The reproductive cycle of seed bearing plants has several stages. Plants flower, fruit, and produce new seeds. Inside each new seed there is a tiny plant embryo which looks like a tiny stem and leaves tucked into a kind of fetal position. The seed also contains enough nutrition for the new plant to use until it sprouts and begins to manufacture its own food by photosynthesis.
The word, embryo, comes from a Greek word which can be translated: something that grows in a body. In botany, the plant embryo looks like a very small plant that is contained inside a seed or, in the case of mosses and ferns, inside an archegonium.
The female reproductive organ of seed bearing plants, pistil, is found in the center of the flower. Ovules are located within the pistil. Ovules are covered by outer layers called integuments which hold a soft nucellus. This is the cavity in which the plant embryo sac fits.
The plant is not mature enough for fertilizaiton until a single large nucleus within the embryo sac divides to form an egg and two synergids. When the pollen tubes in the flower allow sperm cells to enter the embryo sac, the sperm cell unites with the egg. A double fertilization process happens when other sperm cells penetrate deeper into the nucleus.
The plant's fertilized egg is also called a zygote or embryo which remains in and grows to fill the embryo sac. The cells that developed as a result of double fertilization become the endosperm. As the embryo grows, the endosperm surrounds it. The endosperm provides the stored nutrients to feed the plant between the time that it is planted until it sprouts.
Seeds are fully encased plant embryos surrounded by endosperms. You can tell that a seed is mature when the fruit of the plant is ripe. Each plant produces seeds that are unique in size, shape, color and texture. However, hybridized plants that grow from genetic engineering do not produce seeds that will grow. To find out specific planting instructions for particular seeds, consult the information in a seed catalog like Burpee's.