1. Introduction
The root is the bottom part of the plant body axis. It is usually under the ground, although there are some aerial and aquatic roots. The total roots of a plant is known as root system. The main functions of roots are anchoring the plant to the ground and absorption of water and minerals. Roots may perform other functions: storing organ in beets, carrots or sweet potatoes, hormone synthesis, aeration in aquatic plants, etcetera. In many species, roots are associated with some types of fungi to form mycorrhizae, or with some symbiotic bacteria to form nodules. These associations greatly improve the absorption of nitrogenous substances by roots.
Anchoring is an important function performed by roots, which is often taken for granted. However, it is essential for plant survival because it keeps plants erect and allows withstanding adverse environmental conditions like wind, rain, snow, herbivores, supporting the plant body structure, and many others. Indirectly, roots also help to stabilize the soil.
Water absorption is another essential function of roots, although it is not clear what part of the root is more important for this function. Not much water enters at radical meristem level, mostly because there is no functional vascular system. Root hairs are supposed to be a major entry pathway for water. However, it is also plausible that other regions more mature, and even suberized, may have an important role in water absorption because non-suberized regions are scarced and small, so it is difficult to explain how the total water needed can enter through them. In some species, a large part of the water absorption is done by the fungi hyphae associated with roots.
Roots are responsible for finding water by growing in length through the ground. This searching makes roots an important soil transformer, not only because they can break rocks and keeps in place soil, that otherwise is dragged away by rain and wind, but also because they can get associated with fungi and bacteria.
All vascular plants can grow true roots, although they are not formed in some primitive and some epiphyte plants. Some non-vascular plants, like mosses and hepatic plants, show underground structures called ryzhoids that anchor the plant body to the ground and absorb water, but they lack vascular bundles. True roots always have vascular bundles containing xylem and phloem.
Growth rate and extension of roots depend on environment variables, such as moisture, temperature, season, plant species, and many others. It is estimated that corn roots may grow 5 to 6 cm per day, common grass roots around 10 to 12 cm per day and tree roots about 3 to 5 cm per day. The depth that roots may reach in the ground is also variable depending on the ground features and plant species, and it can go from a few cm in herbaceous plants to tens meters in tree roots. The deeper roots usually make plants more resistant to drought.
Root growth and development depend on organic molecules supply and hormones coming from the stem. For example, the root growth is reduced during fruit and seed formation. The relation between the size of stem and the size of the root may vary in different species, and it is influenced by the age of the plant and environment variables. The ratio stem/root may rank be 0,12 in a tropical forest, 0,5 in corn, and 3 inbeet.
Root branches die with high probability, depending on the species and season of the year. So, plants need to produce new branches continuously, not only to increase the size of the root system, but also to maintain the root system. This process demands much energy, mostly coming from photosynthesis. Roots may use 50 % to 70 % of the photosynthesis production of a plant. It is not clear why a plant needs so many roots, but it can be related to the constant searching for water and nutrients.
Some tree species have the ability of fusing the roots between different individuals. This behavior has been observed in some tropical species, many pine species and other angiosperm tree species. These connections allow direct communication of group of individuals through their roots, and therefore they can share water resources, minerals and other nutrients.
2. Organization
The root is the first structure that sprouts from the embryo in the seed. This initial root is known as radicle. The final organization of the root system depends on the species. It may be a main root derived from the radicle that branches many times giving lateral roots (Figure 1). This root disposition, typical of gymnosperm and dicotyledons, is called tap root system. In tap root system, the main root is important during the whole life of the plant and gets very deep in the ground. Although the main root is as old as the plant, the lateral roots may show a high mortality. It is estimated that the smaller lateral roots may live for a few days.
In many monocotyledon species, the primary root is only important during the first stages of development. It is later substituted by many roots emerging from the stem or leaves. All these roots have more or less the same size and length, and form a root system known as fibrous roots (Figure 1). They don't get deep in the soil, but provide a good anchoring for the plant body. That is why species with fibrous roots are suitable to prevent soil lost by erosion.
Adventitious roots are those that sprout from the stem or leaves, but differently when compared with fibrous roots. Adventitious roots emerge after the germination period from cells near the vascular bundles. Some of these roots may be aerial and may contain chloroplasts. For example, ivy has aerial roots developed from leaves or stem. Some plants can propagate by stolons (strawberry), leaves (African violets) or stems (blackberry). Adventitious roots can be developed from each of these plant organs.
Some roots are specialized in less common functions. For example, plants growing in wet or puddled soils usually have aerial roots that facilitate the exchange of gases, as in mangler or in floating aquatic plants. In parasitic plants, roots are adapted to get nutrients from other plants.
3. Regions
From the tip to the more mature parts, several regions can be distinguished in roots (Figure 2). Although these regions are observed in most roots, they may show different length according to the species and environmental variables.
Apical region. The apical root meristem is found in the apical zone, covered by a protecting layer known as root cap. A group of initial cells (undifferentiated cells) are found in the apical root meristem giving rise to every cell type in the root. Initial cells show a low division rate. Near the initial cells, the procambium and protodermis are meristems that differentiate into vascular bundles and epidermis, respectively. Initial cell surrounds the quiescent center, a group of cells that appear to control the behavior of initial cells. Besides protecting root meristems, root cap, or calyptra, releases mucilaginous substances and dead cells that become a lubricant for facilitating the grow and mechanical friction of growing roots.
Root grows by cell proliferation and cell elongation of the cells produced by the apical root meristem. Both processes need organic molecules coming from production or storing places of the plant body through the vascular system. However, the functional vascular system is several millimeters away from the meristem. Hence, proliferating and elongating cells are also involved in feeding the meristem (see below).
In the central zone of the root cap there are two columns of cells, altogether known as columnella. There is a gravity sensor in some columnella cells that allows the root to grow to the center of the Earth. It is the positive gravitropism of roots. The cells that sense gravity are called statocysts. They are large cells with only a few of organelles and cytoskeletal filaments in the center of the cytoplasm. The nucleus is found in the upper half of the cell and the endoplasmic reticulum, and many other organelles, toward the cell periphery. Statocysts contain amyloplasts known as statoliths, which are denser than the cytosol, and hence sediment at bottom part of the cell. Statoliths interact with the plasma membrane triggering a chemical reaction that releases the hormone auxin, which is transported to the lateral parts of the root. The lateral levels of auxin changes according to gravity, and the curvature of the root is modified. This mechanism is absent for sometime in the lateral branches and it appears when they reach some length.
The division zone starts after the initial cells. Here, cells divide quite frequently to increase the total number of cells to form all the root tissues. All cells in this zone are going to be differentiated when they stop proliferating.
The elongation zone is a region of a few millimeters in length where cells increase their size. Root may grow in length by this cell elongation, as well as by the addition of new cells in more apical zones.
In the maturation zone, cells begin to differentiate and get specific cellular features to be functional in each tissue that form the mature root. Root hairs differentiate from epidermal cells in this region.
The borders between these regions are not clearly delimited. For example, cells that become vascular bundles start the differentiation process in the elongation region. In addition, the regions are moving together with the apical root tip during root growing.
Roots may show primary and secondary growth. Primary growth is largely increasing in length, whereas secondary growth leads to larger diameter. The type of growth and the group of plants are the features we are going to consider when studying the microscopic anatomy. Because roots lack nodes and internodes, the general anatomy is quite similar along the root extension.
4. Evolution
All vascular plants have true roots showing positive gravitropism and a root cap protecting the apical tip. Fossils indicate that true roots have been independently discovered several times during evolution. The ancestor of vascular plants lacks true roots, and also those that gave rise to lycophytes and seed plants, therefore true roots in these groups were independently developed.
Lycophytes have roots with ancestral features like branching by bifurcation and lack endodermis. Their root meristems may be uni- or multi-celular. Euphyllophytes do not branch by bifurcation: the roots of ferns generate secondary roots from the endodermis and seed plants from the pericycle.
Rizhoids are unicellular or multicellular root-like structures that emerge from the gametophyte of bryophytes, lycophytes and monilophytes. True roots emerge from the sporophyte of vascular plants.