The adventitious roots of a plant are a type of root system that has numerous plant species and serve for the absorption of water and nutrients from the soil. The origin or formation of the adventitious roots is from the stem, and not as in the primary and secondary root system whose origin is from the radicle or the main root respectively.
In analogy to primary roots, adventitious roots function as structures that allow plant species to explore and colonize other territories. However, each plant can only develop one type of root system, either a primary root system or an adventitious root system..
Adventitious roots originate in principle from the embryonic root or radicle, then all roots are formed from the stem tissue. These adventitious roots develop in species that have a short-lived embryonic root (radicle). Adventitious roots make up the fibrous root systems and do not originate from other roots.
An adventitious root system has many roots of the same size, which have developed from the basal end of the stem. Likewise, adventitious roots can generate smaller lateral roots..
Like the persistent primary root system, adventitious roots are adapted to obtain water from the different areas of the soil with which they come into contact..
In this sense, the organs that develop adventitiously occur in unusual places in the plant. For example, the roots that form from the stem, or the shoots that develop from the roots. Consequently, the different types of adventitious roots have a specific function in plants..
The adventitious roots have several functions: 1) of anchorage type fulcreas or waders; 2) contraction like those found in plants that form corms; 3) photosynthetic like those of some orchids; 4) pneumatophores to capture oxygen such as those found in mangrove plant species; 6) other haustorium types that parasitize the host.
Some species that have adventitious roots are the onion, the Ficus sp., mangroves, grasses and other monocots.
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Adventitious roots are formed in plants whose embryonic root has a very short life. The adventitious root system originates from stem tissues, not from the radicle.
These types of roots do not reach a great depth in the soil, but are adapted to develop close to the surface of the soil and thus obtain rainwater when it seeps over a large area..
In addition to originating from the base of the stem, adventitious roots can also be generated from leaves, branches or other parts of the trunk.
Adventitious roots have specific functions such as absorption, anchoring or support, photosynthesis, storage and gas exchange.
Some adventitious roots specialize in parasitizing other plants and absorbing water and nutrients from their xylem..
Anchoring a plant means that a root modification is required; this occurs in the case of climbing plants, whose roots allow them to anchor themselves on a support, as epiphytes do (for example orchids).
In the case of the plants that live in the mangroves, these are anchored in the flooded banks on the tide along the tropical coasts. These roots are called waders or fulcreas.
Another similar function of the adventitious roots, but in very different climatic conditions, is to support the grassy plants that grow in altitude conditions. These roots are formed by excessive secondary thickening, growing immediately and horizontally below the soil surface..
On the other hand, a very interesting function is performed by the contractile adventitious roots, which pull the underground shoots such as rhizomes, tubers, or bulbs, towards the ground. These roots contract by longitudinally oriented axial extension in cortical cells that respond to an increase in turgor.
They are necessary especially for corms and bulbs, since the growth of each successive year is on the growth of the previous year; thus, corms and bulbs tend to rise in the soil over time, so without the help of contractile roots they would be exposed on the soil surface.
Otherwise, there are adventitious roots with the storage function of some plants that deposit sucrose and starch in root structures.
Another function of the adventitious roots is that of the aerial ones, to protect the base of the stem as observed in certain palms. The adventitious aerial roots also have the function of providing mechanical support to the stems..
For its part, the function of aerial roots is justified by the absorption of water by plants that are unable to absorb water from the ground..
The aerial roots have an outer layer called the velamen, which arises from the protoderm from periclinal divisions. This tissue contains numerous large, short-lived cells with rigid walls and openings in the walls..
The canopy acts as a kind of sponge to absorb rainwater. In some epiphytes, the aerial roots also have a photosynthetic function..
In some cases, aerial roots specialized in absorbing water or substances can also become parasitic roots. This is the case of haustoria, which penetrate plant cells on which they grow and absorb water and substances from their tissues..
Some plants live in environments with waterlogging conditions, such as swamps or subjected to the action of tides. In these cases, the adventitious roots emerge from the ground and rise to the surface so that the plant can obtain the necessary oxygen for its functioning and aerobic respiration. These roots are called pneumatophores.
Anchor or support: like the fulcreas or waders roots of mangroves, Ficus sp., and some monocots such as corn and sorghum.
Pneumatophores: like the roots of the plant Avicennia germinans, bald cypress and black mangrove.
Photosynthetics: like the roots of the orchid Phalaenopsis sp.
Haustoria: like the roots that are present in the evergreen mistletoe and the plant commonly called little bird.
Contractile: as the root system present in plants that develop corms and bulbs.
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