Initially, roots grow in length by the activity of the root apical meristem, which is protected by the root cap. Proliferation and differentiation of the root apical meristem cells give cells that are first organized as the so-called primary root, that is, it shows primary growth. It is a less complex organization than the stem because it lacks nodes, internodes and leaves.
No matter the plant species, primary roots show an epidermis, or rhyzodermis, usually uniseriate (on cell thick layer), lacking stomata, and showing root hairs in the maturation region, just after the elongation region (Figures 1, 2, and 3). In general, near the tip of the root, epidermal cells show a thin cuticle facilitating the entry of water and mineral salts. In some species like xerophyte plants and in those roots near the surface of the soil, a layer of cells known as hypodermis can be found under the epidermis. Hypodermis cells form a thin layer of cells with suberized cell walls. Hypodermis can be further specialized and becomes the exodermis, which is found in some angiosperms, and works as a second barrier to prevent the free diffusion of substances between the root and the soil.
Root hairs absorb water and mineral salts. They are elongated epidermal cells found in the maturation region that increase the absorption surface in contact with the external environment, and therefore the absorption capability. Root hairs are continuously formed and disappear as the root is growing because the maturation region follows the growing of the root tip at a more or less constant distance. The number of root hairs is about 20 to 500 per cm2 in the roots of trees, and about 25000 per cm2 in the winter rye. The number may also varies depending on the environment conditions.
There are three root hair organization patterns (Figure 4). The density of root hairs depends on the environment conditions. For example, in low phosphate soils the number of root hairs is higher to increase the total root absorption surface. Many symbiotic organisms, such as nitrogen fixing bacteria, are associated with root hairs.
The root cortex is found below the epidermis, or hypodermis. In roots, the cortex is thick or very thick (much more than in stems), and it is generally made of parenchyma cells specialized in storing, although it can be photosynthetic parenchyma in aerial roots, and aeriferous parenchyma in aquatic roots. There are many empty spaces between cortical parenchyma cells so that water can be conducted (apoplastic pathway) toward the vascular bundle. Moreover, parenchyma cells are connected between each other by plasmodesmata (symplastic pathway) that allow minerals and water to travel from cell to cell.
A distinct feature of root primary growth is the endodermis, the most inner layer of the cortex. Endodermis is evolutionary conserved from ferns to angiosperms. It is one cell thick layer of tightly compacted cells having cell walls partially impregnated with suberin thar form a thickening, the so-called Casparian stripes. The cell compaction and the impermeability of Caspary stripes make water and solved substances to cross the endodermis through the cytoplasm of endodermis cells. Hence, endodermis is a barrier against free diffusion that controls the substances traveling from the ground to the vascular bundles. In those root regions with secondary growth, both endodermis and cortex are lost (see next page).
Casparian stripes are primary wall impregnations that encircle endodermic cells as a belt arranged longitudinally. They are not secondary wall. The stripes are continuous between neighboring cells through the middle lamella. In 3D view, they form like a cylindrical fishing net, where the cords are the Casparian stripes and the holes are the endodermis cells. Casparian stripes contain lignin, but not suberin. However, suberin can be found in the cell wall of endodemis cells after the Casparian stripes are formed. Suberin is laid as layers over the cell wall surface, with different thickness depending on the cell localization. For example, those endodermis cells found closer to the xylem poles do not synthesize suberin. They are called passage cells. In some plant species, endodermis undergoes an additional development stage where their cell walls become lignified and Casparian stripes show a U shape. Passage cells, however, keep thin cell walls. The continuous growing of roots let to the death and disappearing of endodermis.
Below the endodermis, there is one or two layers of parenchyma cells with very thin cell walls that form the pericycle. Pericycle cells can restart the meristematic activity and form lateral roots. In the older part of the roots, pericycle cells are sclerefied. In plants with secondary root, pericycle contributes to the formation of the vascular cambium and phellogen (cork cambium).
The vascular bundles, xylem and phloem, are found in the inner part of the root. Primary xylem and primary phloem are arranged in separate and alternate rows. According to the number of rows, there are diarch (2), triarch (3), tetrarch (4). Tetrarch organization is typical of dicotyledons and gimnosperms (Figure 5). Monocotyledons are polyarch having many rows of xylem and phloem.
Lateral roots are generated after the embryonary period and determine the root system morphology of the plant. The lateral root formation begins in the pericycle near the apical tip of the root, in the region where the cells become differentiated. This is different from how the branches and leaves are formed. Branches and leaves are differentiated from superficial meristems, in an exogenous way. The location of the starting point of a lateral root is influenced by the organization of the vascular bundle. Lateral root primordia are formed in points opposite to the xylem poles in dicotyledons or to the phloem poles in monocotyledons. In some species, the endodermis also collaborates in the formation of the lateral roots.
Adventitious roots emerge after germination from cells near the vascular bundles, either as a normal process or after an induction process. They can be generated in stems, leaves and roots.
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