supply through its seed leaves, or cotyledons. This nourishes the embryonic plant until it can make its own food through photosynthesis.
Seeds can travel great distances thanks to birds, mammals, and other animals that carry them or disperse them by dropping them. The dustlike seeds of some orchids attach to animals’ fur, while berries and fruits with wings help them travel by air.
How do seeds travel?
Students examine seeds from a variety of plants and try to figure out how their structure helps them travel. They also investigate how seed dispersal methods differ.
Some seeds can float and move long distances on water, like the seeds of coconut trees or sea beans. For this to happen a seed needs a waterproof or water-resistant shell and some way to stay afloat, such as fluff attached.
Wind can carry seeds long distances, too. Plants like dandelions and cottonwood trees make seeds with tufts of hairs called pappi that catch the wind like little parachutes, helping their seeds sail far away from the parent plant.
Other seeds hitch rides on animals, including people, whose fur or clothing they can stick to. This can work even for large animals, like birds like the Clark’s nutcracker, which carries seeds in its mouth for miles before dropping them in a place where they can grow. Some seeds are also hidden in tasty fruit, like tomatoes or blackberries.
How do seeds germinate?
Seeds must have a few conditions to germinate. These include a proper temperature (germination is fastest at the optimum growing temperature), water, oxygen, and a seed coat that has been broken open. During germination, enzymes break down stored energy within the seed to start growing.
When seeds are introduced to water under the right conditions, they absorb it and swell. This is called imbibition. As a result of this, their cells grow rapidly and the outer seed coat breaks apart. A root, known as a radicle, and a shoot, known as a plumule emerge from the seed.
The radicle goes down into the soil looking for water, while the plumule grows up toward sunlight to begin photosynthesis. Interestingly, researchers are starting to understand how gene networks play an important role in seed development. This includes the genes that control a seed’s ovule and embryonic growth. Cecilia and her colleagues are studying this. This work could lead to new ways of planting seeds that have the potential to be more successful.
