The following points highlight the top four types of ground tissues in plants. The types are: 1. Cortex 2. Endodermis 3. Pericycle 4. Pith.
Ground Tissues: Type # 1. Cortex:
The ground tissue found beneath the epidermis which surrounds the central cylinder and is delimited from the cylinder by the endodermis is called the cortex. Usually the cortex of stems consists of thin-walled parenchyma cells having sufficiently developed intercellular spaces among them.
Usually some of the cortical cells or all of them contain chloroplasts at least in young stems. The cortical cells also contain starch, tannins, crystals and other common secretions in them. The cortex may contain collenchyma, sclerenchyma and sclereids in addition to ordinary parenchyma.
Collenchyma is usually arranged as a cylinder or in the form of strands near or beneath the epidermis. In most of the dicotyledonous stems collenchyma is often found in the ridges, in the corners and in other portions to give temporary support to the plant body. Sometimes a few layers of fibres of collenchyma develop just beneath the epidermis forming an outer protective layer called hypodermis.
Just beneath the hypodermis a few layers of parenchyma and chlorenchyma are found. The innermost layer of the cortex is endodermis which is single-layered and sometimes known as starch sheath. The cortex of roots is more homogeneous than that of stems and usually consists of parenchyma only.
The tissues of the cortex are strictly primary and as a whole, mature with the primary tissues of the stele, but there is considerable overlapping of development with secondary-tissue formation within the stele. Collenchyma develops early, but sclerenchymatous cells are usually late in reaching maturity.
The cortex of an axis in which marked secondary growth has occurred has tissues crowded and even more or less crushed radially.
Functions of the Cortex:
In stems it acts as a protective tissue, but secondarily carbon assimilation, storage of water, storage of food and other functions are also carried on. Collenchyma of the cortical region aids in the temporary mechanical support of plant cody. In roots it is a storage tissue and helps in pumping water from hairs to the xylem.
Ground Tissues: Type # 2. Endodermis:
This is a uniseriate layer of cells delimiting the cortex from stele. It consists of barrel-shaped cells arranged quite close to each other having no intercellular spaces among them. The cells of endodermis are elongated and arranged parallel to the long axis of the vascular tissue. They are living cells and the protoplasts are those of typical parenchyma cells.
Starch, tannin, mucilage and nuclei are frequently found in endodermal cells. Due to the presence of starch in the endodermal cells, it is also known as starch sheath. In stems, it is inconspicuous and found in the form of wavy layer, and in certain cases it becomes altogether obliterated, whereas in roots this layer is well-defined and circular in appearance.
Commonly the endodermal cells are of two types— primary or thin-walled and secondary or thick-walled. In primary or thin-walled cells certain thickenings of suberin are developed in the form of a band or strip which run completely around the cell on the radial walls and end walls, are called Casparian strips or Casparian bands.
These bands range in width from minute threads to broad bands that occupy the entire radial wall. In transverse section, the strips are often called Casparian dots or radial dots. In secondary or thick-walled endodermal cells the radial and inner walls and sometimes all the walls are thickened by suberin lamellae laid down over the earlier formed wall with its Casparian strips.
The thick wall is strongly suberized like the Casparian strips.
The Casparian band was first recognized as a wall structure by Caspary (1865-66) and is therefore known as the Casparian strip or band. Among the thick-walled cells of the endodermis, as in many roots, there occur occasionally isolated thin walled cells usually opposite the protoxylem elements, which are known as passage or transfusion cells.
Through these thin walled cells the sap absorbed by root hairs enters the xylary elements.
The endodermis is commonly clearly differentiated in the stems of the vascular cryptogams (pteridohytes) and is found here with Casparian strips and with the additional suberin lamella, but apparently not with the secondary cellulose layer (Guttenburg, 1943).
The endodermis occurs in lower vascular plants around the periphery of vascular cylinder, sometimes also between the pith and the vascular tissues. In some ferns the endodermis encloses individual steles.
In seed plants the endodermis is quite distinct in the roots, but in a number of herbaceous angiosperms, the stems develop an endodermis with Casparian strips, and also with somewhat thickened walls. According to Guttenburg (1943), in underground rhizomes an endodermis develops more frequently than in aerial stems.
In Senecio and Leonurus, the endodermis develops in the herbaceous stem when the plant attains flowering stage (Datta, 1945; Warden, 1935). The woody gymnosperms and the dicotyledons do not possess an endodermis in the aerial stems (Plaut, 1910).
Functions of the Endodermis:
There is a great controversy about the functions of the endodermis. According to some workers it is a protective layer or sort of accessory inner epidermis. According to others it is connected with the maintenance of root pressure. It is also thought that it acts as an air dam which prevents diffusion of air into the vessels and thus they escape from closing. It may serve as storage tissue having starch grains in many dicotyledons.
Ground Tissues: Type # 3. Pericycle:
In dicotyledonous stems, the pericycle is a multi-layered zone found in between the endodermis and the vascular bundles. It always occurs as a thin cylinder of tissue completely encircling the vascular bundles and the pith. Towards inner side this pericyclic zone is limited by the primary phloem, whereas towards outer side it is limited by the endodermis.
It may be a complete sclerenchymatous zone as in many cucurbits or it consists of both sclerenchyma and parenchyma cells (e.g., in sun-flower and in many other members of Compositae). Sometimes when endodermis is altogether absent, the peri-cycle merges with the cortex. Typically the peri-cycle consists of parenchyma, as in most roots and in the stems of the pteridophytes.
Recently it has been shown that the abundant ‘pericyclic fibres’ of some plants are a part of the primary phloem. Certainly many of them—such of those of Cannabis (hemp) and Linum (flax)—belong to the phloem and are known as phloem fibres, bast fibres or hard bast. In many plants such pericyclic fibres are thought to be associated with the phloem of vascular bundles.
It is also claimed that no pericycle is present in the stems of many angiosperms because the fibres which were thought to make up much of this layer belong to the phloem. A narrow well-marked pericycle is present in the pteridophytes, in both root and stem, and in seed plants in the roots.
In the stems of angiospermic seedlings and herbaceous angiosperms, a true or inconspicuous endodermis present and narrow band of parenchyma separates the endodermis from the phloem. This band represents either the outermost layer of the primary phloem or a pericycle. Usually in woody herbs the protophloem fibres lie against the endodermis, and no pericycle is present.
The pericycle of roots consists of thin-walled parenchyma. The lateral roots of angiosperms arise in this tissue. Generally the pericycle is uniseriate in the roots (in Smilax root the pericycle is many layered and sclerenchymatous). Most of the roots possess distinct pericycle. However, the roots of certain angiospermic parasites and aquatic plants lack pericycle.
Functions of Pericycle:
In the dicotyledonous roots the cells of pericycle become meristematic and form the vascular cambium and phellogen. The pericycle gives rise to lateral roots. The adventitious roots originate from pericycle in stems. Laticiferous cells, secretory cells and other specialized cells may occur in the pericycle. Some of the pericyclic cells aid in storage.
Ground Tissues: Type # 4. Pith:
The pith or medulla forms the central region of the stem and the root. Usually the pith of dicot stems is largely parenchyrnatous. It is devoid of chlorophyll in the mature state but starch forming leucoplasts are found in it. The sufficiently developed intercellular spaces are found among the pith cells.
In monocotyledonous stems the vascular bundles are found scattered throughout the ground tissue and pith is not distinguishable. In the dicotyledonous roots the pith is scanty or lacking. However, in monocotyledonous roots it is well developed. In certain monocotyledonous roots (e.g., Canna), the pith is sclerenchymatous.
The extensions of the pith in the form of narrow parenchymatous strips are called medullary or pith rays. The pith of many plants is partially obliterated during the growth of the stem and in such cases the stem becomes hollow. However, in such cases the nodes retain their pith. Certain pith cells possess tannin and crystals.
Certain specialized structures like laticifers or secretory canals may also occur in the pith cells. In many cases the peripheral portion of the pith is demarcated from its central portion, due to the presence of smaller cells, their contents and sometimes even by the presence of chloroplasts in them (e.g., Lantana, Anagalis, etc.).
In certain plants (e.g., Equisetum) there is an inner endodermis. In such cases the medullary rays are not connected with the central pith region. In some plants belonging to Umbelliferae and Compositae the medullary rays are composed of sclerenchymatous cells or of both sclerenchyma and parenchyma cells.
As regards the ontogeny of pith it develops from the ground meristem and may be treated as the inner portion of the ground or fundamental tissue system.
Function of the Pith:
The pith cells mainly serve as storage tissue. In case the pith is sclerenchymatous it acts as mechanical tissue and provides mechanical strength to the plant. The medullary ray consisting of parenchyma cells, serve as channels for the transport of food materials and Water from the central part (pith) to peripheral region (cortex) of the stem.
In dicotyledonous stems some of the parenchyma cells of medullary rays become meristematic and give rise to inter-fascicular cambium.