Anatomy of Flowering Plants Biology Notes - Meristematic and Permanent Tissues, Tissue Systems, Secondary Growth and Dicot Monocot Anatomy

• Anatomy is the study of the internal structure of plants and other organisms.
• A tissue is a group of cells having common origin and function.
• Based on the capability of cell division, plant tissues are divided into two groups: Meristematic and Permanent.

Meristematic Tissues (Meristems)

These are tissues where active cell division and growth occur. Based on position, meristems are classified into three types:

• Apical Meristems: Occur at the tips of roots and shoots, producing primary tissues. They are of two types:
  • Root Apical Meristem: Occupies the tip of a root.
  • Shoot Apical Meristem: Occupies the distant most region of the stem axis. Some cells left behind constitute the axillary bud, present in the axils of leaves, capable of forming a branch or flower.

• Intercalary Meristems: Occur between mature tissues, often in grasses, regenerating parts removed by grazing herbivores.
• Secondary (Lateral) Meristems: Occur in mature regions of roots and shoots, seen in gymnosperms and dicots. Examples include fascicular vascular cambium, interfascicular cambium, and cork cambium, responsible for producing secondary tissues.

Apical and intercalary meristems are primary meristems because they appear early in a plant’s life and contribute to the primary plant body, producing dermal tissues, ground tissues, and vascular tissues.

Permanent (Mature) Tissues

• Cells produced by primary and secondary meristems become structurally and functionally specialized, losing the ability to divide. These are called permanent (mature) cells and constitute permanent tissues.
• They are divided into two types: Simple and Complex.

1. Simple Permanent Tissues

Tissues with all cells similar in structure and function. There are three types: Parenchyma, Collenchyma, and Sclerenchyma.

a. Parenchyma

• Forms the major component within organs.
• Cells are generally isodiametric, spherical, oval, round, polygonal, or elongated.
• Walls are thin and made of cellulose.
• Cells are closely packed or have small intercellular spaces.
• Functions: Photosynthesis, storage, secretion, etc.

b. Collenchyma

• Occurs in layers below the epidermis in dicot plants.
• Found as a homogeneous layer or in patches.
• Cells have thickened corners due to deposition of cellulose, hemicellulose, and pectin.
• Intercellular spaces are absent.
• Cells are oval, spherical, or polygonal and often contain chloroplasts.
• Functions: Provide mechanical support to growing parts like young stems and leaf petioles. Cells with chloroplasts assimilate food.

c. Sclerenchyma

• Consists of long, narrow cells with thick, lignified cell walls having few or numerous pits.
• Cells are usually dead without protoplasts.
• Based on form, structure, origin, and development, sclerenchyma is divided into two types:
  • Fibres: Thick-walled, elongated, pointed cells, generally occurring in groups.
  • Sclereids: Spherical, oval, or cylindrical, highly thickened dead cells with narrow cavities (lumen). Found in fruit walls of nuts, pulp of fruits like guava, pear, and sapota, seed coats of legumes, and leaves of tea.
• Function: Provides mechanical support to organs.

2. Complex Permanent Tissues

These are made of more than one type of cell, working together as a unit. There are two types: Xylem and Phloem.

a. Xylem

• Functions as a conducting tissue for water and minerals from roots to stems and leaves.
• Provides mechanical strength to plant parts.
• Composed of four elements: tracheids, vessels, xylem fibres, and xylem parenchyma.
  • Tracheids: Elongated, tube-like dead cells with thick, lignified walls and tapering ends. Protoplasm absent. Inner wall layers have varying thickenings. Main water-transporting elements in flowering plants, along with vessels.
  • Vessels: Long, cylindrical, tube-like structures made of many vessel members with lignified walls and large central cavities. Protoplasm absent. Vessel members are interconnected through perforations in common walls. Characteristic of angiosperms; gymnosperms lack vessels.
  • Xylem Fibres: Highly thickened walls with obliterated central lumens, septate or aseptate.
  • Xylem Parenchyma: Living, thin-walled cells with cellulosic walls. Store food materials (starch or fat) and substances like tannins. Radial conduction of water occurs via ray parenchymatous cells.
• Primary xylem is of two types:
  • Protoxylem: First-formed primary xylem.
  • Metaxylem: Later-formed primary xylem.
• In stems, protoxylem lies toward the center (pith), and metaxylem toward the periphery, called endarch.
• In roots, protoxylem lies toward the periphery, and metaxylem toward the center, called exarch.

b. Phloem (Bast)

• Transports food materials from leaves to other parts.
• In angiosperms, composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres. Gymnosperms have albuminous cells and sieve cells, lacking sieve tubes and companion cells.
  • Sieve Tube Elements: Long, tube-like structures arranged longitudinally, associated with companion cells. End walls are perforated, forming sieve plates. Mature sieve elements have peripheral cytoplasm and a large vacuole but lack a nucleus. Functions are controlled by companion cell nuclei.
  • Companion Cells: Specialized parenchymatous cells closely associated with sieve tube elements, connected by pit fields in common longitudinal walls. Function: Maintain pressure gradient in sieve tubes.
  • Phloem Parenchyma: Elongated, tapering cylindrical cells with dense cytoplasm and nucleus. Cell walls are cellulosic with pits for plasmodesmatal connections. Absent in most monocots. Function: Stores food materials and substances like resins, latex, and mucilage.
  • Phloem Fibres (Bast Fibres): Sclerenchymatous cells, generally absent in primary phloem but found in secondary phloem. Elongated, unbranched, with pointed, needle-like apices and thick cell walls. Dead at maturity. Used commercially in jute, flax, and hemp. Function: Mechanical support and protection to soft tissues.
• Primary phloem consists of protophloem (narrow sieve tubes) and metaphloem (bigger sieve tubes).

The Tissue System

Based on structure and location, tissue systems are of three types:

• Epidermal tissue system
• Ground (fundamental) tissue system
• Vascular (conducting) tissue system

1. Epidermal Tissue System

• Forms the outermost covering of the whole plant body.
• Comprises epidermal cells, stomata, and epidermal appendages (trichomes & hairs).

Epidermis

• The outermost layer of the primary plant body.
• Usually single-layered.
• Made up of elongated, compactly arranged parenchymatous cells 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 (cuticle) to prevent water loss. The cuticle is absent in roots.

Stomata

• Structures present in the epidermis of leaves.
• Regulate transpiration and gaseous exchange.
• A stoma is made of two bean-shaped cells (guard cells).
• In grasses, guard cells are dumbbell-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.
• Guard cells possess chloroplasts and regulate the opening and closing of stomata.
• Some epidermal cells near the guard cells may become specialized in shape and size, called subsidiary cells.
• The stomatal aperture, guard cells, and surrounding subsidiary cells together form the stomatal apparatus.

Epidermal Appendages

• Root hairs: Unicellular elongations of epidermal cells that absorb water and minerals from the soil.
• Trichomes: Epidermal hairs on the stem, usually multicellular, branched or unbranched, soft or stiff, and sometimes secretory. They help prevent water loss due to transpiration.

2. The Ground Tissue System

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

3. The Vascular Tissue System

• Consists of complex tissues (xylem and phloem).
• Xylem and phloem together constitute vascular bundles.

Based on the presence or absence of cambium, vascular bundles are of two types:

• Open type: Cambium is present between phloem and xylem, allowing the formation of secondary xylem and phloem tissues. Example: dicotyledonous stems.
• Closed type: Cambium is absent, so secondary tissues are not formed. Example: monocotyledons.

Based on the arrangement of xylem and phloem, vascular bundles are of two types:

• Radial type: Xylem and phloem are arranged alternately on different radii. Seen in roots.
• Conjoint type: Xylem and phloem are situated at the same radius. Seen in stems and leaves. Conjoint vascular bundles usually have phloem located only on the outer side of xylem.

Anatomy of Dicotyledonous & Monocotyledonous Plants

Dicotyledonous (Dicot) Root

Transverse section of the sunflower root shows the following tissue organization:

• Epidermis (epiblema): The outermost layer. Many cells of epiblema protrude as unicellular root hairs.
• Cortex: It consists of several layers of thin-walled parenchyma cells with intercellular spaces.
• Endodermis: Innermost layer of the cortex. It comprises a single layer of barrel-shaped cells without intercellular spaces. The tangential and radial walls of the endodermal cells have a deposition of suberin (water impermeable, waxy material) in the form of casparian strips.
• Stele: All tissues on the inner side of the endodermis together constitute stele. They include:
  • Pericycle: A few layers of thick-walled parenchymatous cells next to endodermis. Initiation of lateral roots and vascular cambium during secondary growth takes place in these cells.
  • Pith: Innermost region of the stele. It is small or inconspicuous.
  • Conjunctive tissue: The parenchymatous cells which lie between the xylem and the phloem.
  • Vascular bundles: 2-4 xylem & phloem patches. Later, a cambium ring develops between the xylem & phloem.

Monocotyledonous (Monocot) Root

• It has epidermis, cortex, endodermis, pericycle, vascular bundles, and pith.
• There are usually more than six (polyarch) xylem bundles.
• Pith is large and well developed.
• Monocot roots have no secondary growth.

Dicotyledonous (Dicot) Stem

• Epidermis: Outermost protective layer. Covered with a thin layer of cuticle, it may bear trichomes & few stomata.
• Cortex: Multiple layers of cells arranged between epidermis and pericycle. It consists of three sub-zones:
  • Hypodermis: Outer zone. It consists of a few layers of collenchymatous cells just below the epidermis. It provides mechanical strength to the young stem.
  • Cortical layers: Below hypodermis. They consist of rounded thin-walled parenchymatous cells with conspicuous intercellular spaces.
  • Endodermis: Innermost layer. Cells are rich in starch grains, so it is also called starch sheath.
• Stele: Consists of pericycle, vascular bundles, medullary rays, & pith:
  • Pericycle: Present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
  • Medullary rays: Few layers of radially placed parenchymatous cells between vascular bundles.
  • Vascular bundles: Large in number. Their ring arrangement is a characteristic of dicot stem. Each vascular bundle is conjoint & open. Protoxylem is endarch.
  • Pith: Central portion of the stem. It has many rounded, parenchymatous cells with large intercellular spaces.

Monocotyledonous (Monocot) Stem

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

Dicotyledonous (Dorsiventral) Leaf

The vertical section of a dicot leaf through lamina shows three main parts: epidermis, mesophyll, & vascular system.

• Epidermis: Covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf. It has a conspicuous cuticle. Abaxial epidermis generally bears more stomata. In adaxial epidermis, stomata are fewer or absent.
• Mesophyll: The tissue between the upper and lower epidermis. Made up of parenchyma containing chloroplasts for photosynthesis. It has two types of cells:
  • Palisade parenchyma: Adaxially placed. Made up of elongated cells arranged vertically and parallel to each other.
  • Spongy parenchyma: Oval or round and loosely arranged. Situated below the palisade cells and extends to the lower epidermis. Numerous large spaces and air cavities between these cells.
• Vascular system: Includes vascular bundles seen in the veins and midrib. Size of vascular bundles depends on the size of the veins. The veins vary in thickness in the reticulate venation of dicot leaves. Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells.

Monocotyledonous (Isobilateral) Leaf

The anatomy of a monocot leaf is similar to that of a dicot leaf in many ways, with the following differences:

• Stomata are present on both surfaces of the epidermis.
• Mesophyll is not differentiated into palisade and spongy parenchyma.
• In grasses, certain adaxial epidermal cells along the veins modify into large, empty, colourless cells called bulliform cells. When turgid, they expose the leaf surface; when flaccid due to water stress, they cause the leaves to curl inwards to minimize water loss.
• Parallel venation is reflected in the near-similar sizes of vascular bundles (except in main veins).

Secondary Growth

• The growth of the roots and stems in length with the help of apical meristem is called the primary growth.
• Secondary growth is the increase in girth of dicot plants.
• Tissues involved in secondary growth are the two lateral meristems: vascular cambium and cork cambium.

Vascular Cambium

• It is the meristematic layer responsible for cutting off vascular tissues (xylem and phloem).
• In the young stem, it is present in patches as a single layer between xylem and phloem. Later it forms a complete ring.

Formation of Cambial Ring

• In dicot stems, cells of cambium present between primary xylem and primary phloem is intrafascicular cambium.
• Cells of medullary rays, adjoining this intrafascicular cambium, become meristematic and form interfascicular cambium. Thus, a continuous ring of cambium is formed.

Activity of the Cambial Ring

• The cambial ring becomes active and cuts off new cells, both towards the inner and outer sides. The cells cut off towards the pith mature into secondary xylem. The cells cut off towards the periphery mature into secondary phloem.
• Cambium is more active on the inner side than on the outer. As a result, more secondary xylem is produced than secondary phloem and soon forms a compact mass.
• Primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem. However, primary xylem remains intact, in or around the centre. At some places, 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

• Many physiological and environmental factors control the activity of cambium.
• In spring season, cambium is very active and produces many xylary elements having vessels with wider cavities. This wood is called spring wood (early wood). It is lighter in colour and has a lower density.
• In winter, cambium is less active and forms fewer xylary elements having narrow vessels. This wood is called autumn wood (late wood). It is darker and has higher density.
• These two kinds of woods that appear as alternate concentric rings constitute an annual ring. This is used to estimate the age of a tree (dendrochronology).

Heartwood and Sapwood

• Heartwood: It is the hard, dead, dark brown-coloured, highly lignified, and non-functional central part of the secondary xylem of old trees. The dark colour is due to deposition of organic compounds (tannins, resins, oils, gums, aromatic substances, essential oils, etc.). These substances make it hard, durable, and resistant to attacks by microorganisms and insects.

Function: It gives mechanical support to the stem.

• Sapwood: It is the peripheral region of secondary xylem. It is living and lighter in colour.

Function: 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. It is to be replaced to provide new protective cell layers. Hence, another meristematic tissue called cork cambium (phellogen) develops, usually in the cortex.
• 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. The outer cells differentiate into cork (phellem), while the inner cells differentiate into secondary cortex (phelloderm). Cells of secondary cortex are parenchymatous.
• The cork is impervious to water due to suberin deposition in the cell wall.
• Phellogen, phellem, and phelloderm are collectively known as periderm. Due to the activity of cork cambium, pressure builds up on the remaining layers peripheral to phellogen, and ultimately these layers die and slough off.
• Bark is a non-technical term that refers to all tissues (periderm and secondary phloem) exterior to the vascular cambium. It is of two types:
  • Early (soft) bark: It is formed early in the season.
  • Late (hard) bark: It is formed towards the end of the season.
• Lenticels: At certain regions, phellogen cuts off closely arranged parenchymatous cells on the outer side. These cells rupture the epidermis, forming lens-shaped openings called lenticels. They occur in most woody trees.

Function: Lenticels permit gas exchange between the outer atmosphere and the internal tissue of the stem.

Secondary Growth in Roots

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