The Life Cycle of Plants
Introduction and Summary
The life cycle of plants depends upon the alternation of generations, the fluctuation between the diploid (sporophyte) and haploid (gametophyte) life stages. In bryophytes, the gametophyte stage is dominant and comprises what we think of as the main plant. Conversely, in tracheophytes the sporophyte stage is dominant and the gametophyte is a relatively small extension of the main plant. In the typical plant life cycle, the diploid sporophyte produces (via meiosis) haploid spores, which give rise to gametophytes. These gametophytes generate male and/or female gametes, which join in sexual reproduction (fertilization) to create a zygote. The zygote becomes an embryo (encased in a seed in gymnosperms and angiosperms) that will eventually become a new sporophyte plant.
Fertilization occurs when a male gamete (sperm cell) joins with an egg cell to produce a zygote. In gymnosperms and angiosperms (the seed plants), the ovule containing the egg cell becomes a seed after fertilization takes place. This seed, complete with a food source and a hard outer seed coat, protects the embryo from drying out. In angiosperms (flowering plants), the embryo is given added protection by an ovary, which develops into a fruit.
Plants can also reproduce asexually through vegetative propagation, a process in which plants produce genetically identical offshoots (clones) of themselves, which then develop into independent plants. This asexual means of reproduction can occur via fragmentation or through specialized structures such as tubers, runners, and bulbs. Grafting is an artificial means of vegetative propagation in which parts of two young plants are joined together, first by artificial means and then by tissue regeneration.