Seeds are amazing structures, capable of protecting an embryonic plant for hundreds of years in a state of suspended animation, awaiting the onset of favorable conditions to initiate germination. The seed represents a pause in the lifecycle of plants that is so effective, it conferred a major selective advantage on those species having it, the earliest of which arose in the late Devonian (417-354 Mya) and came to dominate by the early Mesozoic.
Seeds possess a number of improvements over spores that are adaptive, including a seed coat, multicellular embryonic seedling, and a significant reserve of energy. The thickness and material making up the seed coat varies from species to species and plays a role in the particular kind of dormancy experienced by a seed. The nature of the coat also influences the kinds of signals that are able to reach the embryo and release dormancy.
By LadyofHats [Public domain], via Wikimedia Commons
Plants producing thick, lignified seed coats typically undergo coat-imposed dormancy, characterized by the physical exclusion of light and water, extremely limited gas exchange between the embryo and the environment, and the maintenance of high concentrations of the dormancy-promoting hormone abscisic acid. Some seeds having this kind of dormancy can endure for hundreds of years and remain viable . These seeds are “hermetically sealed” packages, and must be broken open in order for germination to proceed. This breakdown may occur through the activity of animals (including exposure to digestive acids), exposure to freeze-thaw cycles, or other weathering processes.
Plants not having a thickened coat remain capable of dormancy, but rather than the physical restriction of exchange, dormancy arises from production of and response to particular chemical messages. In actual fact, both kinds of dormancy use chemical signaling to promote or release dormancy, but seeds with thickened coats have an additional mechanism for protecting the dormant state of the embryo.
In addition to a seed coat, all seeds contain a supply of energy contributed by the parent. Depending on the kind of plant, this energy supply may take one of two different forms. In some plants the energy deposited by the parent remains in a large, starchy mass known as endosperm. Grasses (Family Poaceae) such as wheat (Triticum aestivum), maize (Zea mays), and rice (Oryza sativa) all maintain a prominent endosperm throughout seed maturation and dormancy. It is the endosperm, in fact, that we are eating when we eat grains like rice and wheat. In other plants, the endosperm may be absorbed fully by the developing embryo and stored in specialized seed leaves called cotyledons. Plants such as beans (Family Fabaceae) are representative of this strategy.
The embryo is the result of fertilization, or syngamy (fusion of gametes). Upon the fusion of the sperm nucleus with the egg nucleus inside the ovule, a single-celled zygote forms and begins developing into the embryo. As early as the first division of the zygote, polarity has already been established that will govern the apical-basal axis of the embryo. The embryo is fully dependent upon the parent plant for nutrition during seed maturation and early seedling establishment, until the seedling becomes photosynthetic and therefore autotrophic (self-feeding). Before this, the embryonic plant is in fact heterotrophic, benefiting from the investment of photosynthate from its parent.