Learning Objectives
- Understand different types of plant tissues (meristematic and permanent)
- Learn the tissue systems in plants
- Study the internal structure of dicot and monocot roots, stems, and leaves
- Understand secondary growth in dicot stems and roots
Key Concepts
Meristematic Tissues
Cells that retain the ability to divide. Apical meristems: Present at tips of roots and shoots; responsible for primary growth. Lateral meristems: Vascular cambium and cork cambium; responsible for secondary growth (increase in girth). Intercalary meristems: Present at base of internodes; common in grasses (monocots).
Permanent Tissues
Simple tissues: (1) Parenchyma — thin-walled, living, store food; chlorenchyma (with chloroplasts), aerenchyma (with air cavities in aquatic plants). (2) Collenchyma — thickened at corners, living, provides mechanical support to young stems. (3) Sclerenchyma — thick-walled, dead, lignified; sclereids (stone cells) and fibres.
Complex tissues: (1) Xylem — conducts water and minerals upward; components: tracheids, vessels (unique to angiosperms), xylem parenchyma (only living), xylem fibres. (2) Phloem — conducts food (sucrose) from leaves to other parts; components: sieve tubes, companion cells (unique to angiosperms), phloem parenchyma (absent in monocots), phloem fibres.
Tissue Systems
Epidermal tissue system: Outermost covering; includes epidermis, stomata (guard cells), trichomes (stem hairs), root hairs. Cuticle (waxy layer) prevents water loss. Ground tissue system: All tissues except epidermis and vascular bundles — cortex, pith, medullary rays. Vascular tissue system: Xylem and phloem together form vascular bundles. Open vascular bundles have cambium (dicots); closed lack cambium (monocots).
Anatomy of Dicot and Monocot Organs
Dicot root: Pith small or absent, 2-6 xylem bundles, cambium present (secondary growth possible), cortex well developed with endodermis and Casparian strips.
Monocot root: Pith large, many xylem bundles (polyarch), no cambium (no secondary growth), exodermis present.
Dicot stem: Vascular bundles in a ring, open (with cambium), medullary rays present, epidermis with trichomes.
Monocot stem: Vascular bundles scattered, closed (without cambium), each bundle surrounded by bundle sheath, no secondary growth.
Dicot leaf (Dorsiventral): Different upper (palisade) and lower (spongy) mesophyll, stomata mainly on lower surface.
Monocot leaf (Isobilateral): Similar mesophyll on both sides, stomata on both surfaces, bulliform cells present for rolling.
Secondary Growth
Increase in girth due to activity of vascular cambium (produces secondary xylem inward and secondary phloem outward) and cork cambium (produces cork/phellem outward and secondary cortex/phelloderm inward). Annual rings: Formed due to seasonal variation in cambial activity. Spring wood (early wood) has wider vessels; autumn wood (late wood) has narrower vessels. Counting annual rings helps determine tree age (dendrochronology). Heartwood (duramen): Central, dark, non-functional wood. Sapwood (alburnum): Peripheral, light, functional wood conducting water.
Summary
Plant tissues are meristematic (dividing) or permanent (differentiated). Simple permanent tissues include parenchyma, collenchyma, and sclerenchyma. Complex tissues (xylem and phloem) conduct water and food respectively. Dicot and monocot organs differ in anatomical details. Secondary growth occurs in dicots through vascular cambium and cork cambium activity.
Important Terms
- Casparian strip: Waxy strip in endodermis that regulates water/mineral entry into vascular tissue
- Cambium: Lateral meristem responsible for secondary growth
- Annual ring: One year's growth of secondary xylem visible in cross-section
- Heartwood: Central non-functional wood providing structural support
- Bulliform cells: Large, bubble-shaped epidermal cells in monocot leaves causing leaf rolling
- Collenchyma: Living mechanical tissue with cellulose thickening at corners
- Tracheid: Elongated water-conducting cell in xylem with tapering ends
- Sieve tube: Phloem element for translocation of food
Quick Revision
- Xylem: conducts water upward; only xylem parenchyma is living
- Phloem: conducts food; phloem parenchyma absent in monocots
- Open vascular bundle = with cambium (dicots); Closed = without cambium (monocots)
- Dicot stem: VBs in ring; Monocot stem: VBs scattered
- Dorsiventral leaf = dicot; Isobilateral leaf = monocot
- Secondary growth: vascular cambium + cork cambium
- Spring wood = wide vessels; Autumn wood = narrow vessels
- Heartwood = duramen (non-functional); Sapwood = alburnum (functional)