All organisms strive to survive on an individual and species level by regeneration and reproduction. Sometimes plants can make an exact copy of themselves, with the same genetic makeup. This is called asexual reproduction. Sometimes the genes of two plants mix and produce a new combination, a slightly different plant from either of the parents. This is sexual reproduction, offering the possibility of new characteristics that may be better adapted to new conditions.
Reproduction in plants, as in other organisms, ensures that there are always new individuals growing to replace those lost to accidents, disease or old age. Sexual reproduction maximizes the possibility of new and better adapted varieties but asexual reproduction creates an endless supply of already successful plants. The packaging of genetic material in small, enclosed containers such as spores and seeds allows it to be carried to new environments by wind, water, animals or other means.
The oldest green plants, mosses, liverworts and ferns, are most successful at colonizing environments are moist and lack extremes of heat and cold. Their reproductive structures are exposed and vulnerable. As plants evolved, there were advances in protection for the embryo, a tough seed coat, for example. A method for providing food for the germinating plant, the cotyledons in the seed, also boosted survival. Myriad forms of flowers to attract pollinators, various structures to help seeds in dispersal to new environments and other innovations all allowed plants to reproduce themselves in harsh conditions.
Spores And Seeds
Primitive green plants such as mosses and ferns have a two stage life cycle. The plant we see most often produces spores genetically identical to the parent. These grow into a small, insignificant sexual plant that produces eggs and sperm. The fertilized embryo grows into the larger plant.
Flowering plants have evolved many types of blossoms and fruits but the fertilized embryo is always contained within a protective covering and is provided with a store of food, lacking in spores, to provide energy to the germinating seed.
Runners And Bulbs
Plants often spread by runners, underground stems that grow horizontally. After they grow several inches or more from the original plant, the tip will develop roots and a new shoot, becoming an independent individual.
If it forms bulbs below the surface, the base of each bulb will develop smaller bulbs, or the main bulb may split into two or more. Many lilies also form small bulblets in where the leaves are attached to the stem.
Pieces of stem that break off the parent plant may form roots and then sprout leaves. Low branches of shrubs that lie on the ground may also root themselves and become a new plant. In some species, when the main tree or shrub is cut, the roots near the surface will send up more shoots, forming a thicket of new growth.