Plants may reproduce sexually or asexually. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from either parent. Vegetative reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unless mutations occur. In asexual reproduction, only one parent is involved.
Asexual reproduction
Asexual reproduction does not involve the production and fusion of male and female gametes. Asexual reproduction may occur through budding, fragmentation, spore formation, regeneration and vegetative propagation. Asexual reproduction is a type of reproduction where the offspring comes from one parent only, thus inheriting the characteristics of the parent. Asexual reproduction in plants occurs in two fundamental forms, vegetative reproduction and agamospermy. Vegetative reproduction involves a vegetative piece of the original plant producing new individuals by budding, tillering, etc. and is distinguished from apomixis, which is a replacement of sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species such as dandelions (Taraxacum species) and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms, see parthenogenesis. Natural vegetative reproduction is a process mostly found in perennial plants, and typically involves structural modifications of the stem or roots and in a few species leaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size. A plant that persists in a location through vegetative reproduction of individuals gives rise to a clonal colony. A single ramet, or apparent individual, of a clonal colony is genetically identical to all others in the same colony. The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called vegetative growth. However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. A major disadvantage of vegetative reproduction is the transmission of pathogens from parent to offspring. It is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs. Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. Hawkweeds (Hieracium), dandelions (Taraxacum), some species of Citrus and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring. Other forms of apomixis occur in plants also, including the generation of a plantlet in replacement of a seed or the generation of bulbils instead of flowers, where new cloned individuals are produced.
Structures A rhizome is a modified underground stem serving as an organ of vegetative reproduction; the growing tips of the rhizome can separate as new plants, e.g., polypody, iris, couch grass and nettles. Prostrate aerial stems, called runners or stolons, are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns. Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves. A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include elm (Ulmus) and many members of the rose family such as Rosa, Kerria and Rubus. Bulbous plants such as onion (Allium cepa), hyacinths, narcissi and tulips reproduce vegetatively by dividing their underground bulbs into more bulbs. Other plants like potatoes (Solanum tuberosum) and dahlias reproduce vegetatively from underground tubers. Gladioli and crocuses reproduce vegetatively in a similar way with corms. Gemmae are single cells or masses of cells that detach from plants to form new clonal individuals. These are common in Liverworts and mosses and in the gametophyte generation of some filmy fern. They are also present in some Club mosses such as Huperzia lucidula . They are also found in some higher plants such as species of Drosera.
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