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Basic Plant Tissue Systems and Cell Types
- Tissue Systems:
The plant tissue that actively divide by mitosis are called meristematic tissues located in specific tissues.
Dermal Tissue System: Epidermis
- Forms the outermost layer of cells, usually one cell thick, covering the entire plant body.
- Often flattened and rectangular in shape.
- Include guard cells of stomata, trichomes (hairs) and unicellular root hairs.
- Covered by a waxy cuticle which prevents water loss.
- Provides protection and regulates movement of materials.
Ground Tissue System: Parenchyma, Collenchyma, and Sclerenchyma
Distributed beneath the epidermis and surrounding vascular tissue.
- Parenchyma cells:
- Most common cell.
- Thin-walled with large vacuoles.
- Function in photosynthesis, support, storage of materials, and lateral transport.
- Collenchyma cells:
- Found near surface of stem, leaf perioles and veins. Characterized by an uneven thickening of cell walls.
- Provide flexible support to young plant organs.
- Sclerenchyma cells:
- Thickened cell walls that contain lignin.
- Provide strength and support to mature plant structures.
- May be dead at maturity.
- Common type are the long, thin fibers.
Vascular Tissue System: Xylem and Phloem
- Vascular tissue that functions in the transport of water and minerals throughout the plant and provides support.
- Parenchymal cells function in storage and lateral transport.
- Fibers provide additional strength.
- -Tracheids: long, thin cells with perforated tapered ends.
- -Vessel Elements: larger in diameter, open-ended, joined end to end, forming continuous transport systems referred to as vessels.
- Tissue that transport the products of photosynthesis throughout the plant.
- Phloem parenchyma cells function in storage and lateral transpor and phloem fibers provide additional support.
- -Sieve tube members: lack a nucleus and have sieve plates for end walls, joined end to end ans associated with companion cells.
- -Companion cells: regulate sieve-tube member function.
Meristematic Tissue: Primary Meristem, Cambium, and Pericyle
Primaty Meristems: actively dividing cells located in buds of shoot and in root tips of plants. Produce the primary tissues along the plant axis throughout the life of the plant.
Pericyle: layer of meristematic cells outisde the vascular cylinder in the root. Divide to produce lateral branch roots.
Vascular Cambium: lateral meristem composed of small dividing cells that are located between xylem and phloem vascular tissue. Divide to produce secondary growth which increase in girth.
Cork Cambium: meristem located just inside the cork layer of a woody plant. Divide to produce secondary growth.
- Nodes: regions of the stem from which leaves, buds and branches arise and which contain meristematic tissue.
- Internodes: regions of the stem located between the nodes.
- Terminal buds: located at the tips of stems and branches. Gives rise to leaves, buds and all primary tissue of stem. Only stem produce buds.
- Axillary, or lateral, buds: located in the leaf axes at nodes, give rise to lateral branches.
- Leaves: consist of flattened blades attached at the node of a stem by a stalk or petiole.
- Primary root: first root produce by a plant embryo and may become a taproot.
- Secondary root: arise from meristematic tissue within the primary root.
- Root tips: root apical meristem that gives rise to a root cap (protective layer of cells covering the root tip) and primary tissue of root.
- Root hairs: principal site of water and mineral absorption.
Plant Primary Growth and Development
- Tissues produced from apical meristems are called primary tissues, and this growth is called primary growth.
- Leaf primodia: young developing leaves.
- Bud primodial: between the leaf and the stem.
- Indeterminate growth: meristems of plants continue to grow throughout their lifetime.
Cell Structure of Primary Tissues
- Stems support leaves and conduct water and inorganic substances from the root to the leaves and products of photosynthesis from leaves to roots.
- Section of the stem:
- Dermal tissue system: protective cell layer covering the plant composed of the epidermis and cuticle. May contain trichomes on the outer surface.
- Ground tissue system: background tissue that fills the spaces between epidermis and vascular tissue.
- Cortex region: located between the vascular bundles and the epidermis, composed of parenchyma but outer part contain collenchyma.
- Pith region: center of stem composed of parenchyma; these cells provide support through turgor pressure, also store.
- Vascular system: continuous system of xylem and phloem.
- Vascular bundles: clusters of xylem and phloem; occur in rings that surround the pith.
Phloem Tissue composed of three cells types in the stem.
- Sclerenchyma cells: dead, fibrous, thick-walled cells that provide support of the phloem tissue and appear as a bundle cap.
- Sieve-tube members: large, living, elongated cells that lack a nucleus at maturity; form sieve tubes interconnected through sieve plates.
- Companion cells: small, nucleated parenchyma cells connected to sieve-tube cells by means of cytoplasmic strands.
Xylem tissue composed of two cell types in stem
- Tracheids: elongated, thick-walled cells with closed, tapered ends, dead at functional maturity, their lumens are interconnected through pits in the cell walls.
- Vessel elements: cylindrical cells that are large in diameter and dead at functional maturity. They become joined end to end, lose their end walls, and form long, vertical vessels.
: tissue located between the xylem and the phloem divides to give rise to secondary tissues.
- Rhizomes: grow underground
- Adventitious roots: grow aboveground
- Roots have four primary functions:
- 1. Anchorage of the plant in the soil.
- 2. Absorption of water and minerals from the soil.
- 3. Conduction of water and minerals from the region of absorption to the base of the stem.
- 4. Starch storage to varying degrees, depending on the plant.
- Primary root or radicle: when a seed germinates, this root sends out side branches (lateral roots), and these in turn branch out.
- Taproot: primary root that is the largest and most important part of root system like carrots, dandelions and pine trees.
- Fibrous root: many main roots formed like most grasses.
- Vascular cylinder: vascular tissue located in the center of the root.
- Cortex: composed of large parenchyma cells filled with numerous purple-stained organelles, cortical cells store starch.
- Casparian strip: made of suberin (waxy material) that extends completely around each cell. Forms a barrier to the passage of anything moving between adjacent cells of the endodermis. All water and dissolved materials absorbed must first pass through cytoplasm before entering vascular tissues.
- Pericycle: layer of dividing cells inside endodermis; give rise to lateral roots.
Taproots vs Fibrous Roots
Taproots allow plants to reach water in deep aquifers.
Fibrous roots provide advantage where water is available at soil surface (forest trees).
Roots vs Stems
- Stems have nodes and produce buds; roots don't.
- Tip of root is always covered with cap of some kind; tip of stem isn't.
- Arrangement of internal tissues differs between stem and root.
- Roots have pericycle and endodermis; stems don't.
- Roots have root hairs; stems don't.
Relationship of structure and function of cells in roots
- Root hairs increase surface area available for absorption.
- Endodermis regulates transport of materials into vascular cylinder.
- Cortex provides an extensive storage area.
- Epidermis lacks cuticle, which would prevent absorption.
Importance of endodermis for land plants
Endodermis regulates transport of materials, which otherwise would move indiscriminately through the plant. Absorption occurs only in roots; thus, endodermis provides an important control in the plant's environment.
- Provide a very large surface area for the absorption of light and the uptake of carbon dioxide through stomata.
- The leaf is a layer of parenchyma cells (mesophyll) between two layers of epidermis.
- Guard cells allow the exchange of gases and evaporation of water; they are photosynthetic and are capable of changing shape in response to environment.
- Opening of stomata is the result of the active uptake of K and changes in turgor pressure in the guard cells.
- Cuticle: waxy layer secreted by the epidermis.
- Epidermis: parenchyma with chloroplast (mesophyll).
- Vascular bundle.
- Phloem and xylem.
- Stomata with guard cells and substomatal chamber.
- Veins are vascular bundles.
- Stomata are found on the lower surface where water loss due to evaporation is less.
Cell Structure of Tissues Produced by Secondary Growth
- Vascular cambium: single layer of meristematic cells located between the secondary phloem and secondary xylem. Cells produce new cell toward the inside of stem and another toward the outside, resulting in an increase in stem girth.
- Secondary phloem become sclerenchyma fiber cells, sieve-tube members and companion cells.
- Secondary xylem become tracheids and vessel elements.
- Cork cambium: divides producing cork tissue to the outside of stem and other cells to the inside (peridem).
- Peridem layer replaces epidermis and cortex in stems and roots with secondary growth. Continually broken and sloughed off as the plant grows.
Woody Stem Structure
- Outer cork cells have thick walls impregnated with a waxy material (suberin), dead at maturity.
- Cork cambium.
- Bark: periderm and phloem on the outside of woody plants.
- Sclerenchyma fibers: located in bands in phloem.
- Secondary phloem.
- Vascular cambium.
- Secondary Xylem.
- Lateral rays: radiate from pith through the xylem and expand to a wedge shape in the phloem, forming a phloem ray.
- Annual rings: make up wood of the stem; has several rows of early wood. Early wood: large-diameter cells that grew in the spring.
- Late wood: smaller-diameter cells that grew in the summer, when water is less available.
Primary growth vs Secondary growth
Primary growth is derived from the division from apical meristems in the root tip and shoot tip. It results in the increase in length along plant axis.
Seconday growth is derived from division of vascular cambium and cork cambium. Cells divide and diameter increase.
Woody plants, trees and shrubs continually produce primary tissues as buds grow and develop, new roots are formed. At same time they increase diameter as secondary xylem and phloem are produced by the vascular cambium.
Herbaceous plants have only primary growth.