Internal Structure of Plants characters of Roots Structure of organs of the plant. Each of these organs has The root, stem, and the leaves are the fundamental characteristic internal organisation on the basis of which these can easily recognised. The differences in the internal structure of these organs are because of their specific functions, environment and their relations with other plant organs This chapter deals with the Internal structure of root, stem and leaf.
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Table of Contents
“THE ROOTS OF ALL GOODNESS LIE IN THE SOIL OF APPRECIATION FOR GOODNESS”
ROOT: PRIMARY STRUCTURE
The internal structure of root is comparatively simple than that of the stem since it is not associated with lateral appendages like leaf branch, etc. The root shows following distinctive
characters:
(1) Epidermis (known as epiblema or piliferous layer) is usually not covered with cuticle. It also lacks stomata.
(2) Trichomes or root hairs are always unicellular.
(3) A distinct and well-developed endodermis with characteristic casparian strips is present just outside the Vascular bundles.
(4) Vascular bundles are radial.
(5) Xylem is exarch. Internal structure of a typical dicotyledonous root can be studied by examining the transverse sections of sunflower (Helianthus) or gram (Cicer) root. The following zoncs can be distinguished in a root in the primary state of growth:
(1) Epiblema
(2) Cortex
(3) Pericycle
(4) Vascular system
Epiblema
The epiblema of root is also known as piliferous layer (pilus = hair, ferre = to carry i.e., hair bearing layer). It forms the outermost tissue of the root. It is a continuous layer of compactly arranged thin-walled cells. The epiblema is single layered (uniseriate) and without cuticle and stomata. Many of the cells of epiblema develop root hairs which are characteristically unicellular. The root hairs develop a few millimeters from the root apex in the region of maturation. The cells of the epiblema forming root hairs are known as trichoblasts; these are usually smaller than other epiblema cells. A root hair originates as a small outgrowth and then gradually extends in between the soil particles. Often it becomes
Characteristics of Dicotyledonous Roots
(1) The number of vascular bundles varies from 2-6 (diarch to hexarch condition).
(2) A well developed pericycle gives rise lateral roots, part Of vascular cambium and cork cambium.
(3) Pith is poorly developed or absent.
several millimeters long. It has a thin layer of cytoplasm which encloses a large central vacuole.
[II] Cortex
The cortex is relatively simple lying between epiblema and the endodermis. It is made of thin walled isodiametric parenchymatous cells with intercellular spaces. Thick walled tissue many collenchyma, sclerenchyma) is usually absent. However some roots may have fibers, sclereids, lioblasts and latex cells in the cortex. In aerial roots of Tinospora few outer layers of cortex contain chloroplasts. The cells of the cortex are generally irregularly distributed.
Cortex is chiefly a food storage tissue and the cells are often filled with starch grains. Besides this, the cortex also transfers water and minerals from root hairs to the xylem.
The innermost layer of the cortex is endodermis that separates vascular tissue from the cortex. It consists of a single layer of compactly arranged barrel shaped living cells. These cells are characterised by the presence of
a strip of suberized or lignified material of varying thickness on their radial and tangential walls.
These strips or bands of thickening material are called casparian strips (after Caspary, 1865). The cells of the endodermis opposite the protoxylem elements, lack thickenings and are called passage cells.
[III] Pericycle
Inner to the endodermis lies the pericycle. It forms the outer boundary of the primary vascular cylinder of roots. Generally the pericycle is single lavered (e.g., Helianthus) but in some plants (e.g.Morus alba) two or more layered pericycle is found. It originates from the procambium and also meristematic characteristics. Pericycle retains functions as the site of lateral root initiation.
[IV] Vascular tissue
The vascular tissue of the root is characterised by radial arrangement of vascular bundles, i.e., xylem and phloem occur in separate patches on alternate radii. The parenchymatous cells lying between them form conjuctive tissue. The first formed elements of xylem, called protoxylem is close to pericycle and the later formed xylem, i.e.metaxylem is situated near the center. This arangement of xylem is known exarch and is small and made of thin-walled cells. The outer typical of roots. The phloem patches are relatively part of phloem tissue, next to the pericycle, is the first to be fully differentiated and is called protophloem, while the inner part is the metaphloem. Protophloem and metaphloem are not so readily distinguishable as are protoxylem
In dicot roots, the number of xylem /and metaxylem. phloem groups vary from two to six. The cambium Is absent in young roots but later thin strips of cambium differentiate in the parenchymatous cells on the inner side of the phloem patches and on the outer side of the protoxylem strands. The activity of cambium results in the formation of secondary vascular tissue.
Pith. In the center of the root is present a small parenchymatous pith with large intercellular spaces (e.g., Helianthus, Cicer). These cells store food material and other waste products. In many dicot roots the pith is absent (e.g., Ranunculus).
MONOCOTYLEDONOUS ROOT
Internal structure of monocotyledonous root can be studied in the transverse sections of Zea mays, Liliun, Canna or orchid root. The four zones observed in the dicot roots (i.e., epiblema,cortex, pericycle and vascular tissue) can also be distinguished in the monocot roots.
[Epiblema]
This is the outermost layer of compactly arranged cells. In most of the monocots it is single layered but in the aerial roots of some epiphytic orchids,the epidermis is multiseriate and forms a tissue called velamen. The velamen is composed of compactly arranged non-living cells which often have secondary wall thickenings. Many cells of the epiblema form unicellular root hairs.
Characteristics of Monocotyledonous Roots
(1) The number of vascular bundles is more than six (i.e., polyarch condition).
(2) Pericycle gives rise to lateral roots only.absent, both in
(3) Cambium is always absent, both in young as well as mature roots.
(4) A well developed pith is found in the centre of the axis.
[I] Cortex
The major part of the root is occupied by several layers of parenchymatous cortex. Usually all the cells of the cortex are alike; they are thin walled and developed with well isodiametric intercellular spaces. However, lil some monocotyledons (e.g., Zea maize, Iris,Smilax) one to several sub-epidermal layers of cortex become suberized. These peripheral thick walled cells of the cortex form a protective tissue called exodermis. Endodermis is the innermost layer of the cortex which delimits the cortex from the underlying vascular tissue. It is made of compactly arranged barrel-shaped cells. The radial and tangentical walls of endodermal cells have casparian strips. A few cells of this layer, which lie opposite the protoxylem strands, are without thickenings and function as passage cells.
[III] Pericycle
A distinct pericycle consisting of one or more layers of parenchymatous cells lies just below the endodermis. In some monocots the pericycle is thick walled and multiseriate (e.g.; Smilax).
[IV] Vascular tissue
The vascular tissue consists of six or more vascular bundles which are arranged in a ring just inner to the pericycle. The vascular bundles are radial, i.e., xylem and phloem are situated at different radii. As the number of vascular bundles is more than six in moncotyledons, the condition is known as polyarch.
Xylem consists of tracheids, vessels and xylem parenchyma. Protoxylem elements of each xylemn strand are closer to the endodermis while metaxylem elements are close to the pith; such a xylem is known as exarch. Phloem patches alternate with the xylem strands. It consists of sieve tubes and companion cells. Parenchymatous tissue present between xylem and phloem strands is known as conjunctive tissue. In monocot roots cambium does not develop even after the roots are mature, and therefore, the vascular bundles are closed. It is due to the absence of cambium that monocot roots do not show secondary growth.
Pith. The central portion of monocot roots IS occupied by well developed pith (e.g.; maize,Canna). Sometimes pith becomes the Sclerenchymatous (e.g., Iris, Smilax).
comparison between the internal structure of dicotyledonous and Monocotyledonous Roots
Dicot roots
1. Vascular bundle The number of The vascular bundles varies from 2 to 6 (i.e.diarch to hexarch).
2. Pericycle Lateral roots, vascular cambium and Cork cambium originatee from this layer.
3. Cambium Develops later during secondary growth.
4. Pith Poorly developed or developed. absent
Monocot roots
1. Vascular bundles
are generally more than six (i.e., polyarch).
2.Only lateral roots are produced from this layer.
3. Does not develop at any stage.
4. Large well and developed.