Root Structure and Development in Plants
Root Cap Function and Origin
The root cap, or cofa, is a structure that protects the apical meristem of the root as it grows and penetrates the soil. It is composed of living parenchymal cells derived from the apical meristem. It originates from the caliptrogen in monocots and the calyptra-dermatogen in dicots. The root tip secretes mucilage, which aids in ion exchange, adherence to soil particles, and nutrient chelation. Root georeaction (response to gravity) is controlled by Ca++ and the hormone indole acetic acid (IAA), which inhibits growth.
Lateral Root Formation
Lateral roots originate in the pericycle at variable distances from the apical meristem. Auxin governs their formation, frequency, and distribution. Endogenous inhibition also plays a role. The location of lateral roots depends on the projection of the xylem:
- Diarchic roots: between phloem and xylem.
- Triarch roots: in a direction opposite to the xylem.
- Polyarch roots: in a direction opposite to the phloem.
Adventitious Roots
Adventitious roots are born from any mature part of the plant other than the embryonic radicle. They originate endogenously near vascular tissue, facilitating vascular connection between organs.
Origin:
- Outskirts of the stem.
- Parenchyma near the cambium.
- Interfascicular rays.
- Vascular stem.
- Stem marrow.
- Leaf or stem interstices (e.g., *Salix* (willow)).
Sometimes, they have an exogenous origin in cortical tissue and epidermis. Roots form the entire adventitious vascular system in inferior radical plants (Pteridophytes).
Root Structure in Gymnosperms and Dicots
Roots of gymnosperms and most dicots exhibit secondary growth, characterized by the presence of secondary vascular tissue: secondary phloem and secondary xylem produced by the vascular cambium and cork cambium (periderm). The presence of medullary rays is also notable.
Cambium Initiation: It starts as arches on the edge of the phloem, involving cells from undifferentiated procambium between primary phloem (F1) and primary xylem (X1). Another cambium forms from the pericycle, more internal to the xylem poles, creating a continuous layer surrounding the central xylem body. The shape of the cambium varies:
- Oval in diarchic roots.
- Triangular in triarch roots.
- Polygonal in polyarch roots.
The cambium located in the phloem becomes functional before the xylem. Some secondary pericycle forms in a position opposite to the phloem and moves outward. Wide medullary rays can form in the opposite position of the xylem.
Secondary Vascular Tissue
Changes in the vascular cambium and pericycle are included in the cylinder, which completely internalizes the primary xylem (X1) during secondary growth. The primary phloem (F1) is crushed, and some of its cells differentiate into fibers. The secondary xylem (X2) in the root, compared to the stem, has:
- Less fiber.
- Smaller, uniform-sized vessels.
- Different rings.
- More living parenchyma specialized in storage, containing a great quantity of cells with starch and less tannin.
There is as much secondary phloem (F2) as there is in the stem.
Periderm
The phellogen originates in the outermost cells of the pericycle. In some plants, the root cork cambium can form under the epidermis, where the cortex fulfills the storage function, and the superficial layers of the cortex are preserved. This occurs in *Ipomoea batatas* (sweet potato).
Anomalous Secondary Growth
Some herbaceous dicots with limited secondary growth exhibit characteristic features, particularly in storage roots like carrots. Secondary growth is common but predominantly parenchymal in both xylem and phloem. In beets, anomalous growth occurs as new cambia form from the phloem parenchyma of the last formed ring. These form alternating rings of xylem and parenchyma, separating the phloem (sucrose) for storage.
In *Ipomoea batatas* (sweet potato), additional cambia form within the xylem around individual vessel groups from the paratracheal parenchyma. These cambia produce some vessels inward and sieve elements outward, but mainly produce abundant storage parenchyma cells. This process repeats indefinitely, and no crust is formed. Simple articulated laticifers are present.