In this essay we will discuss about the ground tissue system.
This tissue system includes all the tissues excepting the epidermis and vascular bundles. Therefore, it is the largest tissue system beginning from the layer next to epidermis and continuing up to the centre point of the organ. Ground tissue system is heterogeneous in nature including diverse types of cells specialised for different types of function.
The ground tissue outside the stele is called the cortex or extrastellar ground tissue and that inside the stele is called intrastellar ground tissue or pith. Both the ground tissue are further differentiated into different specialized zones to perform certain particular functions. Ground tissues are derived from the ground meristem of the embryo.
1. Essay on Cortex:
The tissue internal to the epidermis and external to the vascular cylinder is the cortex consisting of various types of cells. The cortex is usually composed of thin walled parenchyma cells only (e.g., Salicornia, Pelargonium etc.). Sometimes the peripheral layers of it, immediately beneath the epidermis, form hypo- dermis with other types of cells of different origin from the epidermis.
This peripheral layer usually contains collenchyma occurring either as a continuous band or discrete groups or as isolated strips. In Cucurbita, patches of collenchyma are present in the projecting ridges of stem. In certain cases the collenchymatous hypodermal cells and peripheral cortical parenchyma cells contain abundant chloroplastids to perform photosynthesis and are known as chlorenchyma.
In the cortex other types of cells like sclereids (e.g., Trochodendron), laticifers (e.g., Calotropis) etc. may also occur. The cortical parenchyma may contain starch, crystals etc.
In monocot stems all the tissues but vascular bundles and epidermis constitute the ground tissue without distinction between cortex and pith. In many genera of Liliaceae (e.g., Asparagus) and Amaryllidaceae, however, there is differentiation of cortex and stele with distinct endodermis and pericycle. In certain cases the outer layers of the cortex may be sclerified parenchyma or sclerenchyma.
In Asparagus sclerenchymatous pericycle is present. The remaining portion of ground tissue is parenchymatous in which the vascular bundles are embedded. The vascular bundles in monocot stem usually remain scattered throughout the ground tissue and this type of distribution is termed as atactostele. But in many genera like Coix, Triticum, Oryza etc. they are arranged more or less in a ring.
The cortex of the root is mainly composed of parenchyma cells. In many monocots sclerenchyma develops in the root cortex in addition to parenchyma. Schizogenous and lysigenous intercellular spaces are frequently formed in the root cortex of some members of Gramineae and Cyperaceae. In many hydrophytes the air containing root cortical cells with regular arrangement form the aerenchyma.
In Iris, Smilax, Phoenix, and Citrus, epiphytic orchids etc. uniseriate or multiseriate exodermis is present forming a kind of hypodermis. The exodermal cell walls are thickened with suberin and lignin to form casparian strips to protect the inner delicate cortical cells. The exodermal cells remain living even after maturity.
Collenchyma cells rarely found in root cortex of monocots e.g., Monstera. The root cortex is usually very wide and the cells often contains starch, crystals etc. The aerial roots of many epiphytes contain chloroplastids in the cortical cells. Several plants possess secretory cells, resin ducts and laticifers in cortex. Trichosclereids are found in the root cortex of Monstera.
In a leaf the ground tissue is the mesophyll tissue that lies internal to epidermis. The cells of mesophyll tissue are parenchymatous. In mesophytic dicotyledons these parenchyma are of two types: palisade parenchyma and spongy parenchyma.
The palisade parenchyma cells are cylindrical, elongated or rod shaped packed densely in one or more rows. Each palisade cell contains numerous chloroplasts. The palisade cells usually remain on the adaxial side of the leaf beneath the epidermis. Leaf of Thymelaea is an exception where there is abaxial palisade only.
Again in Dianthus, Atriplex, and Artemisia etc. palisade parenchyma is present on both adaxial and abaxial sides whereas the strips of spongy parenchyma occur in the middle.
A leaf is said to be isobilateral when the palisade parenchyma occurs on both adaxial and abaxial surface. When the palisade parenchyma occurs only on the adaxial surface of the leaf, it is said to be dorsiventral. Monocotyledonous leaves belong to this category. The cylindrical leaves of Hakea contain palisade tissue along the periphery.
The spongy mesophyll occurs in the abaxial surface of a dorsiventral leaf and may be of various shapes. They may be elongated resembling the palisade or spherical or irregular. The cells contain abundant chloroplastids and are arranged in such a manner that there occur large intercellular spaces.
2. Essay on Endodermis:
The innermost layer of the cortex or the outermost layer of stele is the endodermis. It is the delimiting layer of cortex from the stele. This layer is inconspicuous in aerial stems except in Piper where casparian strips are also observed. Endodermis is prominent in underground stems.
It usually surrounds the entire stele and in certain exceptional polystelic cases it encircles individual vascular strand (e.g., Nymphaea). The endodermal cells of many dicotyledonous stems contain abundant starch and such endodermis is called starch sheath which in some stems may be transformed into endodermis having casparian strips.
An endodermis consists of a layer of compactly set living cells. Lignin and suberin, in addition to cellulose, are deposited on the transverse and radial walls of endodermal cells as strips or bands called Casparian strip (Fig. 5.83A).
In the roots also the endodermis is uniseriate with compactly arranged cells with casparian strips in young cells. The casparian strip is a narrow or wide band of suberin running around the radial and cross walls of the endodermal cells. In roots (e.g., Iris) without secondary growth suberin deposition takes place as lamellae over the whole inner primary wall of the endodermal cells including the casparian strips.
Cellulose is then deposited centripetally on the inner side of suberin lamella to make the radial walls of the endodermal cells considerably thick, which may be further thickened by lignification. Suberin deposition starts initially in the endodermal cells adjacent to the phloem strands.
It then spreads radially to the other adjacent endodermal cells except those opposite to the protoxylem. These thin-walled cells are known as passage cells (Fig. 5.83E) through which water and dissolved substances move from cortex to xylem vessels. The casparian strips prevent such passage of materials. Plasmodesmatal connections exist between cortical and endodermal cells but it is absent in the region of casparian strips.
The endodermis performs the following functions:
1. It accumulates starch, protein granules, fats, and tannins in some cases.
2. The suberised cell wall of endodermis acts as a barrier for the movement of heavy metals into or out of vascular tissues.
3. It secretes water into the adjacent xylem vessels as it possesses more positive water potential value than the neighbouring cortical cells.
4. It controls the conduction of water between the stele and cortex in rhizomes.
5. It forms a barrier for the pathogens as the cells of the endodermis contain large amounts of benzoquinones, napthaquinones and anthroquinones that inhibit the growth of fungi and bacteria.
6. In Cocculus, Paederia foetida stem the cork cambium originates from endodermis.
7. Endodermis may behave as a meristem termed proendodermis that divides to form a part of the cortex.
3. Essay on Pericycle:
The pericycle is the region consisting of one or more layers of cells immediately inner to the endodermis and surrounding the vascular tissues. It is regarded as the limiting layers of stele forming the outermost part of intrastellar ground tissues. The other portions of the stellar ground tissue are the pith and medullary rays.
The pericycle typically consists of parenchymatous cells as found in all roots and pteridophytes. Heterogeneous pericycle with parenchyma and discrete bands of sclerenchyma fibres may also be formed. But modern anatomists have shown that most of the so-called pericyclic fibres of stems are really phloem fibres.
The economically important fibres of hemp, Cannabis sativa of family Urticaceae, and Linum usitatissimum of family Linaceae, are definitely phloem fibres. Pericycle, however, is of universal occurrence in the roots of higher plants except some aquatic plants and parasites and also in the stems and roots of pteridophytes.
From the pericycle the lateral roots originate. Cork-cambium arises in the pericycle layer during secondary growth in thickness. In certain anomalous cases of steles secondary cambium originates from this layer. In monocots the pericycle generally becomes partly or wholly sclerified. The pericycle may also contain laticiferous tissues and secretory ducts.
Pericycle is usually single-layered in the roots of angiosperms but in some cases like monocotyledons (e.g., Smilax, Agave of family Liliaceae, palms and some grasses) and dicotyledons like Morns of family Moraceae, it may be multilayered. The pericycle of gymnosperms is typically multiseriate.
4. Essay on Pith:
Pith is the main internal ground tissue located at the centre of the organs. Very often it is quite large parenchymatous tissue with profuse intercellular spaces. The pith cells are usually isodiametric and sometimes remain arranged in longitudinal series due to their development from rib meristems.
The cellulosic thin-walled cells usually contain colourless leucoplasts. The pith cells may store starch, fatty substances and often crystals and tannins. Secretory tissues are frequently present. Fibres and sclereids may also rarely occur.
Sometimes the outer pith cells are smaller with thicker walls and dense protoplasm and thus become morphologically distinct from the rest. This distinct outer region has been called perimedullary zone or medullary sheath. In many herbaceous plants the pith is destroyed during rapid elongation accompanied by radial expansion of the stem.
In the family Cucurbitaceae and many grasses hollow pith with broken walls lining the cavity may be formed. Pith is absent in many dicotyledonous roots. When present it resembles that of the stem, but is more homogeneous and normally is not destroyed.
5. Essay on Medullary Rays:
Parenchymatous ground tissues present in the interfascicular areas i.e., in between the vascular bundles constitute the medullary rays. These slightly elongated cells link the parenchyma of pith, also called medulla, with those of cortex.
So they often look like radiating strips from the pith. Due to their origin from early meristems, they are referred to as primary medullary rays. During secondary growth in thickness some of these cells become meristematic and produce secondary tissues.