Root

Cross section of the root system of a mango tree (Mangifera indica)

Roots are organs used by the vast majority of plants (the vascular plants) for several purposes, the two main being the absorption of water and inorganic nutrients from the soil and the anchoring of the plant. Roots are usually kept underground, but may in some circumstances for some species be found above the ground; they are then called aerial roots or aerating roots (see section below).

Roots are the first thing to emerge from a seed after germination. It allows the emerging plantling to anchor itself and begin extracting water from the soil to give it all it needs to start a life of its own as a real plant.

Use

For most vascular plants, roots are the main tool used to extract the water they need from the soil. Likewise, they will extract all the inorganic nutrients they require for growth. Roots however have several additional roles. The first, directly linked to the two previously mentioned, is the production of cytokinin. Cytokinin is a hormone produced by the roots in concentrations proportional to that of nutrients in the soil; its role is to tell the vegetative part of the plant (i.e. the part above ground) how fast it can afford to grow.

The second role is a role of storage. The nutritive substances synthesized via photosynthesis by the aerial part of plant migrate towards the roots via canals called phloem. The roots can consume them for their own needs, but more often than not, they are digested and the products of the digestion are sent back to the aerial part of the plant to be consumed, again via the phloem.

A third role is the anchoring of the plant in the ground. Indeed, as sessile organisms (i.e. organisms that cannot move from one place to another), plants need a way to fix themselves in place, in order to avoid being blown away by the wind in places where growth is impossible, or at least less easy. This is especially true for plants already growing in uneasy niches, such as mangrove trees.

Development of roots

The first root to develop when a seed germinates is called the primary root. This root will, in a very large number of plants, keep growing straight downwards. Various new roots eventually branch out of it; these are the secondary roots (or lateral roots). All of these will keep branching themselves out as the plant grows, giving birth to many radicles and forming a taproot system. In this system, the primary root is much more developed than the others and goes deeper in the ground, allowing it to find and use low water tables.

Some plants however, chiefly the monocots, will present a different system. The primary root is usually ephemeral, as the root system of the plant tends to develop from adventitious roots that develop from the stem of the plant.

Switchgrass showing a nice exemple of a diffuse roots system

This creates a dense roots system called diffuse roots, where no root is more developed than the others. This system digs less deep in the soil than the taproot as the plant grows, and is particularly common among plants that creep on the ground. In nature, this root system plays a major role in the protection of the soil against erosion, which would otherwise wash nutrients away by water or wind. By maintaining them in place, the plants secure themselves an access to nutrients that will help them grow; past this selfish consequence, it helps keep the soil fertile for every other plant as well and help avoid the process of desertification.

Types of roots

Besides the two main types described above, a few other types are commonly found not only in the nature, but also on our own houseplants. These sometimes originate from another part of the plant than primary or secondary roots; these are generally called adventitious roots.

Aerial roots

Aerial roots are adventitious roots that develop from the stem, above ground level. These roots essentially have support role, but they can participate in the absorption of water and nutriments when they reach the ground such as those of corn (Zea mays).

Aerating roots

Aerating roots of Taxodium ascendens 'Nutans'

Aerating roots or pneumatophores are roots rising above the ground surface, and especially above the water surface. Commonly found among species growing in partly or completely submerged areas, they are rich in pores that allow the plant to exchange gases.

References

• Raven, P.H., Evert, R.F. and Eichhorn, S.E., 2007. Biologie végétale. de Boeck & Larcier s.a., Bruxelles. ISBN 978-2-8041-5020-4