Lecture 3 The Cell and Tissues Continued (Plant Biology) PDF

Sclerenchyma Sclerenchyma is composed of cells with thick secondary walls that are usually lignified. Sclerenchyma cells are elastic (retain their original size and shape after being stretched) The function of sclerenchyma is support and sometimes protection. mature sclerenchyma is composed of...

Sclerenchyma Sclerenchyma is composed of cells with thick secondary walls that are usually lignified. Sclerenchyma cells are elastic (retain their original size and shape after being stretched) The function of sclerenchyma is support and sometimes protection. mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole cell volume. Two groups of sclerenchyma cells exist: fibres and sclereids. Fibres are long cells while sclereids are short. Fibres develop from meristems while sclereids develop from parenchyma that becomes lignified. Also pits are more numerous in sclereids than in fibres. Fibres occur in different parts of the plant body. They are classified according to their position in the plant body into two basic types: xylary fibres and extraxylary fibres. Xylary fibres are associated with the xylem of the vascular bundles. Extraxylary fibres do not belong to the xylem but rather they exist elsewhere in the plant body. Extraxylary fibres are also termed the bast by industrialists.  Classification of fibres according to their commercial use soft and elastic and are especially well suited for the processing to textiles Their principal cell wall material is cellulose. hard fibres that are mostly found in monocots Their cell walls harbour, besides cellulose, a high proportion of lignin.  Sclereids Sclereids occur in different plant parts. They occur as hard masses of cells within soft parenchyma tissues. Sclereids are usually dead cells. Their origin is parenchyma cells that become lignified. Sclereids are classified into four types: 1- Brachysclereids or stone cells. They are more or less isodiametric and are found in the phloem and in the flesh of stone fruits such as pear. 2- Macrosclereids. They are rod-shaped sclereids. Such sclereids often form continuous layers in the testa of seeds as in the seed coat of legumes. 3- Osteosclereids. These are bone shaped cells with enlarged or branched ends. They are found in seeds coats and in the leaves of some dicotyledons such as leaves of Hakea. 4- Astrosclereids. They are very branched and usually star shaped. Such sclereids are found in leaves such as Ceratophyllum leaves.  Compound tissues Epidermal Xylem cells Phloem  Epidermal cells Stomata Basic epidermal cells Trichomes  Basic epidermal cells The epidermis is the outermost layer(s) of cells of the plant organs above the ground. It covers the shoot, leaves, flowers, fruits, and seeds and serves several functions. Among them are protection from water loss, against physical and chemical influences and from feeding by animals, as well as participation in gas exchange and secretion. Unisereate epidermis Types of epidermis Multisereate epidermis  we distinguish between the cuticle and the cuticle layer, which is found below it. The cuticle is formed by the secretion of cutin on the surface of the epidermal cells, i.e., by cuticularisation. The cuticle layer is formed by deposition of cutin in the interfibrillar spaces of the outermost cell wall layer, i.e., by cutinisation Stomata Guard cells Stoma open Subsidiary cells Sub-stomotal chamber Types of stomata Stomata of grasses Hydathodes  Trichomes A- Non-glandular trichomes B- Glandular trichomes  These trichomes offer three advantages. The lumen of dead cells is air-filled. It gives them a silvery-whitish appearance. A large proportion of the incoming light is thus reflected. A thick layer of wax has the same effect. Only weak circulation takes place at the leaf surface, thus reducing water loss to a minimum. Dying of the hair cells reduces the surface area, where water could be lost drastically. If the cells were living, too a high loss of water loss would occur due to the largely increased surface of the branched hair cells. Function of Glandular trichomes The simple multicellular glandular trichome is composed of a stalk and a head The stinging hair Salt glands Chalk glands Xylem The xylem produced by the procambium in the primary plant body is termed the primary xylem. In many plants, after the completion of the formation of the primary plant body, secondary tissues are developed. The xylem that is produced as a result of the activity of the vascular cambium is termed the secondary xylem. In the primary xylem, the elements that are produced earlier are termed the protoxylem while these produced later are termed the metaxylem.  Tracheary elements: Two basic types of tracheary elements are distinguished namely, tracheids and vessel members. Tracheids are usually more elongate and narrower than Vessel Members. The main difference between tracheids and vessel members is that the end walls of vessel members are perforated while these of tracheids are not. The walls of tracheary elements Phloem The important cells are the sieve elements, of which two types can be distinguished, sieve cells and sieve-tube members. Additional to these elements, phloem also contains typical parenchyma in which reserve substances are stored. More specialized parenchyma cells known as companion cells (in angiosperms) and albuminous cells (in gymnosperms), which assist in transport exist in the phloem. Fibres and sclereids also exist. Irregular phloem Regular phloem Xylem-phloem relationship