This article deals strictly with seed formation in the class of plants called angiosperms, or “enclosed seeds.” It also deals with the further division of the dicots, or plants with two seed leaves.
The seed is the structure that develops from the fertilized ovule of the flower. The seed comprises all of the genetic information required to produce a new plant like the plant from which it originated. It is composed of three distinct structures, the embryo, the endosperm and the seed coat. The formation of these structures occurs during the process called fertilization. Fertilization occurs, as it does with all flowering plants, after a grain of pollen, produced by the anther of a flower, lands on the stigma of a flower of the same species. When this occurs the pollen grains grow a tube that extends down through the style into the ovary. The sperm cells from the pollen travel down the tube and then fuse with the nuclei of an egg cell that is within the ovary.
In angiosperms double fertilization occurs during this process. One fertilization method involves the fusion of sperm cell nuclei with an egg cell. This part of the fertilization forms the zygote which develops into a pro zygote and then into the embryo. A secondary fertilization involves second sperm cell nuclei and the polar nuclei. This fertilization forms the endosperm.
This double fertilization has formed two of the three parts of the seed, the embryo and the endosperm. The seed coat develops over the endosperm and the embryo, protecting them from the elements and holding the parts together.
A zygote is the cell, which forms after sexual fertilization, occurs. This zygote contained within the seed is the undeveloped plant and within it is all the genetic information needed for the plant to grow. This genetic information, or DNA, comes from both parent plants that contributed to the initial fertilization. If self-fertilization has occurred, which happens in many types of plants, the genetic material comes from a single plant. After formation of the seed the zygote develops into a pro zygote that then develops into the embryo. The embryo becomes inactive, waiting for conditions to become satisfactory for germination. Depending upon the plant species and storage conditions a zygote can remain in this inactive state for a period of hours to many, many years after the seed forms. Some plant seeds will germinate immediately after they fall from the plant. Other plant seeds will need a complicated series of developments to trigger germination. The zygote consists of two portions, one of which will form the stem and leaves. The other portion will form the root of the plant. In dicot plants there are two seed leaves present which will emerge upon germination and produce food for the plant until the true leaves develop.
The endosperm is quick to develop after fertilization. Once the endosperm develops it too will remain inactive until after germination occurs. The purpose of the endosperm in the seed structure is to serve as a food source for the embryo to use once germination begins. The embryo will draw upon this food source until the roots and the leaves develop well enough to draw nutrients from the soil and gather light from the sun and manufacture the food the plant needs to survive.
The seed coat’s purpose in the structure of the seed is to protect the embryo and endosperm. Seed coats develop from the ovule of the flower and it will remain in place until germination occurs. The seed coat sheds off at this point. Once the seed forms it enters a state of dormancy. Dormancy can last for hours, days, weeks or years, depending upon the species of the plants and its growth requirements.
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