Adventitious

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 roots, buds and shoots, that are very common in vascular plants.

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Adventitious roots

Root developing from any part other than the radicle is called adventitious root. Adventitious roots develop on stems, leaves and even old roots. The radicle or primary root and its lateral roots 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.

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 maize, screwpine and banyan. Some vine stems cling to their supports using adventitious roots, e.g. English ivy and philodendron. Adventitious roots may also form naturally when an aerial stem contacts the soil, e.g. arching branches of forsythia, blackberry and weeping willow. Some trees, such as willow, naturally shed pieces of twigs, a phenomenon known as cladoptosis. The shed twigs are often transported via rivers and streams and can form new plants if they develop adventitious roots.

Virtually the entire root system is adventitious in many vascular plants, especially ferns, fern allies and most monocots, e.g. grasses. 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 (rhizomes, bulbs, tubers, corms) are all adventitious roots. The tap root systems of most dicots and gymnosperms 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 stolons or runners, usually develop adventitious roots at their nodes, e.g. strawberry.

Adventitious buds and shoots

Adventitious buds 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 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. 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 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 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 clones 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 bundles. 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.^[1]

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, tissue culture. Plant hormones, termed auxins, 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

1. ^ McVeigh, I. 1938. Regeneration in Crassula multicava. American Journal of Botany 25: 7-11. [1]

• 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.