Learn about the structure, functions, and importance of roots in botany, the hidden wonders beneath the surface. Discover the crucial role roots play in plant survival.Root . It is the organ, generally underground, of plants with an essential specialization for their development.

Function

The root is specialized in:

• Fixing the plant to the substrate.
• Absorption of water and dissolved substances.
• Transport of water and solutes to the aerial parts
• Storage: Biennial plants such as carrots store reserves in the roots during the first year that they will use in the second year to produce flowers, fruits and seeds.
• In some plants such as Isoetes (pteridophyte) and Littorella (Plantaginaceae) the roots transport carbon dioxide (CO2) for photosynthesis , since their leaves usually lack stomata .

Location

The root is present in all vascular plants except Psilotales (pteridophytes) which have rhizoids. Certain specialized spermatophytes lack roots because the radical pole atrophies, the embryo does not have a radicle; among them there are aquatic plants such as Wolffia (duckweed), Utricularia and Ceratophyllum demersum and epiphytic plants such as Tillandsia usneoides and some orchids.

Some of them can form adventitious roots. In Salvinia, an aquatic pteridophyte, the radical function is performed by modified leaves. The root is present in all vascular plants except Psilotales (pteridophytes) that have rhizoids. Certain specialized spermatophytes lack roots because the radical pole is atrophied, the embryo does not have a radicle; among them there are aquatic plants such as Wolffia (duckweed), Utricularia and Ceratophyllum demersum and epiphytic plants such as Tillandsia usneoides and some orchids. Some of them can form adventitious roots. ^[1] In Salvinia, an aquatic pteridophyte, the radical function is performed by modified leaves.

Origin

In spermatophytes the radicle or embryonic root located at the radical pole of the embryo gives rise to the primary root after germination. In pteridophytes the embryo is not bipolar, generally the embryonic root is lateral with respect to the stem (Fig. 1.35). In Psilotum, whose embryo does not have a radicle, only rhizoids will be formed. ^[2]

Parts

At the tip of each growing root there is a conical covering called a calyptra. It is usually not visible to the naked eye and consists of soft, undifferentiated tissue. The calyptra covers and protects the meristematic or growth tissue, through the proliferation of which by mitosis the cells that, after their differentiation, form the adult structure of the root are originated.

It protects the meristem by avoiding contact with solid soil particles and preventing injuries. Although new cells are continuously formed in the deep part of the calyptra, it does not increase in size because the outer cells are detached, peeling off, due to gelation of the middle lamellae.

Behind the meristem are: parenchyma, vascular tissues and, in those roots that must thicken in successive years, remnant meristems, responsible for secondary growth.

The calyptra provides mechanical protection to the meristematic cells as the root grows through the soil. These cells are destroyed by root growth and friction with the soil, but are quickly replaced by new cells generated by cell division on the outer surface of the root meristem.

The calyptra is also involved in the production of mucilage, which is a gelatinous substance that covers the newly formed meristematic cells. These cells contain statoliths, which are starch grains found inside the cell and are very dense, so they move in response to the force of gravity, providing the root with the information necessary for its growth.

The outer surface of the root is called the epidermis. New epidermal cells absorb water from the surrounding environment and produce root hairs which increase the water-absorbing surface of the epidermal cell.

Root hairs are very delicate and generally have a very short life of only a few days. When the root grows, it produces new root hairs to replace those that have already died.

The root epidermis, the rhizodermis, is typically unilayered. It is made up of elongated, tightly packed, thin-walled cells, usually without a cuticle. In some cases a cuticle has been described, but it is currently believed that the compounds detected would be precursors of suberin. ^[4]

The process that plants use to absorb water from the soil is called osmosis. This process uses the higher concentration of salt within the root compared to the salt content of the soil to draw water into the root. For this reason, plants have a very difficult time absorbing saline water.

Beneath the epidermis lies the cortex, which comprises most of the root. The main function of the cortex is to store starch. The intercellular spaces in the cortex allow for air circulation within the cells, which is very important for respiration.

The endodermis is a thin layer made up of small cells and is found in the innermost part of the cortex, around the vascular tissue. The cells that make up the endodermis contain a substance called suberin which serves to create a kind of impermeable barrier, this barrier is known as Casparian’s band, the suberin is arranged transversely in the layer of cells that form the band, on the outside and via the apoplast the free space of the root is delimited. Water can only flow in one direction through the endodermis: inwards or in other words towards the center of the root.

Water absorbed by the root hairs passes through the cortex, an area dedicated to the storage of water and nutrients, and passes through the cells of the endodermis. These cells have bands called Casparian bands (formed by suberin), which force the water to pass only through the cells and not between them. ^[5]

The vascular cylinder or stele comprises everything that is inside the endodermis. The outer part is also called the pericycle and surrounds the actual vascular cylinder. In monocotyledonous plants the xylem and phloem are distributed randomly around the center of the vascular cylinder. In eudicotyledons the xylem cells are together forming a single structure.

Adventitious roots

They are not originated in the radicle of the embryo, but in any other part of the plant. They can arise from aerial parts of the plant, in underground stems, and in old roots. They may or may not have branches, but they have a relatively homogeneous shape and size. They do not generally have secondary growth. They are fasciculated roots or fibrous root systems. Their duration varies; in some perennial grasses they can last several years.

In many monocotyledons such as the grass (Cynodon dactylon) and dicotyledons such as the strawberry (Fragaria) that have prostrate stems, the root system is often not unitary, since a fascicle of adventitious roots emerges at each node. Some monostemmed cormophytes such as the palm Socratea and Pandanus, arboreal or shrubby monocotyledons, achieve greater stability by developing adventitious roots called fulcrum roots or stilt roots. These roots also appear in grasses such as corn and sorghum. They are thick, form at the basal nodes, and penetrate the soil where they perform a dual function: support and absorption.

Branch

The degree of branching is influenced by the soil. Roots are sparsely branched if they grow in water or swamp. In aerated and dry soils they are highly branched. Many trees have a divided root system that allows them to make better use of the water supply: horizontal, superficial roots to absorb rainwater, and deep vertical roots to reach water from the inner layers of the soil when the water table drops. In tropical dicotyledons, four root branching patterns have been described for old trees.