Adventitiousness
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'''Adventitious''', in [[botany]], refers to structures that develop in an unusual place, and in [[medicine]], it refers to conditions acquired after birth. This article discusses adventitious [[root]]s, [[bud]]s and [[shoot]]s, which are very common in [[vascular plant]]s.
==Adventitious buds and shoots==
Adventitious [[bud]]s develop from places other than a shoot [[apical meristem]], 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 [[secondary growth]], the axillary buds may be destroyed. Adventitious buds may then develop on stems with secondary growth.
Adventitious buds are often formed after the stem is wounded or [[pruning|pruned]]. 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|Redwood]] (''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.
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 [[Basal shoot|suckers]]. They are a type of natural [[vegetative reproduction]] in many [[species]], e.g. many grasses, [[quaking aspen]] and [[Canada thistle]]. The [[Pando (Quaking Aspen)|Pando quaking aspen]] grew from one trunk to 47,000 trunks via adventitious bud formation on a single root system.
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 [[Cloning|clone]]s of the parent.
[[Coppicing]] is the practice of cutting [[tree]] stems to the ground to promote rapid growth of adventitious shoots. It is traditionally used to produce poles, fence material or [[firewood]]. It is also practiced for [[biomass]] crops grown for fuel, such as [[poplar]] or willow.
==Location of origin==
Adventitious roots and buds usually develop near the existing vascular tissues so they can connect to the [[xylem]] and [[phloem]]. However, the exact location varies greatly. In young stems, adventitious roots often form from [[parenchyma]] between the [[vascular bundle]]s. In stems with secondary growth, adventitious roots often originate in phloem parenchyma near the [[vascular cambium]]. In stem cuttings, adventitious roots sometimes also originate in the [[callus]] cells that form at the cut surface. Leaf cuttings of the ''[[Crassula]]'' form adventitious roots in the epidermis.<ref>McVeigh, I. 1938. Regeneration in ''Crassula multicava''. ''American Journal of Botany'' 25: 7-11. [http://links.jstor.org/sici?sici=0002-9122(193801)25:1%3C7:RICM%3E2.0.CO;2-A]</ref>
==Modification of Adventitious root==
'''Tuberous roots'''-
These are without any definite shape. eg [[Sweet Potato]].
'''Fasciculated root'''-In this type the tuberous root occur in clusters at the base of the stem eg [[asparagus]],[[dahlia]].
'''Nodulose roots'''-This type of roots become swollen near the tips. eg [[turmeric]].
'''Stilt roots'''-These roots arise from the first few nodes of the stem. These penetrate obliquely down in to the soil and give support to the plant. eg [[maize]], [[sugarcane]].
'''Prop roots'''-these roots give mechanical support to the aerial branches. The lateral branches grow vertically downward into the soil and acts as pillars. eg [[banyan]].
==Vegetative propagation==
Adventitious roots and buds are very important when people propagate plants via cuttings, [[layering]], [[plant tissue culture|tissue culture]]. [[Plant hormones]], termed [[auxin]]s, are often applied to stem, [[shoot]] or [[leaf]] cuttings to promote adventitious root formation, e.g. [[African violet]] and [[sedum]] leaves and shoots of [[poinsettia]] and [[coleus]]. Propagation via root cuttings requires adventitious bud formation, e.g. in [[horseradish]] and [[apple]]. 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.
==References==
<references/>
* Esau, K. 1977. ''Anatomy of Seed Plants''. New York: Wiley.
* Hartmann, H.T. and Kester, D.E. 1983. ''Plant Propagation: Principles and Practices''. Englewood Cliffs, NJ: Prentice-Hall.
[[Category:Botany]]
[[Category:Horticulture]]
[[Category:Plant physiology]]
[[Category:Plants]]
[[Category:Plant reproduction]]
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