Plants: Structure & Function
Background Information
Vascular Plants & Non-Vascular Plants
Vascular plants have tissues the support the transfer of water, nutrients and sugar throughout the plant. These types of plants have stems, roots, and leaves. Vascular plants also contain vascular tissue such as phloem and xylem. Non-vascular plants transport water and waste through osmosis. These plants do not roots, stems or leaves and do not contain a vascular system. Below is a great presentation for teaching students about vascular and non-vascular plants.
Overview of Vascular Plant Structure
Vascular plants have 3 main non-reproductive organs. These organs include stems, roots, and leaves. Plant systems are also characterized into two systems. The shoot system includes the stems and leaves and the root system includes all of the root materials. These organs are generally composed of three tissues. These tissues include:
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Dermal Tissue: protect aganist injury, herbivores, disease and water loss
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Vascular Tissue: transports water and nutrients to the plans & supports the plant body
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Ground Tissue: cellular processes to support growth and development, store carbohydrates (especially starch), and support and protect the plant body
Vascular plants contain xylem and phloem. These are the main structures for transporting sugar, water and nutrients in plants. Xylem transports water and minerals from the roots upward, but movement is only in one direction. Phloem transports sugars throughout the plant. Movement in phloem cells is bilateral, so the sugars move up the stem and down the stem.
Plants also contain a tissue known as meristematic tissue. Meristematic tissue is an area of actively dividing undifferentiated cells. These are found in areas of the plant where growth occurs and eventually develop into specialized cells.
Please see the button above for labelling plant strutures
Figure 1: The general structure of a vascular plant
Leaf Structure
The leaves of a plant help carry out its roles in photosynthesis and gas exchange. Most leaves have a flattened area known as the blade. The blade is attached to a stem-like structure known as the petiole. Venation is the arrangement of veins in the leaf. Veins contain the vascular tissue of the leaf. The cells in a leaf are arranged to support photosynthesis and gas exchange. The epidermal cells are tightly packed and covered by a waxy coating called the cuticle. The cuticle prevents water loss and provides a physical barrier against bacteria, moulds and insects. The mesophyll contains chloroplasts are specializes in photosynthesis. There are two parts to the mesophyll. The palisade mesophyll is a layer of elongated photosynthetic cells arranged in columns under the upper surface of a leaf. The spongy mesophyll is a layer of loosely packed photosynthetic cells with large spaces between them, under the lower surface of a leaf. In aquatic plants, the spongy mesophyll is replaced with aerenchyma. Arenchyma is composed of loosely packed parenchyma cells with large pores. The stoma is an opening in the epidermis of the leaf that specializes in gas exchange. The stoma contains two guard cells that control the opening and closes of the stoma.
See the above button for a cross-section diagram of the internal leaf structure
Below see the video above to see how leaves structures interact to carry out photosynthesis.
Stem Structure
Plants stems can be categorized into types. Herbaceous stems do not contain wood. Woody stem do contain wood. Please see the Venn diagram below for an herbaceous and woody stem comparison.
Herbaceous stems contain vascular bundles. Vascular bundles are an arrangement of vascular tissue that consists of xylem and phloem. Vascular bundles run continuously from the root to the leaves. In each vascular bundle, the xylem is always closer to the center of the stem and the phloem is closer to the outside.
Woody stems contain vascular cambium. Vascular cambium is a layer of meristematic cells in vascular tissue that divide to form new xylem and phloem cells. A new layer of xylem and phloem tissue is produced each year. Xylem is on the inside of the vascular cambium and the phloem is on the outside. Sapwood is the younger xylem that transports water and minerals to the leaves. Heartwood is the old xylem layers that fill up with resins and oils that can no longer conduct water. Heartwood is very rigid and helps support the tree. Bark is the protective outermost layer that contains of all the tissues outside of the vascular cambium including phloem, cork cambium and cork.
Stem Structure: Cell Types in Vascular Tissue
Two types of xylem tissue:
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tracheid - long, cylindrical cell with tapered ends
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all cell walls of a tracheid have pits (small holes) that allow water and solutes in
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vessel element - shorter and wider than a tracheid and has less tapered ends
Three types of phloem cells:
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sieve cells - narrow pores in all their cell walls
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sieve tube elements - contains pores in its side walls and a sieve plate at the end walls
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companion cell -small, nucleated phloem cell that is alwasy associated with a sieve tube element
Root Structure
Roots can be classfied into two systems. A taproot system has a large, thick, main root that grows straight downwards. The taproot may also have lateral roots branching from it. Both taproots and lateral roots have root hairs. Root hairs is an extension of the root that allows for increased surface area, resulting in higher absorption of water and dissolved nutrients. A fibrous root system has many small roots. Fibrous roots tend to be shallower than taproots and also contain root hairs.
The root cortex is a region of parenchyma cells beneath the epidermis of root. The root cortex ends at the endodermis. The walls of the endodermis are wrapped with a wax-like substance, forming a barrier called the casparian strip.
Transport in Vascular Plants
Each organ in a plant has a different method of transport. The roots use osmosis and active transport. The stem use capillary action/root pressure. The leaves use transpiration.
Roots
In the roots, the cytoplasm of the plant cells has a lower concentration of water molecules than the soil water, and therefore uses osmosis to allow water to enter the roots. Roots also use active transport to transport nutrients to the endodermis. The endodermis contains the Casparian strip which prevents all substances from passing through the spaces between the endodermal cells.
Stem
In the xylem, capillary actions occurs to transport liquid. This happens due to attractive forces in the liquid molecules. A column of liquid is held together by a weak attractive forces between molecules and rises because of attractive forces between the liquid and the sides of the tube. The water molecules in the xylem sap stick to each other and are also down up the sides of the xylem tubes.
Leaves
The main driving force of transport in the plant is in the leaves by a process known as transpiration. Transpiration occurs through the stomata. The guard cells in the stomata allow for the opening and closing of the stomata. When the guard cells are swelled with water they allow the stomata to open and gas exchange to occur. When the guard cells lose water, they become limp and flaccid, forcing the stomata closed and reducing gas exchange.