A seed is the embryo of a plant enclosed in a protective covering. It is the product of a mature ovule fertilized by sperm from pollen.
Despite being dormant, seeds are highly complex with resumption of physiological processes and a surprisingly long lifespan. The physiology of these features is important for understanding how seeds survive and germinate.
What is a seed?
The seed of a plant has three basic parts: a protective outer covering, some stored food and an embryo that is the beginnings of a new plant. Its functions are to be dispersed to a new location, remain dormant until conditions are favorable for germination, and nourish the embryo once it starts growing.
Seeds come in a huge variety of shapes and sizes. Some have appendages to entice animal dispersers; others have hooks, barbs or sticky hairs that attach to fur or feathers; and still others have wings for wind dispersal.
Some seeds, like those of the coco de mer, float to be carried considerable distances. They can also be ellipsoid, square or ovate (broader below the middle) or kidney-shaped (reniform). Many seeds are found inside fruits, which are the mature ripened ovaries of flowers.
How do seeds germinate?
The seed germination process begins when the embryo in a dormant seed absorbs water. This water uptake, called imbibition, causes the seed to swell and soften its tough outer seed coat. Then a series of hydrolytic enzymes begin to break down the stored food reserves in the seed embryo and release nutrients for growth.
The rate of germination is greatly affected by internal physiology and the environment, including temperature, moisture, light exposure and oxygen availability. Some seeds have a physical dormancy that requires mechanical action to break, such as breaking open the thick seed coat. Others have a physiological dormancy that requires specific environmental conditions, such as warm soil and cool temperatures.
During the germination process, the primary root anchors the seed to the ground and begins to grow, and the radicle breaks out of the cotyledons to form a small embryonic stem and leaves. The seedling is then ready to grow and utilize sunlight energy for photosynthesis.
What are the different types of seeds?
Seeds come in a variety of shapes, sizes and colors and are surrounded by a protective outer covering. They contain nutrients that the young plant needs to grow and are the primary way that angiosperms propagate themselves.
A seed contains an embryo and endosperm. The embryo consists of one or more cotyledons (embryonic leaves) in monocots and two or more cotyledons in dicots. The cotyledons absorb nutrients from the endosperm and provide food for the growing embryo.
The seed coat is a thick membranous layer that performs many different functions, including protecting the embryo from physical, mechanical and temperature-related damage. It also protects the embryo from microorganisms that would otherwise cause it to rot or die. A scar remains on the seed coat, called the hilum, from where it fused with the testa during ovule development.
How do seeds stay viable?
Seeds can stay alive a long time if they are stored properly. On average, seeds will remain viable for about three years after the “packed for” date on the packet. However, the viability of seeds depends on the type of seed and how they are stored.
The biggest threat to seed viability is moisture. Keep your seeds dry by storing them in a dark, cool place with low humidity. You can also use silica gel packets or powdered milk to absorb any moisture from the seeds’ storage containers. Stored in this way, seeds can last 10 years or more. The germination rate of seeds will decrease over time, but they will still be viable for a few years after their package date. This is a natural process due to the DNA damage that accumulates during seed dormancy.
What are the benefits of saving seeds?
Seed saving is a practice of gardening that allows gardeners to take control over their food and limit their dependence on large seed companies. It is an ancient tradition that has been revived by those who want more variety and better control of their crops.
By saving seeds from plants that perform well in your garden, you will have a greater chance of replicating that success season after season. This is especially true for fruiting crops like tomatoes, peppers, squash, melons, and cucumbers.
Another benefit is the ability to share your seeds with others in your community or swap for new varieties. This can also save money since seeds are much cheaper than plant starts. Saving and sharing open-pollinated seeds is part of a larger movement to reclaim the commons and fight against patenting and copyright laws.
