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#REDIRECT ] |
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'''Adventitious''', in ], refers to structures that develop in an unusual place, and in ], it refers to conditions acquired after birth. This article discusses adventitious ]s, ]s and ]s, that are very common in ]s. |
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==Adventitious roots== |
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{{R from related word}} |
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Adventitious roots develop on ], ] and even old roots. The ] or primary root and its ] are the only nonadventitious roots. Many aerial stems naturally form aerial roots. There are two types of adventitious roots. Aerial stems often have '''preformed root initials''' at nodes that will develop into roots given the proper conditions. Other adventitious roots are termed '''wound roots''', because they do not form until the stem, root or leaf is wounded. |
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{{R to section}} |
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{{R with history}} |
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In some species, adventitious roots form on aerial stems, grow down into the soil and help keep the stems upright, e.g. prop roots of ], ] and ]. Some ] stems cling to their supports using adventitious roots, e.g. ] and ]. Adventitious roots may also form naturally when an aerial stem contacts the ], e.g. arching branches of ], ] and weeping ]. Some trees, such as willow, naturally shed pieces of twigs, a phenomenon known as ]. The shed twigs are often transported via rivers and streams and can form new plants if they develop adventitious roots. |
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Virtually the entire root system is adventitious in many ]s, especially ]s, ] and most ]s, e.g. ]es. In most monocot species, the radicle never develops to any extent, and adventitious roots form at the base of the stem. Such adventitious root systems are also termed fibrous root systems. Root systems of specialized underground stems (]s, ]s, ]s, ]s) are all adventitious roots. The tap root systems of most ]s and ]s develop from the primary root but even they often develop some adventitious roots when the old roots or lower stem is wounded. Horizontal, aboveground stems, termed ]s or runners, usually develop adventitious roots at their nodes, e.g. ]. |
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==Adventitious buds and shoots== |
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Adventitious ]s develop from places other than a shoot ], which occurs at the tip of a stem. They may develop on stems, roots or leaves. Shoot apical meristems produce one or more axillary or lateral buds at each node. When stems produce considerable ], the axillary buds may be destroyed. Adventitious buds may then develop on stems with secondary growth. |
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Adventitious buds are often formed after the stem is wounded or ]. The adventitious buds help to replace lost branches. Adventitious buds and shoots also may develop on mature tree trunks when a shaded trunk is exposed to bright sunlight because surrounding trees are cut down. ] (''Sequoia sempervirens'') trees often develop many adventitious buds on their lower trunks. If the main trunk dies, a new one often sprouts from one of the adventitious buds. Small pieces of redwood trunk are sold as souvenirs termed redwood burls. They are placed in a pan of water, and the adventitious buds sprout to form shoots. |
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Some plants normally develop adventitious buds on their roots, which can extend quite a distance from the plant. Shoots that develop from adventitious buds on roots are termed ]. They are a type of natural ] in many ], e.g. many grasses, ] and ]. The ] grew from one trunk to 47,000 trunks via adventitious bud formation on a single root system. |
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Some leaves develop adventitious buds, which then form adventitious roots, e.g. piggyback plant (''Tolmiea menziesii'') and mother-of-thousands (''Kalanchoe daigremontiana''). The adventitious plantlets then drop off the parent plant and develop as separate ]s of the parent. |
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] is the practice of cutting ] stems to the ground to promote rapid growth of adventitious shoots. It is traditionally used to produce poles, fence material or ]. It is also practiced for ] crops grown for fuel, such as ] or willow. |
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==Location of origin== |
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Adventitious roots and buds usually develop near the existing vascular tissues so they can connect to the ] and ]. However, the exact location varies greatly. In young stems, adventitious roots often form from ] between the ]s. In stems with secondary growth, adventitious roots often originate in phloem parenchyma near the ]. In stem cuttings, adventitious roots sometimes also originate in the ] cells that form at the cut surface. Leaf cuttings of the '']'' form adventitious roots in the epidermis.<ref>McVeigh, I. 1938. Regeneration in ''Crassula multicava''. ''American Journal of Botany'' 25: 7-11. </ref> |
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==Vegetative propagation== |
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Adventitious roots and buds are very important when people propagate plants via cuttings, ], ]. ], termed ]s, are often applied to stem, ] or ] cuttings to promote adventitious root formation, e.g. ] and ] leaves and shoots of ] and ]. Propagation via root cuttings requires adventitious bud formation, e.g. in ] and ]. In layering, adventitious roots are formed on aerial stems before the stem section is removed to make a new plant. Large houseplants are often propagated by air layering. Adventitious roots and buds must develop in tissue culture propagation of plants. |
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==References== |
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<references/> |
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* Esau, K. 1977. ''Anatomy of Seed Plants''. New York: Wiley. |
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* Hartmann, H.T. and Kester, D.E. 1983. ''Plant Propagation: Principles and Practices''. Englewood Cliffs, NJ: Prentice-Hall. |
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