The below mentioned article provides an overview on the ground tissue system of plants.
This system includes all the tissues excepting the epidermis and the vascular bundles. Thus it is the largest or the most exhaustive system, which begins from the layer next to epidermis and continues right up to the centre of the organs in cylindrical bodies.
Ground tissue system is heterogeneous in nature, consisting of diverse types of cell elements adapted to carry on different functions. In the axis of higher plants the vascular bundles occupy a restricted position inside the stele or central cylinder.
The ground tissues occurring outside the stele, and, in fact, surrounding it, form the cortex, what may be called external or extrastelar ground tissue. Similarly there are internal or intrastelar ground tissues inside the stele, e.g., pith.
Both external and internal ground tissues are further differentiated to specialised zones. These tissues are derived from the ground meristem of the embryonic region.
Cortex:
Cortex constitutes that part of the axis which envelopes the stele. It begins from the layer next to epidermis and solely consists of primary tissues. During growth in thickness secondary tissues are added to the cortex, forming what is known as secondary cortex.
The cortex may be rather thin, consisting of only a few layers of cells, or it may be quite massive. It is essentially composed of parenchyma which is the most predominant cell type.
Collenchyma very often occurs in the superficial regions of herbaceous stems and other rapidly elongating bodies like leafstalks and floral stalks. It may be present as discrete patches at the ridges in the bodies with wavy outline, at the corners in the angular stems, or as continuous bands consisting of a few layers of cells beneath the epidermis.
Sclerenchyma fibres are also common as continuous sheets or in patches. In some organs sclerenchyma forms a band of supporting tissue just beneath the epidermis.
The term hypodermis is applied to that zone of protecting or supporting tissues composed of collenchyma or sclerenchyma. The parenchyma cells of the cortex are loosely-arranged isodiametric ones usually containing chloroplasts.
Sclereids, resin ducts, oil-cavities and laticifers may also occur. Starch, tannins and crystals are the common inclusions. The cortex of root is more homogeneous, as it consists of only parenchyma cells.
Though cortex is often composed of various types of cells, naturally with different functions, but it is primarily a protective layer. Other functions like support, photosynthesis, storage, etc., are secondary.
Endodermis:
It is a uniseriate zone composed of modified parenchyma cells which are compactly set, so that intercellular spaces are absent. Endodermis is considered to be the innermost layer of cortex; but some anatomists are of opinion that it is the limiting layer of stele.
Though ontogenetic studies are not conclusive on this point, its intimate association with the vascular system is undeniable.
The endodermal cells are elongate ones with long axes parallel to the vascular tissues. In cross-section the cells are essentially tabular in shape (Fig, 568).
A very distinctive feature of the endodermal cells in the presence of a waxy substance like suberin in form of bands or strips on the radial and transverse walls. These are known as Casparian strips or bands (Fig. 568 A, B & C), so named after Caspary who first noted them in 1865. The bands also show lignin reactions.
In some cases the radial and inner, and often, all walls (Fig. 568 E & F) become very much thickened, so much so that the lumen of the cells may be obliterated. These walls are also strongly suberised.
In this type of endodermis, what is common in monocotyledonous roots, thin-walled cells occasionally occur here and there. These cells, called passage cells or transfusion cells (Fig. 568E), are supposed to facilitate flow of water and solutes.
In roots passage cells are located just opposite to the protoxylem vessels. The terms primary and secondary have also been attributed to the thin-walled and thick-walled endodermis respectively.
The first type is common in pteridophytes and most dicotyledons and the second type in the monocotyledons. In an endodermal cell the cytoplasm usually remains firmly attached to the Casparian strips (Fig. 568C), so much so that they do not easily separate even on plasmolysis.
Some worker believe that it is due to strong penetration of cytoplasm into the band, while others are of opinion that it is for higher viscocity of cytoplasm of endodermal cells. This phenomenon is of considerable importance in interpreting the functions of endodermis.
The endodermis lies internal to the cortex (recognised as the innermost layer of cortex by some authors) and surrounds the vascular tissues. Another layer, known as pericycle, is often placed between the two—the endodermis and vascular tissues.
At any rate, endodermis delimits the stele from the cortex. In some stems, as in Equisetum, Marsilea, etc., another layer of endodermis occurs, which limits the vascular tissues from the internal ground tissue, pith. Such a layer is known as inner endodermis (Fig. 568D).
In some ferns and angiosperms individual bundles remain surrounded by endodermis. It is of universal occurrence in all roots and in all the organs of most of the pteridophytes. In many gymnosperms it is absent in the stems, though it occurs in the leaves.
Endodermis is present in the stems of majority of herbaceous angiosperms, in aquatic plants, in creepers and rhizomes. It is absent in the woody stems of dicotyledons and gymnosperms, and leaves of angiosperms.
In young stems of some angiosperms the innermost layer of cortex contains abundant starch grains. This layer is referred to as starch sheath. In view of the fact that it occupies the position of the endodermis, starch sheath is considered homologous with the endodermis, the distinct wall character, the Casparian strips, are of course absent.
Moreover, there are cases when the starch sheath in the young stem may lose starch and become thickened at a later stage. The starch grains present in the root of some plants are often large, and they move to the lowest side of the cell. They have been called statoliths by some authors, who have further suggested that they have a function in connection with geotropism, being receptors of gravitational impulse.
The function of endodermis has been a subject of long-drawn controversy, but no agreement could really be reached. Mainly on the basis of its relation to water and vascular tissues, quite a good number of functions have been ascribed to it. It is often considered as a ‘watertight’ layer between vascular system and surrounding tissues. Due to absence of intercellular spaces and occurrence of highly thickened walls it is said to be ‘a sort of a water-dam’.
Endodermis has also been called a protective layer, something like an accessory epidermis; its connection with root-pressure has been suggested, it has been called an ‘air-dam’ meant for preventing the conducting elements being clogged with air. The cells of thin-walled endodermis retain the power of cell division. The lateral roots and adventitious buds often originate from this layer.
Pericycle:
The pericycle is the region consisting of one or more layers of cells which surround the vascular tissues. It is located between the endodermis and the vascular bundles. Pericycle is regarded as the limiting layer of stele; thus it forms the outermost part of intrastelar ground tissues, the other portions being the pith and medullary rays. Though occurring inside the stele it is considered to be distinctly separate from the vascular tissues.
It typically consists of parenchyma, as found in all roots and pteridophytes. Heterogeneous pericycle or pericycle consisting of parenchyma and sclerenchyma or sclerenchyma in form of discrete bands has been recognised since a long time.
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 flax, Linum usitatissimum of family Linaceae, definitely belong to phloem.
So it is absent in many angiospermic stems. Pericycle, however, is of universal occurrence in the roots of higher plants, only some aquatic plants and parasites excepted, and also in the stems and roots of pteridophytes.
Pericycle is the seat of origin of lateral roots. During secondary growth in thickness phellogen or cork-cambium arises in this layer. In case of anomalous steles it forms secondary cambium.
The term ‘pericambium’ was formerly used for the pericycle of roots because of its property of becoming meristematic and giving rise to new tissues. In monocotyledons, which usually have no secondary increase, the pericycle generally becomes sclerified, either partly or wholly. The parenchyma cells of pericycle serve for storage. They may contain laticiferous tissues and secretory ducts.
It is usually one-layered in the roots of angiosperms. But in quite a good number of monocotyledons like Smilax, Agave of family Liliaceae, palms and some grasses and in some dicotyledons like Morus of family Moraceae, it may be multilayered (Fig. 568F). The pericycle of gymnosperms is typically multiseriate.
Pith:
The pith is the main internal ground tissue present at the central portion of the organs limited by the vascular bundles. Often it is quite massive, consisting of parenchyma cells with profuse intercellular spaces.
The cells are isodiametric in shape and sometimes remain arranged in longitudinal series due to their development from rib meristems. At the mature stage the cells of pith lack chloroplasts, when colourless leucoplasts are usually present.
The walls are thin and made of cellulose. The pith cells serve as organs for storage of starch, fatty substances and often of crystals and tannins. Secretory tissues are frequently present. Fibres and sclereids may also occur, though rarely.
In some cases the outer part of the pith is composed of smaller cells 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.
The cells may be parenchymatous, as found in many members of families Euphorbiaceae, Boraginaceae; sclerenchymatous, as in Compositae, Umbelliferae; or both parenchyma and sclerenchyma may be present.
In many herbaceous plants the pith is destroyed during rapid elongation accompanied by radial expansion of the stem. Hollow pith, as it is called, with broken walls lining the cavity may be seen in the family Cucurbitaceae and many grasses. Pith is absent in many dicotyledonous roots. When present it resembles that of the stem, but is more homogeneous and normally is not destroyed.
Medullary Rays:
Parenchymatous ground tissues passing in between the vascular bundles, thus occupying the interfascicular areas, constitute the medullary rays. These cells, often slightly elongate, link the parenchyma of pith, also called medulla, with those of cortex.
So they often look like radiating portion of pith. Due to their origin from early meristems, they are referred to as primary medullary rays. During secondary increase in thickness some of these cells become meristematic and produce secondary tissues.
Ground Tissues of the Leaf:
Ground tissue system of the foliage leaves markedly differs from that of stem and root. In leaves the ground tissue is called mesophyll tissue which is mainly composed of thin-walled parenchyma cells (chlorenchyma) with profuse chloroplasts, so that photosynthesis may be effectively carried on.
In many monocotyledonous leaves with parallel venation mesophyll consists of more or less isodiametric parenchyma cells with intercellular spaces (Fig. 618). But in most of the dicotyledons mesophyll is differentiated into two types of cells.
Beneath the adaxial or upper epidermis mesophyll cells are elongated or columnar ones arranged more or less at right angles to the epidermis. These are known as palisade parenchyma (Fig. 613).
Those occurring towards abaxial or lower epidermis are isodiametric or irregular in shape with more intercellular spaces. They are called spongy parenchyma (Fig. 613). In some leaves, as for example Eucalyptus, palisade cells may be present on both the sides.
Chloroplasts are abundant in mesophyll cells, which are in fact, the region of most active photosynthesis. Secretory cells and sclereids may often be present.