Permanent Tissues

6.2.2 Permanent Tissues

Permanent tissues having all cells similar in structure and function are called simple tissues. Permanent tissues having many different types of cells are called complex tissues. Permanent tissues are derived from meristems and have lost the ability to divide.

6.2.2.1 Simple Tissues

A simple tissue is made of only one type of cells. The various simple tissues in plants are parenchyma, collenchyma, and sclerenchyma.

Parenchyma

Parenchyma forms the major component within organs. The cells of parenchyma are usually isodiametric. They may be spherical, oval, round, polygonal, or elongated in shape. Their walls are thin and made of cellulose. They may either be packed closely or have small intercellular spaces.

Parenchyma performs various functions like photosynthesis, storage, and secretion. Modified parenchyma:

• Chlorenchyma — parenchyma with chloroplasts; performs photosynthesis (e.g., in leaf mesophyll)
• Aerenchyma — parenchyma with large air spaces; provides buoyancy in aquatic plants (hydrophytes)

Collenchyma

Collenchyma occurs in layers below the epidermis in dicotyledonous plants. It is found either as a homogeneous layer or in patches. It consists of cells which are much thickened at the corners due to a deposition of cellulose, hemicellulose and pectin. Collenchymatous cells may be oval, spherical or polygonal and often contain chloroplasts. These cells assimilate food when they contain chloroplasts. Intercellular spaces are absent. They provide mechanical support to the growing parts of the plant such as young stems and petiole of leaves.

Sclerenchyma

Sclerenchyma consists of long, narrow cells with thick and lignified cell walls having a few or numerous pits. They are usually dead and without protoplast. On the basis of variation in form, structure, origin and development, sclerenchyma may be either fibres or sclereids.

The fibres are thick-walled, elongated and pointed cells, generally occurring in groups, in various parts of the plant. The sclereids are spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities (lumen). These are commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea. Sclerenchyma provides mechanical support to organs.

6.2.2.2 Complex Tissues

The complex tissues consist of more than one type of cells. These work together as a unit. Complex tissues are concerned with transportation of water, minerals, and food. There are two types: xylem and phloem.

Xylem

Xylem functions as a conducting tissue for water and minerals from roots to the stem and leaves. It also provides mechanical strength to the plant. It is composed of four different kinds of elements: tracheids, vessels, xylem fibres, and xylem parenchyma.

Tracheids are elongated, tube-like, dead cells with tapering ends and thick lignified walls. Vessels are long cylindrical tube-like structures, dead, made up of many cells called vessel members, each with lignified walls and large lumen. Tracheids and vessel members are perforated. Xylem fibres have thick walls and obliterated lumen and provide mechanical strength. Xylem parenchyma cells are living and store food (starch, fats) and other substances like tannins.

Primary xylem is of two types — protoxylem (the first formed primary xylem) and metaxylem (the later formed primary xylem). In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery — this arrangement is called endarch. In roots, the protoxylem lies towards the periphery and metaxylem lies towards the centre — this is exarch.

Phloem

Phloem transports food materials, usually from leaves to other parts of the plant. Phloem is composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres.

Sieve tube elements are long, tube-like structures, arranged longitudinally and are associated with the companion cells. End walls of sieve tube elements are perforated to form sieve plates. A mature sieve tube element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus. The functions of sieve tubes are controlled by the nucleus of companion cells.

Companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements. Phloem parenchyma stores food materials. Phloem fibres provide mechanical support. In gymnosperms, companion cells are absent; instead albuminous cells are associated with sieve cells. In monocots, phloem parenchyma is also absent.

Worked Examples