That perfect package of plant life we call an angiosperm seed is an amazing and complex structure. Not only are seeds critical in the continuation of flowering plant species, they are also of great value to humans.
The production of edible seeds is unquestionably the most important agricultural activity humans engage in. Seeds such as wheat, corn, rice, and rye are basic items in our daily diet and they hold equal importance as food for many other animals.
The External Structure of a Seed
A seed is composed of a plant embryo (miniature new plant) and stored food wrapped in a protective seed coat. If we start on the outside and work our way down into a typical dicot seed, such as a bean, we find the seed is wrapped externally in a thin, waxy seed coat.
The seed coat functions mainly to prevent the seed from dehydrating. The seed coat is usually thin, as in the bean, and in the peanut, in which it forms the brown, papery layer around the seed, but it may be thick and hard or stony as in the Brazil nut. The seed coat of the cotton plant produces the cotton fibers, which are spun into thread and woven into cloth.
The micropyle, the site of pollen tube penetration into the ovule prior to fertilization, is still evident as a tiny pore or dark patch and is located just above the hilum. The hilum is a scar that marks the position of the stalk that that attached the seed to the ovary wall.
The Internal Structure of a Seed
Internally is the embryo consisting of the radicle, or embryonic root; the hypocotyl, or embryonic lower stem; the epicotyl or embryonic upper stem. The epicotyl, along with the embryonic leaves, makes up the plumule.
In a dicot seed, such as a bean, two cotyledons fill most of the interior of the seed. The cotyledons, sometimes known as “seed leaves,” are technically part of the embryo. They function as food storage and supply the developing embryo with nourishment until its own leaves become functional
A monocot seed, such as corn, possesses only a tiny cotyledon that does not store nutrients. Technically, a corn grain is a fruit, but the seed occupies almost the entire grain. The wall of the fruit is very thin and fused to the seed coat. A single umbrella-shaped cotyledon presses close to the endosperm.
The cotyledons of a monocot seed do not store nutrients as dicot cotyledons do. Rather, they absorb nutrients from a structure called the endosperm and transfer them to the embryo.
Seed Germination
The story of the seed is completed only when the seed has germinated and the seedling has become established. Germination is resumption of growth of the embryo, and is completed with the appearance of the radicle (embryonic root) outside the seed coat.
The developing embryo then enters the establishment phase, a period beginning with the end of germination and ending with a seedling independent of the accumulated food in the seed. Germination and establishment are crucial in the life of the plant, for it is in these stages of the life cycle of any species that the greatest numbers of individuals are lost.
The depth to which a seed is buried in the soil, either by accident or in planting, is an important factor in germination. Seeds on the surface may not have sufficient water to complete their germination. Those buried too deeply may deplete their reserves of food before they break through the soil and reach the light.
Certain environmental conditions are necessary for seed germination:
- moisture,
- oxygen, and
- favorable temperatures.
Moisture. The seeds of most plants, when ripe, have a low water content, and germination cannot occur until the seed coat, and later, other tissues have imbibed (taken in) water.
Oxygen. The seeds of various species differ in their oxygen requirements, but oxygen is essential to germination.
Temperature. Seeds also differ greatly in their temperature requirements for germination, and many will germinate over a considerable range of temperatures.
In addition, the germination of seeds of many species is favored by light, while in others germination is retarded by light.
While seeds come in a wide variety of sizes, shapes, and colors, inside every seed is a tiny embryonic plant awaiting its turn to become a member of the plant world.
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