Anatomy of Flowering Plants - Detailed Notes

Anatomy of Flowering Plants

Introduction

• Plants and animals show structural similarities and variations in external morphology.
• Internal structure reveals similarities and differences upon study.
• This chapter introduces the internal structure and functional organization of higher plants.
• Anatomy is the study of the internal structure of plants.
• Plant organization:
  • Cells are the basic unit.
  • Cells organize into tissues.
  • Tissues organize into organs.
• Different organs show differences in internal structure.
• Monocots and dicots are anatomically different within angiosperms.
• Internal structures show adaptations to diverse environments.

The Tissues

• A tissue is a group of cells with a common origin and function.
• Plants are made of different kinds of tissues.
• Tissues are classified into:
  • Meristematic tissues: Cells capable of dividing.
  • Permanent tissues: Cells that have lost the ability to divide.

Meristematic Tissues

• Growth in plants is restricted to meristems, regions of active cell division (Gk. meristos: divided).
• Types of meristems:
  • Apical meristems:
    • Located at the tips of roots and shoots.
    • Produce primary tissues.
    • Root apical meristem: Occupies the tip of a root.
    • Shoot apical meristem: Occupies the distant most region of the stem axis.
    • During leaf formation and stem elongation, some cells are left behind from the shoot apical meristem, forming the axillary bud.
    • Axillary buds: Present in the axils of leaves, capable of forming a branch or a flower.
  • Intercalary meristem:
    • Occurs between mature tissues.
    • Found in grasses.
    • Regenerates parts removed by grazing herbivores.
  • Primary meristems:
    • Apical and intercalary meristems.
    • Appear early in the life of a plant.
    • Contribute to the formation of the primary plant body.
  • Secondary or lateral meristem:
    • Occurs in mature regions of roots and shoots of many plants, particularly those that produce woody axis.
    • Appear later than primary meristem.
    • Cylindrical meristems.
    • Examples: Fascicular vascular cambium, interfascicular cambium, and cork-cambium.
    • Responsible for producing the secondary tissues.
• Following divisions in primary and secondary meristems, newly formed cells become specialized and lose the ability to divide, becoming permanent or mature cells.
• Apical meristem produces dermal, ground, and vascular tissues during the formation of the primary plant body.

Permanent Tissues

• Cells of the permanent tissues generally do not divide further.
• Types of permanent tissues:
  • Simple tissues: Composed of cells similar in structure and function.
  • Complex tissues: Composed of many different types of cells.

Simple Tissues

• Made of only one type of cells.
• Types: Parenchyma, collenchyma, and sclerenchyma.
  • Parenchyma:
    • Forms the major component within organs.
    • Cells are generally isodiametric (spherical, oval, round, polygonal, or elongated).
    • Thin walls made of cellulose.
    • Closely packed or have small intercellular spaces.
    • Functions: Photosynthesis, storage, secretion.
  • Collenchyma:
    • Occurs in layers below the epidermis in dicotyledonous plants.
    • Found as a homogeneous layer or in patches.
    • Cells are thickened at the corners due to deposition of cellulose, hemicellulose, and pectin.
    • Cells may be oval, spherical, or polygonal.
    • Often contain chloroplasts, assimilating food when they do.
    • Intercellular spaces are absent.
    • Provides mechanical support to growing parts of the plant such as young stem and petiole of a leaf.
  • Sclerenchyma:
    • Consists of long, narrow cells with thick and lignified cell walls having a few or numerous pits.
    • Usually dead and without protoplasts.
    • Types: Fibres and sclereids, based on variation in form, structure, origin, and development.
      • Fibres: Thick-walled, elongated and pointed cells, generally occurring in groups in various parts of the plant.
      • Sclereids: Spherical, oval, or cylindrical, highly thickened dead cells with very narrow cavities (lumen).
      • Commonly found in fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea.
    • Provides mechanical support to organs.

Complex Tissues

• Made of more than one type of cells working together as a unit.
• Types: Xylem and phloem.
  • Xylem:
    • Functions as a conducting tissue for water and minerals from roots to the stem and leaves.
    • Provides mechanical strength to the plant parts.
    • Composed of four different kinds of elements: Tracheids, vessels, xylem fibres, and xylem parenchyma.
    • Gymnosperms lack vessels in their xylem.
    • Tracheids:
      • Elongated or tube-like cells with thick and lignified walls and tapering ends.
      • Dead and without protoplasm.
      • Inner layers of the cell walls have thickenings which vary in form.
    • Vessels:
      • Long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity.
      • Vessel cells are also devoid of protoplasm.
      • Vessel members are interconnected through perforations in their common walls.
      • Presence of vessels is a characteristic feature of angiosperms.
    • Xylem fibres:
      • Have highly thickened walls and obliterated central lumens.
      • May either be septate or aseptate.
    • Xylem parenchyma:
      • Cells are living and thin-walled, and their cell walls are made up of cellulose.
      • Store food materials in the form of starch or fat, and other substances like tannins.
      • Radial conduction of water takes place by the ray parenchymatous cells.
    • Primary xylem:
      • Protoxylem: First formed primary xylem elements.
      • Metaxylem: Later formed primary xylem.
      • In stems, the protoxylem lies towards the center (pith) and the metaxylem lies towards the periphery of the organ (endarch).
      • In roots, the protoxylem lies towards periphery and metaxylem lies towards the center (exarch).
  • Phloem:
    • Transports food materials, usually from leaves to other parts of the plant.
    • In angiosperms, it is composed of sieve tube elements, companion cells, phloem parenchyma and phloem fibres.
    • Gymnosperms have albuminous cells and sieve cells and lack sieve tubes and companion cells.
    • Sieve tube elements:
      • 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.
      • The functions of sieve tubes are controlled by the nucleus of companion cells.
    • Companion cells:
      • 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.
      • Help in maintaining the pressure gradient in the sieve tubes.
    • Phloem parenchyma:
      • 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.
      • Stores food material and other substances like resins, latex and mucilage.
      • Absent in most of the monocotyledons.
    • Phloem fibres (bast fibres):
      • Made up of sclerenchymatous cells.
      • Generally absent in the primary phloem but are found in the secondary phloem.
      • 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.
    • Primary phloem:
      • Protophloem: First formed primary phloem, consists of narrow sieve tubes.
      • Metaphloem: Later formed phloem, has bigger sieve tubes.

The Tissue System

• Tissues vary depending on their location in the plant body.
• Structure and function depend on location.
• Three types of tissue systems based on structure and location:
  • Epidermal tissue system.
  • Ground or fundamental tissue system.
  • Vascular or conducting tissue system.

Epidermal Tissue System

• Forms the outer-most covering of the whole plant body.
• Comprises epidermal cells, stomata and the epidermal appendages – the trichomes and hairs.
• Epidermis:
  • Outermost layer of the primary plant body.
  • Made up of elongated, compactly arranged cells, which form a continuous layer.
  • Usually single- layered.
  • Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and a large vacuole.
  • The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water.
  • Cuticle is absent in roots.
• Stomata:
  • Structures present in the epidermis of leaves.
  • Regulate the process of transpiration and gaseous exchange.
  • Each stoma is composed of two bean- shaped cells known as guard cells which enclose stomatal pore.
  • In grasses, the guard cells are dumb-bell shaped.
  • The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.
  • The guard cells possess chloroplasts and regulate the opening and closing of stomata.
  • Sometimes, a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells.
  • The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.
• Epidermal appendages:
  • Hairs.
    • The cells of epidermis bear a number of hairs.
    • The root hairs are unicellular elongations of the epidermal cells and help absorb water and minerals from the soil.
  • Trichomes.
    • On the stem the epidermal hairs are called trichomes.
    • The trichomes in the shoot system are usually multicellular.
    • They may be branched or unbranched and soft or stiff.
    • They may even be secretory.
    • The trichomes help in preventing water loss due to transpiration.

The Ground Tissue System

• All tissues except epidermis and vascular bundles constitute the ground tissue.
• Consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.
• Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays, in the primary stems and roots.
• In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called mesophyll.

The Vascular Tissue System

• The vascular system consists of complex tissues, the phloem and the xylem.
• The xylem and phloem together constitute vascular bundles.
• Types of vascular bundles:
  • Open vascular bundles:
    • In dicotyledonous stems, cambium is present between phloem and xylem.
    • Possess the ability to form secondary xylem and phloem tissues.
  • Closed vascular bundles:
    • In the monocotyledons, the vascular bundles have no cambium present in them.
    • Do not form secondary tissues.
  • Radial:
    • When xylem and phloem within a vascular bundle are arranged in an alternate manner on different radii, the arrangement is called radial such as in roots.
  • Conjoint:
    • The xylem and phloem are situated at the same radius of vascular bundles.
    • Common in stems and leaves.
    • The conjoint vascular bundles usually have the phloem located only on the outer side of xylem.

Anatomy of Dicotyledonous and Monocotyledonous Plants

• Study transverse sections of mature zones of roots, stems, and leaves for better understanding of tissue organization.

Dicotyledonous Root

• Outermost layer is epidermis.
• Many epidermal cells protrude in the form of unicellular root hairs.
• Cortex:
  • Several layers of thin-walled parenchyma cells with intercellular spaces.
  • Innermost layer is called endodermis.
• Endodermis:
  • Comprises a single layer of barrel-shaped cells without any intercellular spaces.
  • The tangential as well as radial walls of the endodermal cells have a deposition of water- impermeable, waxy material-suberin-in the form of casparian strips.
• Pericycle:
  • Next to endodermis.
  • A few layers of thick-walled parenchymatous cells.
  • Initiation of lateral roots and vascular cambium during secondary growth takes place in these cells.
• Pith:
  • Small or inconspicuous.
• Conjunctive tissue:
  • The parenchymatous cells which lie between the xylem and the phloem.
• Vascular bundles:
  • Usually two to four xylem and phloem patches.
• Cambium ring:
  • Later, a cambium ring develops between the xylem and phloem.
• Stele:
  • All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith.

Monocotyledonous Root

• Similar to dicot root in many respects.
• Has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.
• Vascular bundles:
  • More than six (polyarch) xylem bundles.
• Pith:
  • Large and well developed.
• Monocotyledonous roots do not undergo any secondary growth.

Dicotyledonous Stem

• Epidermis:
  • Outermost protective layer of the stem.
  • Covered with a thin layer of cuticle.
  • May bear trichomes and a few stomata.
• Cortex:
  • The cells arranged in multiple layers between epidermis and pericycle constitute the cortex.
  • Sub-zones:
    • Hypodermis: A few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem.
    • Cortical layers: Below hypodermis, consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.
    • Endodermis: Innermost layer of the cortex. The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.
• Pericycle:
  • Present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
• Medullary rays:
  • A few layers of radially placed parenchymatous cells, which lie in between the vascular bundles.
• Vascular bundles:
  • A large number of vascular bundles are arranged in a ring; the ‘ring’ arrangement of vascular bundles is a characteristic of dicot stem.
  • Each vascular bundle is conjoint, open, and with endarch protoxylem.
• Pith:
  • A large number rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem.

Monocotyledonous Stem

• Sclerenchymatous hypodermis.
• A large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath.
• A large, conspicuous parenchymatous ground tissue.
• Vascular bundles are conjoint and closed.
• Peripheral vascular bundles are generally smaller than the centrally located ones.
• The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.

Dorsiventral (Dicotyledonous) Leaf

• Vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll and vascular system.
• Epidermis:
  • Covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf and has a conspicuous cuticle.
  • The abaxial epidermis generally bears more stomata than the adaxial epidermis. The latter may even lack stomata.
• Mesophyll:
  • The tissue between the upper and the lower epidermis.
  • Possesses chloroplasts and carry out photosynthesis, is made up of parenchyma.
  • Types of cells:
    • Palisade parenchyma: The adaxially placed palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other.
    • Spongy parenchyma: The oval or round and loosely arranged spongy parenchyma is situated below the palisade cells and extends to the lower epidermis. There are numerous large spaces and air cavities between these cells.
• Vascular system:
  • Includes vascular bundles, which can be seen in the veins and the midrib.
  • The size of the vascular bundles are dependent on the size of the veins.
  • The veins vary in thickness in the reticulate venation of the dicot leaves.
  • The vascular bundles are surrounded by a layer of thick walled bundle sheath cells.

Isobilateral (Monocotyledonous) Leaf

• The anatomy of isobilateral leaf is similar to that of the dorsiventral leaf in many ways.
• Characteristic differences:
  • The stomata are present on both the surfaces of the epidermis.
  • The mesophyll is not differentiated into palisade and spongy parenchyma.
• Bulliform cells:
  • In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells.
  • When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed.
  • When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.
• Vascular bundles:
  • The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves.

Secondary Growth

• The growth of the roots and stems in length with the help of apical meristem is called the primary growth.
• Apart from primary growth most dicotyledonous plants exhibit an increase in girth. This increase is called the secondary growth.
• The tissues involved in secondary growth are the two lateral meristems: vascular cambium and cork cambium.

Vascular Cambium

• The meristematic layer that is responsible for cutting off vascular tissues – xylem and pholem – is called vascular cambium.
• In the young stem it is present in patches as a single layer between the xylem and phloem. Later it forms a complete ring.

Formation of cambial ring

• Intrafascicular cambium:
  • In dicot stems, the cells of cambium present between primary xylem and primary phloem.
• Interfascicular cambium:
  • The cells of medullary rays, adjoining these intrafascicular cambium become meristematic.
• Thus, a continuous ring of cambium is formed.

Activity of the cambial ring

• The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides.
• The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem.
• The cambium is generally more active on the inner side than on the outer.
• As a result, the amount of secondary xylem produced is more than secondary phloem and soon forms a compact mass.
• The primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem.
• The primary xylem however remains more or less intact, in or around the center.
• At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.

Spring wood and autumn wood

• The activity of cambium is under the control of many physiological and environmental factors.
• In temperate regions, the climatic conditions are not uniform through the year.
• Spring wood or early wood:
  • In the spring season, cambium is very active and produces a large number of xylary elements having vessels with wider cavities.
  • Lighter in colour and has a lower density.
• Autumn wood or late wood:
  • In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels.
  • Darker and has a higher density.
• Annual ring:
  • The two kinds of woods that appear as alternate concentric rings.
  • Annual rings seen in a cut stem give an estimate of the age of the tree.

Heartwood and sapwood

• Heartwood:
  • In old trees, the greater part of secondary xylem is dark brown due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem.
  • These substances make it hard, durable and resistant to the attacks of micro- organisms and insects.
  • This region comprises dead elements with highly lignified walls.
  • Does not conduct water but it gives mechanical support to the stem.
• Sapwood:
  • The peripheral region of the secondary xylem, is lighter in colour.
  • It is involved in the conduction of water and minerals from root to leaf.

Cork Cambium

• As the stem continues to increase in girth due to the activity of vascular cambium, the outer cortical and epidermis layers get broken and need to be replaced to provide new protective cell layers.
• Hence, sooner or later, another meristematic tissue called cork cambium or phellogen develops, usually in the cortex region.
• Phellogen is a couple of layers thick. It is made of narrow, thin-walled and nearly rectangular cells.
• Phellogen cuts off cells on both sides.
• Outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.
• The cork is impervious to water due to suberin deposition in the cell wall.
• The cells of secondary cortex are parenchymatous.
• Periderm: Phellogen, phellem, and phelloderm are collectively known as periderm.
• Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral to phellogen and ultimately these layers die and slough off.
• Bark:
  • A non-technical term that refers to all tissues exterior to the vascular cambium, therefore including secondary phloem.
  • Refers to a number of tissue types, viz., periderm and secondary phloem.
  • Early or soft bark: Bark that is formed early in the season.
  • Late or hard bark: Towards the end of the season.
• Lenticels:
  • At certain regions, the phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells.
  • These parenchymatous cells soon rupture the epidermis, forming a lens- shaped openings.
  • Permit the exchange of gases between the outer atmosphere and the internal tissue of the stem.
  • These occur in most woody trees.

Secondary Growth in Roots

• In the dicot root, the vascular cambium is completely secondary in origin.
• It originates from the tissue located just below the phloem bundles, a portion of pericycle tissue, above the protoxylem forming a complete and continuous wavy ring, which later becomes circular.
• Further events are similar to those already described above for a dicotyledon stem.
• Secondary growth also occurs in stems and roots of gymnosperms.
• However, secondary growth does not occur in monocotyledons.