Root Anatomy Lecture 5 PDF

The root: Anatomy The root is the lowermost portion of the plant that normally develops below the soil surface. Two types of roots can be distinguished on the basis of origin: tap roots and adventitious roots. Tap roots develop from the apex of the embryo that is determined to produce roots (the...

The root: Anatomy The root is the lowermost portion of the plant that normally develops below the soil surface. Two types of roots can be distinguished on the basis of origin: tap roots and adventitious roots. Tap roots develop from the apex of the embryo that is determined to produce roots (the radicle) and from the pericycle of mature roots. Adventitious roots develop from other tissues of mature roots or other parts of the plant body such as stems and leaves.  The roots of most plants are covered at the very top with a specialised tissue-the root cap. It consists of parenchyma cells that contains starch and secretes mucilaginous materials to facilitate the penetration of the root through the soil. The outer layers of the root cap are replaced by new layers from the initials as they die as a result of friction with soil particles. Root caps are found on the roots of all plants except for some mycorrhizal roots. Arrangement of the primary tissues in the mature root 1- The epidermis The epidermal cells of the root are thin walled and devoid of cuticle. The root epidermis is usually unisereate but in the aerial roots of some plants such as Orchidaceae, it may be multisereate where it forms a velamen. The most characteristic feature of the root epidermis is the production of root hairs that are adapted to the uptake of water and nutrients. They are restricted to a few centimetres from the root apex and they die on the more mature parts of the root. In some plants all the epidermal cells have root hairs while in others only some cells, trichoplasts, may do so. 2- The root cortex The root cortex of dicotyledons and gymnosperms is formed mainly of parenchyma. The cortical parenchyma lacks chlorophyll except in some aerial roots of epiphytes and usually contains starch. Resin ducts and secretory cells can exist in the cortex. Sclerenchyma may exist in roots of monocotyledons in addition to parenchyma. The root cortex is normally wider than that of stem  The exodermis In some plants such as Iris and Citrus, the cell walls of the outer subepidermal layers of the cortex become suberized to form a protective layer, the exodermis. The exodermal cells contain viable protoplasms even when mature. The exodermis ranges in thickness from a single layer to many layers 3- The endodermis The endodermis is a single layer of cells that separates the cortex form internal tissues. The endodermis controls the passage of water and nutrients from the cortex to the vascular cylinder. Water can pass through the cortex through the cytoplasms of the cortical cells and also by diffusion through their walls and intercellular spaces. The endodermal cells are compact and their walls are thickened in such a way that prevents the passage of water and nutrients except through their cytoplasms. The endodermal are characterised by the development of casparian strips on their radial and cross walls but not on the tangential walls, which face the cortex and vascular cylinder. These strips are composed of lignin and suberin. The casparian strip is the primary stage of wall thickening of the endodermal cells. In plants with secondary growth, the thickening on the endodermal cell walls stops at this stage and it is shed together with the cortex with the development of the secondary thickening and periderm In plant with no secondary growth such as monocotyledons, an entire suberin lamella develops on the inner side of the endodermal cell wall including casparian strip. This lamella is the second stage of wall development. In the third stage, a layer of cellulose is formed centripetally on the subrin lamella. The cellulose layer reaches a considerable thickening on all walls of the endodermal cells except the outer tangential walls where the cells become cup-shaped. Because of the delay in thickening of the cell walls of endodermal cells opposite to the xylem, these cells usually have casparian strips only and are termed passage cells. Passage cells are thought to facilitate the passage of water through the endodermis as it is retarded through the other endodermal cells with thick walls. 4- The vascular cylinder The vascular cylinder occupies the central part of the root. It is clearly delimited from the cortex by the endodermis. Immediately inside the endodermis is a single layer of thin walled parenchyma, the pericycle. It is a meristematic tissue from which the primordia of the lateral roots of all spermatophytes develop. The pericycle may be unisereate or multisereate The vascular tissues of the root, xylem and phloem, are arranged in separate alternating strands at the periphery of the vascular cylinder. The root xylem is exarch i.e., protoxylem is external to metaxylem The number of protoxylem groups in the root i.e., whether one, two, three etc., is expressed by the terms monarch, diarch, triarch respectively. A root with many protoxylem groups is termed polyarch. Polyarch arrangement is characteristic of monocotyledons. In Triticum, one large metaxylem vessel is found in the centre of the vascular cylinder and is surrounded by many protoxylem vessels on the periphery. Roots with fewer protoxylem groups, up to six groups, are characteristic of dicotyledons Cambium exists in roots with secondary growth such as tap roots of dicotyledons. It appears on the inner side of phloem. Later, the Cambium cells on the outer side of protoxylem start to divide and then the resulting cells unite with these on the inner side of phloem. Therefore cambium becomes an undulating cylinder between xylem and phloem. In the centre of the vascular cylinder, pith may exist. It is formed of parenchyma. Its size depends on the size of the whole cylinder. Bigger vascular cylinders usually have bigger pith Epiphytic roots The velamen is composed of compact dead cells that store the rainwater. The velamen contains specialised cells (pneumatodes) that allow gas exchange through it when it is filled with water.  Roots of Cleisostoma yersinii. (A) Cross-section of the stele, green- excited autofluorescence showing the distribution of chloroplasts (in red). (B)The same section as in A, UV- excited autofluorescence showing anatomical details, including phloem and xylem strands and endodermis with passage cells. (C) Cross-section of the root. (D) Two-layered velamen with exodermis. (E) Tracheoidal idioblast (water storage cell) in root cortex. (F) Root cortex with exodermis (blue) and numerous tracheoidal idioblasts (UV-excited autofluorescence). (G) Growing tip of a root. (H). Wet root showing white pneumatodes in translucent (green) velamen. Storage roots big  The underground roots may become very much thickened and serve as organs for the storage of food. Such is the case in sweet potatoes, radishes, turnips, carrots and dahlias. In such roots the food may be stored largely in the cortex or xylem region or in both.  In turnips food is stored largely in the xylem, and the phloem and cortex are relatively narrow. In the radish and sweet potato the xylem is also the chief region of food storage, but food is also stored outside the xylem. In the carrot there is a more even distribution between xylem and bark. Respiratory roots Hydrophytes roots  Parasitic roots Al-Wakeel, S. A., Moubasher, H., Gabr, M. M., & Madany, M. M. (2013). Induced systemic resistance: an innovative control method to manage branched broomrape (Orobanche ramosa L.) in tomato. European Journal of Biology, 72(1), 9-21.