ANATOMY OF FLOWERING PLANTS

Study of internal structure of plants is called anatomy. Plants have cells as the basic unit, cells are organised into tissues and in turn the tissues are organised into organs. Different organs in a plant show differences in their internal structure. Internal structures also show adaptations to diverse environments.

  • Tissues are mainly divided into three categories:

(A) Meristematic tissues or Meristems

(B) Permanent tissue

(C) Secretory tissue


 

THE TISSUE

A tissue is a group of cells having a common origin and usually performing a common function.

Meristematic Tissues

This tissue is responsible for active cell division which results in Growth in plants. Based on location and origin, Plants have different kinds of meristems.

  • Apical meristem –

The meristems which occur at the tips of roots and shoots and produce primary tissues.e.g., root and shoot apical meristem. During the formation of leaves and elongation of stem, some cells ‘left behind’ from shoot apical meristem, constitute the axillary bud. Such buds are present in the axils of leaves and are capable of forming a branch or a flower.

  • Intercalary meristem –

The meristem which occurs between mature tissues is known as intercalary meristem. They occur in grasses and regenerate parts removed by the grazing herbivores. Both apical meristems and intercalary meristems are primary meristems because they appear early in life of a plant and contribute to the formation of the primary plant body.

  • Lateral meristem –

The meristem that occurs in the mature regions of roots and shoots of many plants, particularly those that produce woody axis and appear later than primary meristem is called the secondary or lateral meristem.

Fascicular vascular cambium, interfascicular cambium and cork-cambium are examples of lateral meristems. These are responsible for producing the secondary tissues.

Permanent Tissues

  • The cells of the permanent tissues do not generallydivide further.

  • 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.                                                                                                                                  Simple Tissues

Parenchyma –

  • It forms the majorcomponent within organs.

  • The cells of theparenchyma are generally isodiametric.

  • Their walls are thin and madeup of cellulose.

  • They may either be closely packedor have small intercellular spaces.

  • Theparenchyma performs various functions likephotosynthesis, storage, secretion.

Collenchyma –

  • It is present in layers below theepidermis (hypodermis) in dicotyledonous plants.

  • It is foundeither as a homogeneous layer or in patches.

  • Itconsists of cells which are much thickened at thecorners due to a deposition of cellulose,hemicellulose and pectin.

  • Collenchymatous cellsmay be oval, spherical or polygonal and oftencontain chloroplasts.

  • Intercellularspaces are absent.

  • They provide mechanicalsupport to the growing parts of the plant such asyoung stem and petiole of a leaf.

Sclerenchyma –

  • It consists of long, narrow cellswith thick and lignified cell walls having a few ornumerous pits.

  • They are usually dead and withoutprotoplasts.

  • It provides mechanical support to organs.

  • On the basis of variation in form,structure, origin and development, sclerenchymamay be either fibres or sclereids.

  • Fibers– these arethick-walled, elongated and pointed cells,generally occuring in groups, in various parts ofthe plant.

  • Sclereids – theseare spherical, oval orcylindrical, 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.

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 Complex Tissues

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 parts.

  • It is composed of four different kinds of elements, namely, tracheids, vessels, xylem fibres and xylem parenchyma.

  • Tracheids –

    • Tracheids are elongated or tube like cells with thick and lignified walls and tapering ends.

    • These are dead and are without protoplasm.

    • The inner layers of the cell walls have thickenings which vary in form.

    • In flowering plants, tracheids and vessels are the main water transporting elements.

  • Vessels –

    • Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity.

    • The vessel cells are also devoid of protoplasm.

    • Vessel members are interconnected through perforations in their common walls.

    • Gymnosperms lack vessels intheir xylem. The presence of vessels is a characteristic featureof angiosperms.

  • Xylem fibres –

    • They have highly thickened walls and obliterated central lumens.

    • These may either be septate or aseptate.

  • Xylem parenchyma –

    • Cells are living and thin-walled,and their cell walls are made up of cellulose.

    • They store food materials in the form of starch or fat, and other substances like tannins.

    • The radial conduction of water takes place by the ray parenchymatous cells.

 

  • Primary xylem is of two types – protoxylem and metaxylem. The first formed primary xylem elements are called protoxylem and the later formed primary xylem is called metaxylem.

  1. Endarch –Instems, the protoxylem lies towards the centre (pith) and themetaxylem lies towards the periphery of the organ. This typeof primary xylem is called endarch.

  2. Exarch –In roots, the protoxylemlies towards periphery and metaxylem lies towards the centre.Such arrangement of primary xylem is called exarch.

Phloem –

  • It transports food materials, usually from leaves toother parts of the plant.

  • Phloem in angiosperms is composedof sieve tube elements, companion cells, phloem parenchyma and phloem fibres.

  • Gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells.

  • Sieve tube elements –

    • They are also long, tube-like structures, arranged longitudinally and are associated with the companion cells.

    • Their end walls are perforated in a sieve-like manner to form the sieve plates.

    • A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.

  • Companion cells –

    • The functions of sieve tubes are controlled by the nucleus of companion cells.

    • The companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements.

    • The sieve tube elements and companion cells are connected by pit fields present between their common longitudinal walls.

    • The companion cells help in maintaining the pressure gradient in the sieve tubes.

  • Phloem parenchyma –

    • Itis made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus.

    • The cell wall is composed of cellulose and has pits through which plasmodesmatal connections exist between the cells.

    • The phloem parenchyma stores food material and other substances like resins, latex and mucilage.

    • Phloem parenchyma is absent in most of the monocotyledons.

  • Phloem fibres (bast fibres)–

    • They are made up of sclerenchymatous cells.

    • These are generally absent in the primary phloem but are found in the secondary phloem.

    • These are much elongated, unbranched and have pointed, needle like apices.

    • The cell wall of phloem fibres is quite thick.

    • At maturity, these fibres lose their protoplasm and become dead.

    • Phloem fibres of jute, flax and hemp are used commercially.

                        

  Special or secretory tissues

These tissue perform special function in plants, e.g., secretion of resins gum, oil and latex. These tissues are of two types :

(1) Laticiferous tissues

(2) Glandular tissues

(1) Laticiferous tissues: They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called latex. These occur irregularly distributed in the mass of parenchymatous cells. latex is contained inside the laticiferous tissue which is of two types:

(i) Latex cells:

(a) A laticiferous cell is a very highly branched cell with long slender processes ramifying in all directions in the ground tissue of the organ.

(b) Plants having such tissues are called simple or non-articulated laticifers. e.g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).

(ii) Latex vessels:

(a) They are formed due to fusion of cells and form network like structure in all directions.

(b) Plants having such tissues are called compound or articulated laticifers. e.g., Argemone, Papaver (Papaveraceae), Sonchus (Compositae), Hevea, Manihot (Euphorbiaceae).

(2) Glandular tissue: This is a highly specialized tissue consisting of glands, discharging diverse functions, including  secretory and excretory. Glands may be external or internal.

(i) External glands: They are generally occur on the epidermis of stem and leaves as glandular hair in Plumbago and Boerhaavia.

(ii) Internal glands: These are present internally and are of several types. e.g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus.

 

The Tissue System

A collection of tissues performing the same general function is known as a “Tissue System''. According to Sachs (1975) there are three major tissue systems in plants as follows:

(1) Epidermal tissue system    (2) Ground or fundamental tissue system   (3) Vascular tissue system

(1) Epidermal tissue system: The tissues of this system originate from the outermost layer of apical meristem.

(i) Epidermis: Epidermis is composed of single layer cells.

(ii) Cuticle and Wax: In aerial parts, epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. it helps in reducing the loss of water by evaporation.

(iii) Stomata: Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. Stomata are absent in roots.

(2) Ground or Fundamental tissue system: Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of body. This tissue system mainly originates from ground meristem.

(3) Vascular tissue system: The central cylinder of the shoot or root surrounded by cortex is called stele. The varying number of vascular bundles formed inside the stele constitute vascular tissue system. Xylem, phloem and cambium are the major parts of the vascular bundle.