Gymnosperms and angiosperms are distinguished from other groups of plants by their ability to produce seeds. Seeds are neat little packages that allow for the survival and dispersal of subsequent sporophyte generations.

      Plants have a wide variety of means of seed dispersal. Some have structures that allow the seeds to be carried on the wind. Maple trees produce seeds in fruits called samaras that twirl like the blades of a helicopter, carrying the seeds farther away from the mother tree to a location with enough sunlight for a seedling to grow. Samaras of elm trees increase the surface area of their seeds so that they too can be carried on the wind. The same samara that carries the seed on the wind may also keep the seed afloat on a meandering stream.

      Dandelions provide their seeds with increased surface area by developing a plume called a pappus that carries the seeds like parachutes on the wind.

      Other plants depend on animals to disperse their seeds far and wide. Ask any dog owner who has ever gotten a burr stuck in its fur. Awns of grasses also get caught in the fur of animals that carry the seeds to new locations. Birds eat fruits containing seeds and deposit them in their droppings.

      Seeds can survive unfavorable conditions such as extreme temperatures and periods of drought. Many will go dormant and wait until favorable conditions occur before they begin to germinate. This is known as quiessence. Seeds are also "equipped" with a seed coat that protects the developing embryo within.

Developmental Origin of Seed Tissues: Gymnosperm vs. Angiosperm

      The details of fertilization and seed formation vary per gymnosperm group. The seeds of conifers, the most ecologically important group of gymnosperms, form inside ovulate or female cones. The microsporangiate or male cone produces pollen that is dispersed on the wind. When a pollen grain lands on the scales of an ovulate cone, it is drawn through the micropyle, an opening found on the ovule, by the contraction of pollination drops (liquid that exudes from the micropular canals at the open end of the ovule). The pollen grain then produces a pollen tube that carries the sperm to the egg in the ovule. One sperm nucleus unites with the egg nucleus, and the other degenerates.

      The resulting seed consists of 3 generation of tissue, two different diploid sporophytic generations and a haploid gametophytic generation. The seed coat and remnants of the nucellus are part of the old sporophyte. The haploid megagametophyte serves as a food source for the newly formed diploid embryo, the new sporophyte.

      Seeds of angiosperms, the flowering plants, develop from ovules that include layers of tissue called the nucellus and integuments. Ovules are found inside the ovaries. Flowers may be self-pollinating or they may require pollen from another plant. This varies from species to species. A pollen grain that has landed on the stigma of the pistil grows a pollen tube, which delivers two sperm through the micropyle.

      One of the sperm fertilizes the egg and the other unites with the polar nuclei to form the endosperm. The resulting seed consists of 2 generations of tissue. The diploid seed coat develops from the parental integuments, and the triploid endosperm results from the fusion of two haploid polar nuclei and the haploid sperm. The embryo resulting from the fusion of the haploid sperm and egg is the new diploid generation. The ovary of the flower makes up the fleshy fruit that encloses the seeds.

Food Storage

      There are three common patterns of food storage found in the angiosperm seeds. Perispermous storage involves the tissue surrounding the embryo called the nucellus. Water lilies and peperomia plants exhibit this type of storage in their seeds. Albuminous storage incorporates nutrients such as oils, starches, and proteins in copious amounts of endosperm. This can be found in the palms and grasses. In cases of ex-albuminous storage, the embryo has taken up all available food sources and therefore, the embryo itself stores the nutrients. Peas, beans, composites, and squash are some examples of plants incorporating ex-albuminous storage systems.

Germination

      Seeds germinate when environmental conditions are favorable. The dormant embryo resumes growth inside the seed and expands to a point where the seed coat splits apart, allowing the plant to emerge. Imbibition (water uptake) facilitates this process also. The radicle, or embryonic root, is generally the first organ to emerge from the seed and it promptly begins embedding itself into the substrate.

      Epigeal germination occurs when the hypocotyl extends and the cotyledon(s) emerge above the surface of the soil. There are protective advantages to having the cotyledons precede the first vegetative leaves. For example, unfavorable weather conditions may occur, which may cause damage to the tissue of the cotyledons, yet the apical meristem lies protected by the cotyledons. Herbivores may also damage the cotyledons and not destroy the entire plant.

      Hypogeal germination occurs when the cotyledon(s) remains under the surface of the soil, and/or inside the seed itself. Germination is hypogeal when the epicotyl elongates rather than the hypocotyl.

      These "seed leaves" are one of the criteria that distinguish the monocots, which have one cotyledon, from the eudicots, which have two. Frequently, as seen in Pinus, Ricinus, and Phaseolus, all which exhibit epigeal germination, the seed coat stays on the new shoot, offering protection to the developing plant. The cotyledons may be very different in appearance to the vegetative leaves of a plant or they may be very similar to the vegetative leaves. Some cotyledons act as food storage vessels that provide nourishment to the rapidly growing seedling. Once this food source is depleted and photosynthetic leaves have emerged, the cotyledons fall away from the plant.