Seed is a plant spore that produces a new plant when it germinates under proper environmental conditions. Each seed consists of an embryo (the fertilized ovule), endosperm, and a seed coat or coats.
Seeds are found in a variety of shapes and sizes. Some have fleshy appendages that entice animal dispersers to eat them; some have hooks, barbs, or sticky hairs that attach to fur or feathers; and some have wings for wind dispersal.
What is a seed?
A seed is the reproductive structure of a plant that disperses and can survive for some time. It contains an embryo, a food reserve and some kind of protective outer covering called the seed coat.
The seed coat protects the embryo and encapsulates the endosperm, which serves as the initial food source for the embryo. It also protects the seed from disease and insects, which might cause it to germinate before it is ready.
In most seeds, germination depends on specific conditions that must be met for the seed to germinate. These conditions include temperature, moisture and sunlight.
There are many different kinds of seeds, with various functions. Some are edible, like sesame seeds and peanuts; others are commercially valued for their oils (such as canola).
Germination
When all of the proper conditions are in place, a seed will start to germinate. This process can be very quick for some seeds (such as cabbage, kale, and cauliflower) or very slow for others (such as pepper, eggplant, and fennel).
During germination, seeds go through different stages, from dormancy to emergence. During dormancy, the seed is covered with a protective layer called the seed coat.
After a period of dormancy, the seed will take up water and begin to swell. It may even break open!
Then, the cells in the embryo will start dividing rapidly and a root called the radicle will emerge. The root will then grow down into the soil, looking for water and minerals.
Once the roots are strong, they will send up leaves that will use sunlight to make food and help the plant stay healthy. These leaves will eventually grow to be a full-size plant. When a plant reaches this stage, it is considered to have “established.”
Perennation
The term perennation is used to describe the ability of some organisms, such as plants, to survive periods of unfavourable conditions. During these times, organisms develop perennating organs that store enough nutrients to sustain them and produce one or more new plants the following year.
These organs are different in form but commonly include tubers, rhizomes and corms. They are modified plant structures that can distend to store carbohydrates, nutrients and water for future use in the next growing season.
They are usually located below ground and may have adaptations such as insulating characteristics from plant litter or snow cover that provide additional protection against winter frost damage. These changes in structure can increase the storage capacity of these organs to allow them to persist for long periods under harsh winter weather conditions.
Seeds are small, often negligible and eminently suited to perform a number of functions that are not always obvious. These include multiplication, survival during seasons of stress (perennation), dormancy and dispersal.
Dormancy
Seed dormancy is a state in which seeds are prevented from germinating even under conditions that are favourable for germination. These conditions include temperature, water, light, gas, seed coats and hormone structures such as auxin and ABA.
Dormancy occurs in many species in response to environmental cues that are divergent from those that trigger germination. Such environmental cues may be seasonally characteristic or integrated by the seed over time and are triggered by a combination of different physiological mechanisms.
In response to these divergent environmental signals, plants have evolved multiple behavioural and genetic strategies for maintaining control of their progeny seeds. These include maternal-derived hard outer tissues such as the seed coat and pericarp (the fruit tissue surrounding the embryo) that act to suppress progeny dormancy, as well as mechanisms for gene imprinting and silencing.
These mechanisms elicit a gradual increase in the proportion of nondormant seeds over time, and after-ripening removes dormancy. The mechanism by which afterripening terminates dormancy is not entirely understood, but a signal specific to afterripened seeds activates ABA catabolism. This enables GA synthesis to begin and the events that elicit germination to subsequently take place.
