ANATOMY OF FLOWERING PLANTS
THEORETICAL PARTS :
1. TISSUES
- Meristematic tissues (Dividing tissues): It consists of undifferentiated actively dividing cells.
- Permanent tissues (Non-dividing tissues): It consists of differentiated cells that have lost the ability to divide.
2. TYPES OF MERISTEMATIC TISSUE
- Apical Meristems: They are found at the growing tips of roots and stems. The shoot apical meristem results in the elongation of the stem and formation of new leaves. The root apical meristem helps in root elongation.
- Lateral Meristems: They occur in the mature region of roots and shoots. They are called secondary meristems as they appear later in a plant's life. It helps in adding secondary tissues to the plant body and in increasing the girth of plants. E.g. Fascicular cambrum, inter fascicular cambium and cork cambium.
- Intercalary Meristems: It is present between the masses of mature tissues present at the bases of the leaves of grasses. It helps in the regeneration of grasses after they have been grazed by herbivores. Since the intercalary meristem and the apical meristem appear early in a plant’s life, they constitute the primary meristem.
3. TYPES OF PERMANENT TISSUE
Depending on the structural differences (type of cells), permanent tissues are divided into two types:
ANATOMY OF FLOWERING PLANTS
- Simple tissue
- Complex tissue.
Simple permanent tissue is made up of only one kind of cells forming a uniform mass.
Simple permanent tissues are of three types: Parenchyma, Collenchyma and Sclerenchyma
a) Parenchyma: Parenchyma is widely distributed in plant bodies such as stem, roots, leaves and flowers. They are found in the cortex of root, ground tissue in stems and mesophyI of leaves.
Characteristic features
- Cells are isodiametric 1.e. equally expanded on all sides.
- Cells may be oval, round or polygonal in outline.
- Nucleus is present and hence, living.
- The cell walls are thin and made of cellulose.
- Cytoplasm is dense with a central vacuole.
- Cells are loosely packed with large intercellular spaces between the cells.
- It may contain chlorophyll. Parenchyma containing chlorophyll is called chlorenchyma.
- It is the seat of photosynthesis.
- Parenchyma that encloses large air cavities is known as aerenchyma. Aerenchyma
- provides buoyancy to aquatic plants.
Functions
- Parenchyma store and assimilate food.
- They give mechanical support to the plant body by maintaining turgidity.
ANATOMY OF FLOWERING PLANTS
- Presence of intercellular spaces in between parenchyma cells helps in exchange of gases.
- It prepares food if chlorophyll is present.
- It stores waste products like gum, crystal, tannin and resins.
b) Collenchyma: Collenchyma is a strong and flexible tissue that provides flexibility to soft aerial parts. They are found below the epidermis in leaf stalks, leaf mid-ribs, and herbaceous dicot stems.
Characteristics features
- Collenchyma cells are elongated cells with thick primary walls.
- Cell wall is unevenly thickened with cellulose at the corners.
- Intercellular spaces are absent.
- Nucleus is present and hence the tissue is living.
- Few chloroplasts may be present in the cells.
Functions
- Collenchyma provides mechanical support to the growing parts of the plant such as young stem and petiole of a leaf.
- It provides flexibility to soft aerial parts to that they can bend without breaking.
c) Sclerenchyma: Like collenchyma, sclerenchyma is also a strengthening tissue. It is found in and around the vascular tissue, under the skin i.e. the epidermis in dicot stems.
Characteristics feature
- Cells are long, narrow, thick and lignified usually pointed at both ends.
- The cell wall is evenly thickened with lignin. Lignin is a water proof material.
- Intercellular spaces are absent.
- Nucleus is absent and hence the tissue is made up of dead cells.
- Middle lamella i.e. the well between adjacent cells is conspicuous.
On the basis of variation in form, structure, origin and development, sclerenchyma cells are of two types:
(1) Fibers: They are long, narrow, thick walled and pointed cells.
Location: Fibers are found in and around the vascular tissue. It may also occur below the epidermis. Fibers help in transportation of water in plant.
(2) Sclereids: They are spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities. The cells are hard and strongly lignified. They are shorter, 1iso-diametric or irregular cells.
ANATOMY OF FLOWERING PLANTS
Location: 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. Sclereids give firmness and hardness to the part concerned.
Functions
- Sclerenchyma gives mechanical support to the plant by giving rigidity, flexibility and elasticity to the plant body.
- It forms a protective covering around seeds and nuts.
3.2 Complex Permanent Tissues
Complex tissue is made up of more than one type of cells that work together to perform a particular function.
Complex tissues are of two types: Xylem and Phloem.
(a) Xylem (Greek xylo wood):
Xylem is a complex permanent tissue that conducts water and mineral upward from root to the plant. It is also known as wood.
Xylem is composed of four different kinds of elements, namely, tracheids, vessels, xylem fibres and xylem parenchyma.
(1) Tracheids:
- Tracheids are long, tubular dead cells with wide lumen and tapering ends.
- The cell wall is thick with lignin. They have pores in their walls.
(2) Vessels:
- Vessel is a long, cylindrical tube like structure that are placed one above the other end to end.
- It is non-living cell with lignified walls.
- Vessel members are interconnected through perforations in their common walls. The presence of vessels is a characteristic feature of angiosperms.
(ii) Xylem fibers:
- They are long, non-living cells with very thick lignin deposition on the walls.
- They have narrow lumen and tapering ends.
Function: Xylem fibers provide mechanical support to the plant.
(iv) Xylem parenchyma: They are living cells with cellulosic cell wall.
Function:
- They help in storage of starch or fats and other materials like tannins.
- They also help in lateral conduction of water by the ray parenchymatous cells.
ANATOMY OF FLOWERING PLANTS
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.
- In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch.
- In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre.
- Such arrangement of primary xylem is called exarch.
(b) Phloem (Greek = Phloeis = Onner bark):
- Phloem is a complex permanent tissue that conducts food synthesized in the leaves to different parts of the plant body.
- Phloem in angiosperms is composed of sieve tube elements, companion cells, phloem parenchyma and phloem fibres.
- Gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells.
(1) Sieve tubes: Sieve tube cells have vacuolated cytoplasm and lacks nucleus. The functions of sieve tubes are controlled by the nucleus of companion cells.
(2)Companion cells: Companion cells are associated with sieve tubes. They are thin walled cells which lie on the sides of sieve tube cells. They help sieve tubes in the conduction of food material by maintaining a proper pressure gradient in the sieve tube cells.
(3) Phloem Parenchyma: The phloem parenchyma cells are thin-walled and living. They help in storage and slow lateral conduction of food.
ANATOMY OF FLOWERING PLANTS
(4) Phloem fibers: They are the only non-living (dead) component of phloem. Phloem fibers provide mechanical support to the tissue. Phloem fibers are a source of commercial fibers.
E.g. Jute, hemp, flax etc.
4, TISSUE SYSTEMS
On the basis of their structure and location, there are three types of tissue systems.
4.1 Epidermal Tissue System :
Epidermal tissue system forms the outer-most covering of the whole plant body and comprises epidermal cells, stomata and the epidermal appendages such as the trichomes and hairs.
(a) Epidermis:
- It comprises the outer covering of the plants, such as epidermis, cuticle, stomata, epidermal extensions; trichomes in stem and root hairs.
- Stomata regulate gaseous exchange and water loss by transpiration.
- Guard cells are specialized epidermal cells which help in exchange of gases. It is also the seat of transpiration.
- Guard cells are kidney shaped in dicots and dumb-bell shaped in monocots.
- The guard cells possess chloroplasts and regulate the opening and closing of stomata.
- Epidermal cells, in the vicinity of the guard cells become specialized 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.
- The root hairs are unicellular elongations of the epidermal cells and help absorb water and minerals from the soil.
- On the stem the epidermal hairs are called trichomes.
- The trichomes help in preventing water loss due to transpiration
ANATOMY OF FLOWERING PLANTS
4.2 The Ground Tissue System
- All tissues except epidermis and vascular bundles constitute the ground tissue.
- It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.
The Vascular system consists of vascular bundles, the phloem and the xylem.
There are two types of vascular bundles:
(i) Open vascular bundles: In open vascular bundles, cambium is present between phloem and xylem. It is found in dicotyledonous plants.
ANATOMY OF FLOWERING PLANTS
(ii) Closed vascular bundles: Closed vascular bundles lack cambium. Hence, since they do not form secondary tissues they are referred to as closed. It is found in the monocotyledons plants.
Xylem and phloem can be arranged in two different kinds of arrangement within a vascular bundle.
(i) Radial arrangement: When xylem and phloem within a vascular bundle are arranged
in an alternate manner on different radii, the arrangement is called radial. Such vascular
bundles are common in roots.
(ii) Conjoint arrangement: When the xylem and phloem are situated at the same radius
of vascular bundles, the arrangement is called conjoint. Such vascular bundles are common
in stems and leaves. The conjoint vascular bundles usually have the phloem located only
on the outer side of xylem.
5. ANATOMY OF DICOT AND MONOCOT ROOTS
5.1 T.S. of Dicot Root :-
- Epidermis: It has a single layer of epidermal cells, many of which protrude in the form of unicellular root hairs.
- Cortex: The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces.
- Endodermis: The innermost layer of the cortex is called endodermis. It comprises a single layer of barrel-shaped cells without any intercellular spaces.
- Pericycle: Next to endodermis lie a few layers of thick-walled parenchymatous cells known as pericycle.
- Pith: The pith is small or inconspicuous.
- Vascular bundles: Vascular bundles is single (Monoarch). There are usually two to four xylem and phloem patches. Later, a cambium ring develops between the xylem and phloem.
- Conjunctive tissue: The parenchymatous cells which lie between the xylem and the phloem are called conjunctive tissue.
ANATOMY OF FLOWERING PLANTS
- Stele: All tissues on the inner side of the endodermis such as pericycle, vascular bundles and pith constitute the stele.
5.2 T.S. of Monocot Root
The anatomy of the monocot root is similar to the dicot root in many respects. Epidermis, cortex, and endodermis: Same as that of dicot root.
Pericycle: Monocotyledonous roots do not undergo any secondary growth.
Pith: Pith is large and well developed.
Vascular bundle: There are usually more than six (polyarch) xylem bundles in the monocot root.
6. ANATOMY OF DICOT AND MONOCOT STEMS
6.1 T.S. of Dicot Stem
- Epidermis: It is covered with a thin layer of cuticle. It may bear trichomes and a few stomata.
- Cortex: The cells arranged in multiple layers between epidermis and pericycle constitute the cortex.
- It consists of three sub-zones: Hypodermis, cortical layers and endodermis.
- Hypodermis: The outer hypodermis consists of a few layers of collenchymatous cells just below the epidermis
- Cortical layers: It is found below hypodermis and consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.
- ⮚Endodermis: The innermost layer of the cortex is called the endodermis.
- The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.
- Pericycle: It is present on the inner side of the endodermis and above the phloem in the form of semilunar patches of sclerenchyma.
- Vascular bundles: A large number of vascular bundles are arranged in a ring. This arrangement is characteristic of dicot stem. Each vascular bundle is conjoint, open, and with endarch protoxylem.
- Medullary rays: In between the vascular bundles there are a few layers of radially placed parenchymatous cells, which constitute medullary rays.
- Pith: A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.
- Epidermis: It is covered with a thin layer of cuticle. It may bear trichomes and a few stomata.
- Cortex: Cortex contains few layers of sclerenchymatous cells.
- Pericycle: Same as that of dicot stem.
- Vascular bundles: A large number of scattered vascular bundles are present. Vascular bundles are conjoint, closed with peripheral vascular bundles generally smaller than the centrally located ones. Each vascular bundle is surrounded by sclerenchymatous bundle-sheath cells. Phloem parenchyma is absent, and water-containing cavities are present.
- Pith: Pith is absent in monocot stem .
- Epidermis: Epidermis is present on both the upper surface (adaxial epidermis) and the lower surface (abaxial epidermis). The epidermis on the outside is covered with a thick cuticle. Abaxial epidermis bears more stomata than the adaxial epidermis.
- Mesophyll: Mesophyll is a tissue of the leaf present between the adaxial and abaxial epidermises. It is differentiated into the palisade parenchyma (composed of tall, compactlyplaced cells) and the spongy parenchyma (comprising oval or round, loosely-arranged cells with inter cellular spaces). Mesophyll contains the chloroplasts which perform the function of photosynthesis.
- Vascular system: The vascular bundles present in leaves are conjoint and closed. They are surrounded by thick layers of bundle-sheath cells.
7.2 T.S. of Monocot Leaf
- Isobilateral leaf is found in monocot leaf.
ANATOMY OF FLOWERING PLANTS
- The anatomy of isobilateral leaf is similar to that of the dorsiventral leaf in many ways.
- It shows the following characteristic differences.
- In an isobilateral leaf, the stomata are present on both the surfaces of the epidermis.
- The mesophyll is not differentiated into palisade and spongy parenchyma.
- Bulliform cells, which are modified adaxial epidermal cells, are present along the veins, which absorb water and make the cells turgid.
- When they are flaccid due to water stress, they make the leaves curl inwards to minimize water loss.
- 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.
- The growth of the roots and stems in length with the help of apical meristem is called the primary growth.
- Increase in girth exhibited by most of the dicot plants is known as secondary growth.
- The tissues involved in secondary growth are together known as lateral meristem.
- Lateral meristem consists of vascular cambium and cork 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.
- In dicot stems, the cells of cambium present between primary xylem and primary phloem is the intrafascicular cambium.
- The interfascicular cambium is formed from the cells of the medullary rays adjoining the interfascicular cambium, as they become meristematic.
- This results in the formation of a continuous ring of cambium.
- The cambial ring becomes active and cuts off new cells, towards its either sides.
- The cells present toward the outside differentiate into the secondary phloem, while the cells cut off toward the pith give use to the secondary xylem.
- The amount of the secondary xylem produced is more than that of the 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 phloem gets gradually crushed due to the continued formation and accumulation of secondary xylem.
- The primary xylem however remains intact, in or around the centre.
- At some places, the cambium forms narrow band of parenchyma, known as secondary medullary rays.
ANATOMY OF FLOWERING PLANTS
- In the dicot root, the vascular cambium is completely secondary in origin.
- It originates from a portion of pericycle tissue, above the protoxylem and below phloem bundles.
- They form a complete and continuous wavy ring, which later becomes circular.
- Further events are similar to those already described above for a dicotyledonous stem.
- Secondary growth also occurs in stems and roots of gymnosperms.
- However, secondary growth does not occur in monocotyledons.
- Spring wood (Early wood): In the spring season, cambium is very active and produces a large number of xylary elements having vessels with wider cavities. This wood is called spring wood or early wood. The spring wood is lighter in colour and has a lower density.
- Autumn wood (Late wood): In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels. This wood is called autumn wood or late wood. The autumn wood is darker and has a higher density.
- Annual ring: When two kinds of wood appear as alternate concentric rings, it is known as annual ring. Annual rings seen in a cut stem give an estimate of the age of the tree.
- It is composed of deed elements with highly lignified walls.
- It imparts dark brown colour to the older trees, due to the deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem.
- It makes the plant hard, durable and resistant to the attacks of microorganisms and insects.
- The heartwood does not conduct water but it gives mechanical support to the stem.
- It constitutes the peripheral region of the secondary xylem.
- It is lighter in colour.
- It is involved in the conduction of water and minerals from root to leaf.
- The outer cortical and epidermis layers break down.
- These are replaced down 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 composed of thin-walled, narrow and rectangular cells.
- Phellogen cuts off cells on its either side.
- The cells cut off toward the outside give rise to the phellem or cork. The suberin deposits in its cell wall make it impervious to water.
- The inner cells give rise to the secondary cortex or phelloderm. The secondary cortex is parenchymatous.
- 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.
- It is a non-technical term that refers to all tissues exterior to the vascular cambium, therefore including secondary phloem.
- Bark is divided into two types: Early (or soft bark) and late (or hard bark).
- Early or soft bark: Bark that is formed early in the season is called early or soft bark.
- Late or hard bark: Bark that is formed towards the end of the season late or hard bark is formed.
- 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 lens-shaped openings called lenticels.
- Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem.
- It is found in most woody trees.
ANATOMY OF FLOWERING PLANTS
