Table of Contents
- In this simple tissues and complex tissues in plants post we have briefly explained about simple tissues, complex tissues, and outer covering.
- Around each plant cell is a cell wall. Living cells filled with water exert force against their walls, making each cell a rigid box. Plant cells are glued to each other by a material called pectin, and collectively they form a very strong yet flexible plant body. Plants consist of many different types of cells that are organized into aggregates called tissues.
- The organs of the plant, leaves, stems, roots, and flower parts are composed of tissues arranged in different patterns. Tissues in the plant body are made up of both living and dead cells. The dead cells, often with thick, strong cell walls, are retained as strengthening cells.
- The main tissues of plants may be grouped into three systems. The ground tissue system is the most extensive, at least in leaves (mesophyll) and young green stems (pith and cortex). The vascular tissue system contains two types of conducting tissues that distribute water and solutes (xylem) and sugars (phloem) through the plant body. The dermal tissue system (epidermis and periderm) covers and protects the plant surface.
- Some of the tissues are composed mostly of a single cell type; these are called simple tissues. Tissues made from aggregates of different cell types are called complex tissues. Tissues, simple or complex, act together as a unit to accomplish a collective function and are derived from meristems.
Simple Tissues and Complex Tissues in Plants
- There Are Three Types of Simple Tissues:
- The simple tissues are made of cells that are the workhorse cells of the plant body. They do the photosynthesis, load things in and out of the vascular system, hold up the weight of the plant, store things, and generally conduct the important business and housekeeping chores needed to keep the plant body healthy and functioning.
- Parenchyma cells are commonly isodiametric (as broad as they are long) and have thin walls, living protoplasts, and conspicuous intercellular spaces. Parenchyma is regarded as an unspecialized tissue. If the cells have chloroplasts, the tissue is called chlorenchyma.
- In potatoes and fruits, the parenchyma cells store starch. Parenchyma is the main tissue of apples, pears, plums, and tomatoes. One of the most significant features of parenchyma is that it retains living protoplasts at maturity.
Parenchyma tissue found in cells responsible for storage.
- Collenchyma is the first of the supporting tissues found in young stems and leaves. These cells retain living protoplasts in maturity, as do parenchyma cells. Wall thickenings are conspicuous at the corners of cells. While these thickenings contribute to the strength of cells, they also remain plastic so the cells can stretch.
- The walls are composed primarily of cellulose but also contain some Pectic compounds. These cells become elongated (often as many as two millimetres long) and frequently have chloroplasts. Intercellular spaces are sometimes noted.
Collenchyma cells are thin walled with thickened corners.
- The cells making sclerenchyma tissue are rigid and function to support the weight of a plant organ. There are two types of sclerenchyma cells:
Sclerenchyma tissue provides support in plants.
- Fibers can occur in aggregates forming a continuous cylinder around stems; they may connect end-to-end to form multicellular strands acting like strengthening cables like re-bar in concrete; or they can form a component of vascular tissues. They are long, narrow cells with thick, pitted cell walls and tapered ends. Fibers are sometimes very elastic and can be stretched to a degree, but they will snap back to their original lengths.
- Sclereids sometimes occur as sheets (an example being the hard outer layer of some seed coats), but they usually occur in small clusters or as solitary cells. Sclereids have many striking shapes, from elaborately branched cells, to star-shaped cells, to the simple stone cells that give a gritty texture to pear fruits. Sclereid cell walls are often thicker than the walls of fibers.
- Complex Tissues Make Up the Plant’s Vascular System and Outer Covering. The vascular system consists of an interconnected network of cells that traverse the entire body of the plant. All cells of the plant require minerals and water, which are absorbed by the roots and transported by the xylem. Sugars are manufactured in the leaves and transported by the phloem.
- Xylem is a complex tissue that is mainly responsible for the conduction of water and mineral salts from roots to other parts of the plant. The xylem, which is derived from procambium, is called primary xylem and the xylem, which is derived from vascular cambium, is called secondary xylem. Earlier formed xylem elements are called protoxylem, whereas the later formed xylem elements are called metaxylem. Xylem is made up of four kinds of cells – tracheids, vessels or tracheae, xylem fibres and xylem parenchyma.
Longitudinal section through a xylem vessel to show hollow lumen to allow for transport of water and nutrients.
- Like xylem, phloem is also a complex tissue. It conducts food materials to various parts of the plant. The phloem elements which are formed from the procambium of apical meristem are called primary phloem. The phloem elements which are produced by the vascular cambium are called secondary phloem. The primary phloem elements that develop first from the procambium are smaller in size called the protophloem, whereas those develop later are larger in size called metaphloem. The protophloem is short lived. It is crushed by the developing metaphloem.
Longitudinal section: phloem tissue transports nutrients throughout the plant.
- Phloem is composed of four kinds of cells: sieve elements, companion cells,phloem parenchyma and phloem fibres. Companion cells are present only in angiosperms. Companion cells are absent in pteridophytes and gymnosperms. Phloem fibres are absent in the primary phloem of most of the angiosperms. But they are usually present in the secondary phloem.
- The epidermis is the outer covering of the plant. It is a complex tissue composed of epidermal cells, guard cells, and trichomes (hairs) of various types. The epidermis is usually one layer of cells, but it may be as many as five or six layers in the leaves of some succulent plants and in the aerial roots of certain orchids. The epidermis protects the inner tissues both from drying and from infection by some pathogens. It also regulates the movement of water and gases out of and into the plant.
- Epidermal cells are the main cell type making up the epidermis. These cells are living, lack chloroplasts, are usually somewhat elongate, and often have walls with irregular contours. The outer walls of epidermal cells are often thicker than the inner and sidewalls. The outer epidermal wall is coated with a waxy substance (cutin) forming an impermeable layer called the cuticle. All parts of the plant, except the tip of the shoot apex and the root cap, have a cuticle. In roots, the cuticle is often very thin (and has been reported to be totally lacking on the surface of root hairs).
- Young stems, leaves, flower parts, and even some roots in exceptional instances have specialized epidermal cells called guard cells. Between each pair of guard cells is a small opening, or pore, through which gases enter and leave the underlying tissues. Two guard cells plus the pore constitute one stoma. Guard cells differ from other epidermal cells by their crescent shape and the fact that they contain chloroplasts. Another type of epidermal cell, the subsidiary cell, forms in close association with guard cells and functions in stomatal opening and closing. Trichomes are epidermal outgrowths and may be a single cell or multicellular. In roots, for example, root hairs are extensions of single epidermal cells that increase the root surface area in contact with soil water. In some leaves, very elaborate multicellular trichomes may form.
- The periderm is a protective layer that forms in older stems and roots after those organs expand and the epidermis splits and is lost. It is a secondary tissue. This tissue is several cell layers deep and is composed of phellem (cork) cells on the outside, a layer of dividing cells (phellogen or cork cambium), and the phelloderm toward the inside. Phellem cells are dead at maturity, and have a waxy substance (suberin) embedded in their cell walls. Phelloderm cells live longer than phellem cells and are parenchymalike.