Functions, origin of lateral roots and secondary growth in Dicot Roots
Functions of Roots
Roots perform two kinds of functions; primary functions and secondary functions.
Primary functions are performed by the normal roots of almost all the plants, these functions include; anchorage of the plant to the substratum, absorption of water and minerals, conduction of the absorbed water and minerals to the stem.
Secondary functions are performed by certain roots, which are specifically modified for the purpose only in some plants, such functions include; storage food, provision of additional support, absorption of atmospheric moisture, assimilation/photosynthesis, sucking food from host, reproduction, respiration, better gaseous exchange and mechanical functions like floating and balancing.
Origin of Lateral Roots
The lateral roots are endogenous in origin (from a deeper layer). The seat of its origin is cells of pericycle, usually opposite a protoxylem group, become meristematic and go on dividing periclinally and anticlinally. Lateral Root showing the Vascular Connection between it and the parent Stele. Lateral roots are produced in a definite pattern. The youngest roots are closing (proximal) to the apex. The pattern of lateral root production is readily observed with tap root systems. The tap root can also be called the primary root while its branches are called secondary or lateral roots. The origin of lateral roots can be unraveled for species that have fibrous root systems. A clear pattern emerges with careful study. The hump penetrates into the cortex, and emerges as a lateral branch. Later, the hump differentiates into 3 regions of the root apex- dermatogen, Periblem and Plerome. Finally the lateral root comes out. The number of lateral roots corresponds to the number of xylem bundles.
Secondary Growth in Dicot Roots
The roots grow in length with the help of apical meristem. It is called primary growth. Apart from primary growth, roots grow in width i.e., they increase in girth. This increase is called secondary growth. It is found only in dicot roots.
Secondary growth in dicot roots occurs, following an increase in the diameter of the stem by secondary growth. Secondary growth in dicot roots is initiated by certain cells that become meristematic by dedifferentiation as there is no cambium.
Certain cells of the conjunctive tissue on the inner edge of phloem and certain cells of the pericycle lying opposite to the xylem groups, become meristematic. These strips of meristematic cells join to form a cambial ring.
The cambial ring is wavy in outline initially, but later it attains a circular outline. The cells of the cambial ring undergo mitotic divisions and produce new cells both on its outer side and inner side.
The cells on the outerside differentiate to form secondary phloem while the cells produced on the inner side differentiate into secondary xylem. Secondary tissue formed outer to the protoxylem bundle differentiates into prominent primary medullary ray thus, protoxylem does not get crushed.
Later, cork cambium (Phellogen) also differentiates in the pericycle. The cork cambium divides and gives rise to cork (Phellem) towards outside and secondary cortex (Phelloderm) towards inside. All the three i.e. Phellogen, Phellem and Phelloderm together form the Periderm of the root and has protective function.
Secondary protective tissues are also formed to replace the epidermis that becomes broken, with the increase in the girth/diameter of the root.