Seed is the food supply that enables a new plant to grow. It contains all the nutrients that it will need until it is able to make its own food through photosynthesis.
Seeds are found in a wide variety of shapes and sizes, but they all have the same basic structure. They have a protective outer coating that encloses an embryo and food reserves.
Definition
Seed is the fertilized ovule of a plant with stored food materials and enclosed within a protective seed coat. It is a means of reproduction for vascular plants, including gymnosperms and angiosperms.
An embryo is a miniature undeveloped plant and the food reserves are in the endosperm of the seed. The seed coat is usually a thick, toughened covering that can retain moisture and protect the embryo during storage. It also acts as a barrier to dehydration. Seeds are the primary source of grains, fruits and legumes. Seeds from plants like soybeans, cotton, sunflower and coconut provide cooking oil.
Open pollinated seeds, such as heirloom varieties of vegetables, will grow true to type if another variety does not cross-pollinate them. Hybrid seeds, however, will produce a plant with the genetic characteristics of both parents. The term seed is also used figuratively, as in the idea of a person or thing being ranked and spread out across early tournament rounds.
Origin
Until seed plants evolved, most land plants reproduced by throwing out single-celled spores into the air to be dispersed and grow into new plants. The advantage of seeds over this method of reproduction is that the reserve food materials accumulated in the endosperm give the emerging plant an excellent start.
The structure of a seed is complex. It consists of an embryo, a storage tissue (endosperm) and a seed coat. The embryo consists of an immature root (the radicle) and an embryonic shoot (the plumule or epicotyl). The cotyledons—one in monocotyledons, two in dicotyledons, and two to several in gymnosperms—are attached to the epicotyl.
The evolutionary origin of the seed is difficult to determine. Some authors have suggested that seed ferns—Bennettitales and Gnetales—are angiosperm ancestors because of their flower-like structures, but fossils have not proved this hypothesis. Furthermore, if molecular analyses are correct, Gnetales should be more closely related to conifers than Bennettitales, and the Gnetales ovules would be better homologous with the cupule of angiosperms.
Function
The primary function of seed is to store a reserve supply of food that supports the embryo plant until it develops roots and can make its own food. The outer seed coat protects the embryo from damage and helps to retain moisture. The seed coat also contains a layer of food storage called endosperm.
A seed’s multicellular structure provides an advantage over single-celled spores for dispersal. The nutrient material stored in the seed enables the new plant to grow quickly and establish itself in a new environment.
The three parts of a seed include the radicle, cotyledons, and epicotyl. The cotyledons are the first leaves of the seedling. The epicotyl is the stem that grows above the cotyledons. Many seeds have a hard outer seed coat that is impervious to water. This condition, called physiological dormancy, is usually broken by a period of moist chilling or stratification. Other seeds, such as soybeans (a dicot), are able to break down their own dormancy with the help of enzymes that facilitate DNA repair.
Dispersal
Many plants depend on seed dispersal to survive. As seeds spread out over a region they can avoid overcrowding and competition for water, sunlight and space for growth.
Some seeds, such as those of dandelions, have feathery bristles that float away on the wind (anemochory). Others like maples have wings that flutter when the fruits open and fall from the tree (epiphyte).
Other seeds, primarily those of gymnosperms such as ginkgo and spruce, have hard coatings adapted to stick to fur, feathers or even animal dung. Still others have fleshy appendages that entice animals to eat them and then carry the seeds along, or have hooks and barbs that allow them to pass through the digestive tract without being digested.
Hydrochory is the seed dispersal method used by plants that live in or near the water, such as lilies, palm trees and many aquatic and freshwater plants. This type of dispersal sends seeds out into the waters where they can be carried by ocean currents over long distances, allowing them to land at other locations.
