Sexual Reproduction Without Seeds
Angiosperms, or flowering plants, became the most numerous—and most diverse—division of the plant kingdom by the sexual reproduction of seeds; they give birth using a fruit or flower that carries a fertilized seed, the genetic child of two parent plants. Before evolution of the angiosperms, though, early plants used a method called “alternation of generations” in which cell division created spores that then produced either male or female “gametes.” Gametes join together to form a complete juvenile plant. Algae, mosses, ferns and other ancient forms all reproduced themselves by the alternation of generations method without producing seeds. Unlike sexual reproduction practiced by angiosperms that creates new plants from cross-pollination, the simpler sexual reproduction processes of these less-developed plants results in less diversity.
Natural Asexual Reproduction
Reproduction that uses cellular duplication and does not require fertilization or any outside intervention is called “asexual natural vegetative” reproduction. Asexual reproduction perpetuates specific varieties (or cultivars) of species because plants are able to reproduce themselves exactly. Perennial angiosperms that grow from bulbs, corms and tubers—specialized roots that store food and genetic material—can reproduce by simple vegetative reproduction of these large roots. Plants that reproduce by cloning this way include lilies, narcissus, gladiola and many other familiar flowers. Other plants, like peonies, daylilies and hostas, propagate vegetatively by growing additional “crowns” within clumps of the parent plants. Angiosperms are highly developed plants because the can easily reproduce vegetatively in addition to the sexual reproduction process using seeds; this versatility insures both the survival of original cultivars and creation of new, better natural hybrids.
Artificial Asexual Reproduction
Many herbaceous and woody plants reproduce simply and without the bother of waiting for seeds to germinate or crowns to multiply by artificial methods. New plant matter is created by action from outside intervention—either human, animals or weather-related. The most common type of artificial vegetative reproduction is stem cutting; the taking of established (hard) or new (soft) growth on a plant, shrub or tree. The cutting is cleaned and planted in a moist growing medium where it develops roots and becomes a new clone of the parent. When branches are cut and wrapped in moist moss or other media, roots may form on living branches in a procedure called “air layering.” Stem cuttings are successful with annuals like coleus, perennials like geraniums, shrubs like lilacs and mock orange and fruit and flowering trees. Many tropical plants like aralias and rubber plants can be easily air layered. Cuttings and air layering of branches broken by animals or storms occur occasionally; the practice of artificial vegetative propagation by hobbyists, horticulturists and botanists preserves many heirloom or old cultivars that might have died out without human intercession.