PAB 307: PLANT ANATOMY

THE PRIMARY STRUCTURE OF THE ROOT

General Introduction

The root develops from the radicle of the embryo and grows down into the soil to form the primary root. In dicotyledons, the primary root grows further deep into the soil, developing a number of secondary and tertiary branches to form a well-developed tap root system. Conversely in monocotyledons, the primary root is soon replaced by a bunch or cluster of roots, forming the adventitious root system. In some cases, the roots become specialized for specific functions e.g. in epiphytes and mangrove plants, aerial and respiratory roots are formed and in case of turnips, carrots and radish, fleshly roots are stored for storage. In roots, the position of apical meristem is sub-terminal, due to terminal position of the root cap.

We shall in the subsequent sections, discuss an overview of the tissue systems found in the roots of both plant groups (dicots and monocots), as well as providing anatomical description of the tissue system arrangement and function in detail. It is also worthy of note that the general method for analyzing root (and stem) tissues of most plants is by cutting a section, double staining and examining the anatomical details carefully under the microscope.

Anatomy of Dicotyledonous Roots

Anatomy of a Dicotyledenous Root: Sunflower root before secondary growth

The following important anatomical features are noted in the root section of the sunflower plant:

I. Epidermis

The epidermis is also called epiblema or piliferous layer, which is the outermost single layer composed of thin walled, compactly packed parenchymatous cells, without intercellular spaces. Here and there, the epidermal cells give rise to unicellular root hairs. Epiblema of roots is characterized by absence of cuticle and stomata. Root hairs increase the absorbing surface of the root and mainly function to absorb water and mineral salts from the surrounding soil. II. Cortex This lies below the epiblema and consists of many layers of thin-walled, loosely arranged, parenchymatous cells, with numerous intercellular spaces between them. However, outer few layers may be compactly arranged. The cells of the cortex contain leucoplasts (amyloplasts) and store starch grains.

III. Endodermis This is the innermost layer of cortex, which demarcates the cortex from the stele and surrounds the stele. The cells are more or less barrel shaped, parenchymatous cells which fit closely without intercellular spaces. The endodermis behaves like a water-tight jacket around the stele. Their radial and inner walls are often thickened due to deposition of lignin and suberin, forming Casparian strips, a characteristic feature of the endodermis. The walls abutting upon the protoxylem are often provided with simple pits. Cells lying opposite the protoxylem elements are thin walled and called passage cells, because they allow the passage of water from cortex to xylem.

IV. Vascular Tissue System (Stele) This includes the pericycle, vascular bundles, conjunctive tissue and pith. The stellar type is a tetrarch actinostele. The components parts are discussed briefly as follows: i. Pericycle: This lies internal to the endodermis and consists of a single, uniseriate layer of thin-walled, parenchymatous cells containing dense protoplasm. This layer is usually two to three layered opposite the protoxylem elements which show the point of origin of lateral roots. ii. Vascular bundles: The vascular bundles are arranged in a ring, four in number and of the radial type. Each bundle has a patch of xylem and phloem placed on different radii and alternate in their position. (a) Phloem: This lies alternate to xylem patches in alternate radi and is composed of only thin and cellulose-walled elements. It consists of sieve elements, companion cells and phloem parenchyma. Phloem is present in form of small patches, the outer part (towards pericycle), being called protophloem and inner part (towards metaphloem) called metaphloem. (b) Xylem: This is conical in shape with metaxylem vessels towards centre and protoxylem towards periphery, thus exarch in nature. In mature and well developed roots, the metaxylem elements meet in the centre, and as a result, the pith soon becomes obliterated. Fibres and xylem parenchyma are absent. Xylem tracheids are found present around the vessels

iii. Conjunctive tissue: These are parenchymatous cells, lying between patches of xylem and phloem

iv. Pith: In young roots, the central part of the stele is the pith which consists of few parenchymatous cells compactly arranged without intercellular spaces. In older roots, the pith may be altogether absent due to development of metaxylem.

Anatomy of Monocotyledenous Roots

General Account of Monocot Roots

The roots of Zea mays (maize) have been selected to study the anatomical features of typical monocotyledonous roots taken as type examples. Sections are cut and double-stained for study.

Anatomy of a Monocotyledenous Root: Maize

A section of the young stem of the maize plant reveals the following important structures:

I. Epidermis, Cortex and Endodermis

The above named tissue systems are structurally similar to dicot roots. In maize, endodermal cells are thick walled and Casparian strips are indistinct.

II. Vascular Tissue System (Stele) This includes the pericycle, vascular bundles, and pith. The stellar type is an actinostele. The components parts are discussed briefly as follows: i. Pericycle: This lies internal to the endodermis and is usually single layered. It is partly composed of parenchymatous cells and partly of sclerenchymatous cells. ii. Vascular bundles: The vascular bundles are found in large numbers (more than eight) and each one is a radial type (as described in dicot root). These are arranged in a ring around the central pith. The xylem is composed of single, rounded metaxylem vessels towards the pith, and one or sometimes two, small, polygonal or rounded thick-walled protoxylem vessels towards the periphery. Thus the xylem shows exarch arrangement. The phloem lies in small patches alternate to xylem, and is composed of sieve tubes, companion cells and phloem parenchyma. Parenchyma cells associated with xylem undergo sclerosis and thus become thick walled iii. Pith: Occupies the central portion of the stele. It is composed of thin walled, parenchymatous cells containing abundant starch grains. Cells are loosely arranged.

Development of Lateral Roots

The lateral roots of both gymnosperms and angiosperms arise from an inner layer (endogenously) the pericycle, and grow through the cortex. A few mature cells of the pericycle lying against the protoxylem become meristematic and begin to divide tangentially, and lead eventually to the formation of a root primordium. This primordium appears as a protrusion which pushes the endodermis outwards and tends to grow through the cortex. During the process, cells of the cortex are either crushed or pushed aside. The passage of the lateral roots through the cortex is by mechanical penetration, but enzyme action may also facilitate the growth.

The primordium soon takes shape of a growing point with its initial cells, the root-cap and other histogens. The root-cap and the promeristem of the growing lateral root develop while still in the cortex. The vascular elements differentiating in the lateral roots become connected with that of the parent root through the cells of pericycle. The endodermis sometimes participates in the formation of the primordium, in which case, they eventually forms a layer surrounding the primordium. These cells however die, and are shed when lateral roots come out. The lateral roots usually develop in acropetal order that is, older at the base and younger at the apex. At times, there is mno regularity in this order with reference to each other.

REFERENCES (For further reading and diagrams)

1. Dutta, A.C. (1964) Botany for Degree Students. 708 pp. Sixth Edition revised in 2009 by T.C. Dutta. Oxford University Press

2. Pandey, S.N. and Chadha, A. (1996). Plant Anatomy and Embryology. 468pp. Vikas Publishing House PVT Ltd