Seeds are like tiny just-add-water kits for new life. They contain an embryo and starchy food, all kept safe inside a protective shell. Some seeds need special light or temperature conditions or a period of dormancy to spring into action.
Cecilia Zumajo and her colleagues are studying how seeds take that first step from dormancy to seedling. Their research is helping us understand gene networks that control seed development and evolution.
What is a seed?
A seed is a miniature plant in a state of arrested growth (called dormancy) with stored food for its future development. Its food is contained within a nutrient reserve called endosperm that can be made of starch, carbohydrates or proteins. It is protected by a seed coat and its outer shell.
A seed can also have a tiny hole, called the micropyle, through which water and nutrients enter the embryo. This allows for the process of imbibition, which reactivates enzymes in the seed that converts storage compounds into usable nutrients. This process is important to the plant’s germination. After germination, the seed can grow into a mature, reproducing plant that will produce more seeds and continue the cycle of life. Seeds vary in size, shape and color. They are often able to remain dormant, or inactive, for years until conditions are right for them to grow. This is one of the reasons gardeners can plant heirloom varieties, which tend to “grow true” or produce offspring that resemble the parent plants.
What are the parts of a seed?
A seed contains an embryo and food reserve enclosed in a protective outer covering. Under favorable conditions seeds give rise to a new plant. There are three main parts of a seed; the seed coat, the endosperm and the embryo.
The seed coat is typically hard, thick and brown in color. It consists of two layers; the outer layer is known as the testa and the inner layer as tegmen. The hard seed coats prevent germination of the embryo in unfavorable environmental conditions.
Inside the seed coat lies a soft, water-absorbent tissue called the endosperm. It stores carbohydrates, proteins and other nutrients for the embryo. The cotyledons, which can resemble tiny leaves or be fleshy, provide nourishment to different parts of the embryo during germination. There is a single cotyledon in monocots and two in dicots. The embryo contains the genetic instructions for the new plant. It is surrounded by the cotyledons and the endosperm.
How do seeds grow?
Seeds need a lot to grow into a plant: Water, the right temperature, and sunlight. They also need a food store–which most seeds have inside them, called endosperm. The food stores all the carbohydrates and protein the embryo needs to start growing.
Once a seed has the right conditions it can grow roots and become a tiny seedling. The process is called germination.
The seed coat lets water into the embryo, which then swells up and splits open. The embryo grows into a tiny plant, with one or more leaf-like parts, called cotyledons.
The cotyledons give the seedling nourishment until it can make its own food, using sunlight and water, through photosynthesis. The seed also has a food reserve, stored in the embryo, that it can use until its leaves are large enough to get food from outside the seed.
How do seeds disperse?
Seeds can’t move themselves, so they rely on external entities to hitchhike them around the landscape—wind, water and animals. Seeds dispersed further away from the parent plant are more likely to survive, as they have less competition with siblings and parents, better access to light conditions and avoidance of predators and pathogens.
Wind dispersal (anemochory) is the most common method of seed transport. For example, the feathery pappus that opens and catches the wind on a dandelion can transport its seeds far from its parent plant. Similarly, maples use winged fruits (samaras) that flutter to the ground on winds.
Some plants also make their seeds a bit more attractive to potential dispersers by enclosing them in fleshy fruit that is appealing to animals, such as squirrels. The seeds may have hooks or burs that catch on fur or feathers, or they might have structures designed to break open under the force of a bird’s beak.