The Tissues
Meristematic Tissues
Meristems are specialized regions of active cell division responsible for plant growth. Meristems exhibit totipotency NEET 1993.
- Apical Meristem: Found at root and shoot apices. Contributes to primary growth (elongation).
- Crucial detail: Axillary buds and terminal buds are cells "left behind" from the shoot apical meristem during leaf formation and stem elongation NEET 1993, 1998, 2023.
- Intercalary Meristem: Occurs between mature tissues (e.g., at the base of internodes in grasses). Regenerates parts removed by grazing herbivores.
- Both apical and intercalary meristems are Primary Meristems because they appear early in plant life NEET 1988, 2010.
- Lateral Meristem (Cylindrical Meristem): Appears later in mature regions of roots and shoots. Responsible for secondary growth (increase in girth) NEET 1988, 2010, 2024 Re.
- Examples: Fascicular vascular cambium, interfascicular cambium, and cork cambium NEET 2024 Re.
- Exception/Crucial detail: The Tunica-Corpus theory (by Schmidt) explains the organization of the shoot apex based on planes of cell division NEET 1988.
Permanent Tissues
Composed of cells that have temporarily or permanently lost the power of division.
- Simple Tissues: Made of only one type of cell.
- Complex Tissues: Made of more than one type of cell working together as a unit.
Simple Tissue
| Tissue | Cell Wall Nature | Distinguishing Features & Occurrence |
|---|---|---|
| Parenchyma | Thin, cellulosic | Isodiametric, living cells with intercellular spaces. Universal tissue. Main function: Storage, photosynthesis. |
| Collenchyma | Thickened at corners (Cellulose, Hemicellulose, Pectin) | Living, flexible mechanical tissue. Occurs in young dicot stems and petioles of herbaceous climbers NEET 1990. Angular collenchyma occurs in Cucurbita NEET 1991. Absent in monocots and roots. |
| Sclerenchyma | Thick, Lignified | Dead, without protoplasts. Very narrow lumen. Provides purely mechanical support NEET 2019, 2021. |
Types of Sclerenchyma:
- Fibres: Long, highly thick-walled with pointed (tapering) ends.
- Sclereids: Spherical, oval, or cylindrical, extremely thick-walled dead cells with narrow cavities (lumen). Cause the gritty texture in the fruit walls of nuts, and pulp of guava, pear, and sapota NEET 1996, 2022 Re, 2024, 2024 Re.
Complex Tissue
Xylem
Conducts water/minerals. Elements: Tracheids, Vessels, Xylem Fibres, Xylem Parenchyma.
- Tracheids: Elongated, dead, lignified with tapering ends. Main water-conducting element in gymnosperms NEET 2010. Unicellular with narrow lumen NEET 2002. Differ from vessels by being imperforate (having pitted end walls) NEET 2012, 2014.
- Vessels: Multicellular, long cylindrical tubes with a large central cavity, devoid of protoplasm (enucleate) NEET 2006. Possess perforated end walls. Presence of vessels is a characteristic feature of angiosperms NEET 2002, 2023.
Phloem
Conducts food. Death of protoplasm is not a pre-requisite for food transport NEET 1989.
- Sieve Tube Elements: Long, tube-like, arranged longitudinally. Have perforated cross-walls (sieve plates). Mature elements lack a nucleus (enucleate) but have peripheral cytoplasm NEET 1991, 1997, 2006. Interconnected lumen facilitates efficient translocation NEET 1989, 2010.
- Companion Cells: Specialized living parenchyma connected to sieve tubes via pit fields. They maintain the pressure gradient in sieve tubes NEET 2012, 2024 Re and load sucrose into sieve elements NEET 2011.
Exception/Crucial detail: Gymnosperms lack vessels in their xylem NEET 2023 and lack sieve tubes and companion cells in their phloem NEET 2019. Instead, they possess albuminous cells and sieve cells NEET 2019, 2024 Re.
Crucial detail: Tyloses are balloon-shaped extensions of xylem parenchyma cells into the lumen of vessels, effectively blocking them in heartwood NEET 2016. Annular and spirally thickened conducting elements develop in elongating root/stem protoxylem NEET 2009.
The Tissue System
Epidermal Tissue System
Forms the outermost covering. Includes epidermis, stomata, and epidermal appendages (trichomes and root hairs).
- Stomata: Regulate transpiration and gas exchange.
- Crucial detail: Guard cells are bean-shaped in dicots but dumb-bell shaped in grasses NEET 2018, 2024. Specialized epidermal cells closely surrounding the guard cells are known as subsidiary cells NEET 2016.
- Root Hairs: Always unicellular elongations of epidermal cells; absorb water.
- Trichomes: Usually multicellular appendages on the stem; prevent water loss.
Ground Tissue System
Includes all tissues except epidermis and vascular bundles NEET 2011.
- Composed mainly of simple tissues (parenchyma, collenchyma, sclerenchyma).
- Consists of cortex, endodermis, pericycle, pith, and medullary rays.
- Crucial detail: The cortex is the region found strictly between the epidermis and stele NEET 1993.
Vascular Tissue System
Consists of complex tissues (xylem and phloem).
- Radial Arrangement: Xylem and phloem are arranged in an alternate manner on different radii. Characteristic of roots NEET 2022.
- Conjoint Arrangement: Xylem and phloem are situated on the same radius. Characteristic of stems and leaves.
- Open Vascular Bundles: Possess intrafascicular cambium between xylem and phloem. Capable of producing secondary tissues (Dicot stems) NEET 2011.
- Closed Vascular Bundles: Lack cambium. Do not show secondary growth (Monocots) NEET 2012, 2015.
- Crucial detail: Water-containing cavities are present within the vascular bundles of monocot stems like maize NEET 2012.
Anatomy of Dicotyledonous and Monocotyledonous Plants
Dicotyledonous vs Monocotyledonous Root
| Feature | Dicotyledonous Root | Monocotyledonous Root |
|---|---|---|
| Xylem Bundles | Diarch to hexarch (2 to 6 bundles). | Polyarch (Usually more than 6 bundles) NEET 2022. |
| Pith | Small or inconspicuous. | Large and well-developed NEET 2012. |
| Secondary Growth | Present. Vascular cambium is completely secondary in origin. | Absent. |
| Pericycle Activity | Initiates lateral roots and vascular cambium during secondary growth NEET 2021, 2022 Re. | Initiates lateral roots only. |
Crucial detail: The Endodermis comprises barrel-shaped cells featuring water-impermeable, waxy Casparian strips made of suberin on their radial and tangential walls NEET 1990, 1994, 2018. It acts as a water-tight jacket.
Dicotyledonous vs Monocotyledonous Stem
| Feature | Dicotyledonous Stem | Monocotyledonous Stem |
|---|---|---|
| Hypodermis | Collenchymatous (provides flexibility). | Sclerenchymatous (provides rigidity). |
| Vascular Bundles | Wedge-shaped, arranged in a ring. Conjoint, collateral, open. | Scattered, oval. Conjoint, collateral, closed. Peripheral bundles are smaller than central ones. |
| Bundle Sheath | Absent (but pericycle forms a sclerenchymatous 'bundle cap' above phloem). | Present (sclerenchymatous). |
| Endodermis | Present, rich in starch grains (starch sheath). | Absent / Undifferentiated. |
| Pith & Rays | Well-developed parenchymatous pith and medullary rays. | Absent (Ground tissue is undifferentiated). |
Crucial detail: Primary xylem arrangement in stems is Endarch (protoxylem towards center), whereas in roots it is Exarch (protoxylem towards periphery). Transition from exarch to endarch occurs in the hypocotyl NEET 1999.
Dorsiventral (Dicot) vs Isobilateral (Monocot) Leaf
| Feature | Dorsiventral Leaf (Dicot) | Isobilateral Leaf (Monocot/Grass) |
|---|---|---|
| Stomata Location | Mostly on abaxial (lower) surface. | Equal on both surfaces. |
| Mesophyll | Differentiated into Palisade (adaxial, vertically elongated parallel cells NEET 2024 Re) and Spongy (abaxial, round cells with air cavities). | Undifferentiated. |
| Specialized Cells | Absent. | Bulliform (motor) cells present on adaxial epidermis. |
Crucial detail: Bulliform cells are large, empty, colourless cells. During dry weather, water stress makes them flaccid, causing the grass leaves to curl inwards to minimize water loss NEET 2019, 2020, 2024.
Secondary Growth
Vascular Cambium
Responsible for increasing the girth of dicot stems and roots.
- Formation in Dicot Stem: The intrafascicular cambium (primary meristem) and the interfascicular cambium (secondary meristem originating from medullary ray cells NEET 2021, 2022 Re) join to form a continuous cambial ring.
- The cambial ring cuts off cells on both sides:
- Cells cut off towards the center mature into secondary xylem (wood).
- Cells cut off towards the periphery mature into secondary phloem (bark).
- The cambium is generally more active on the inner side, hence more secondary xylem is produced than secondary phloem.
- Crucial detail: The terms "endarch" and "exarch" only apply to primary xylem, not secondary xylem NEET 2023.
Spring Wood and Autumn Wood
| Feature | Spring Wood (Early Wood) | Autumn Wood (Late Wood) |
|---|---|---|
| Season of Formation | Spring (Cambium is highly active). | Autumn/Winter (Cambium is less active). |
| Vessels | Large number of xylary elements with wider cavities/vessels NEET 2023. | Fewer xylary elements with narrow vessels NEET 2023. |
| Physical Properties | Lighter in colour, lower density NEET 2023. | Darker in colour, higher density NEET 2023. |
The two kinds of wood appear as alternate concentric rings constituting an Annual Ring, used to estimate the age of the tree (Dendrochronology).
Heartwood and Sapwood
| Feature | Heartwood (Duramen) | Sapwood (Alburnum) |
|---|---|---|
| Position & Colour | Central region, Dark brown. | Peripheral region, Light coloured. |
| Composition | Highly lignified dead elements. Filled with secondary metabolites (tannins, resins, essential oils). | Living parenchyma and functioning vascular elements. |
| Function | Provides mechanical support. Does NOT conduct water. Highly resistant to insects and microbes NEET 2022. | Actively conducts water and minerals from root to leaf. |
Cork Cambium (Phellogen)
As secondary growth continues, outer cortical and epidermal layers are crushed. Phellogen (cork cambium) develops in the outer cortex to replace them.
- Phellogen cuts off cells on both sides:
- Outer cells differentiate into Phellem (Cork). Cork is impervious to water due to suberin deposition in the cell wall NEET 2021.
- Inner cells differentiate into Phelloderm (Secondary Cortex).
- Periderm = Phellogen + Phellem + Phelloderm NEET 2021.
- Bark: A non-technical term referring to all tissues exterior to the vascular cambium (includes periderm and secondary phloem) NEET 2021, 2023.
- Lenticels: Lens-shaped openings formed when phellogen cuts off closely arranged parenchymatous complementary cells instead of cork cells. They permit exchange of gases between the atmosphere and internal tissues NEET 2013, 2021.
Secondary Growth in Roots
In dicot roots, the vascular cambium is completely secondary in origin.
It originates from the tissue located just below the phloem bundles (conjunctive tissue) and a portion of pericycle tissue above the protoxylem, forming a wavy ring that later becomes circular.