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A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in vascular tissue, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues. Also, it is a vein in the leaf that contains conducting tissues.
The xylem typically lies adaxial with phloem positioned abaxial. In a stem or root this means that the xylem is closer to the centre of the stem or root while the phloem is closer to the exterior. In a leaf, the adaxial surface of the leaf will usually be the upper side, with the abaxial surface the lower side. This is why aphids are typically found on the underside of a leaf rather than on the top, since the sugars manufactured by the plant are transported by the phloem, which is closer to the lower surface.
The position of vascular bundles relative to each other may vary considerably: see stele.
Bundle-sheath, or sheath-bundle cells are contained in certain C4 plants. These plants use C4 carbon fixation to carry out photosynthesis when normal conditions for C3 plants aren't present (e.g. normal water levels, CO2 levels, heat, light).
Bundle-sheath cells are photosynthetic cells arranged into tightly packed sheaths around the veins of a leaf. They form a protective covering on leaf veins, and consist of one or more cell layers, usually parenchyma. Loosely arranged mesophyll cells lie between the bundle sheath and the leaf surface. The Calvin cycle is confined to the chloroplasts of these bundle sheath cells.
Nervous tissue is the main component of the nervous system - the brain, spinal cord, and nerves-which regulates and controls body functions. It is composed of neurons, which transmit impulses, and the neuroglial cells, which assist propagation of the nerve impulse as well as provide nutrients to the neuron.
Nervous tissue is made of nerve cells that come in many varieties, all of which are distinctly characteristic by the axon or long stem like part of the cell that sends action potential signals to the next cell.
Functions of the nervous system are sensory input, integration, controls of muscles and glands, homeostasis, and mental activity.
All living cells have the ability to react to stimuli. Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialized nerve cells called neurons. Neurons are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibers (neurons) bound together by connective tissue. A sheath of dense connective tissue, the epineuriumsurrounds the nerve. This sheath penetrates the nerve to form theperineuriumwhich surrounds bundles of nerve fibers. Blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers.
The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules called Nissl bodiesare found in the cytoplasm of the cell body. Within the cell body, extremely fineneurofibrils extend from the dendrites into the axon. The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called theneurilemmaor sheath of Schwann cells. The myelin sheath together with the neurilemma is also known as themedullary sheath. This medullary sheath is interrupted at intervals by thenodes of Ranvier.
Nerve cells are functionally made to each other at a junction known as a synapse, where the terminal branches of an axon and the dendrites of another neuron lie close to each other but normally without direct contact. Information is transmitted across the gap by chemical secretions called neurotransmitters. It causes activation in the post-synaptic cell.All cells possess the ability to respond to stimuli. The messages carried by the nervous system are electrical signals called impulses.
Classification of Neurons
Neurons are classified both structurally and functionally.
Structural Classification Neurons are grouped structurally according to the number of processes extending from their cell body. Three major neuron groups make up this classification: multipolar (polar = end, pole), bipolar and unipolar neurons.
- Multipolar Neurons (3+ processes) :
- The are the most common neuron type in humans (more than 99% of neurons belong to this class) and the major neuron type in the CNS
- Bipolar Neurons :
- Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other . An example can be found in the light-sensitive retina of the eye.
- Unipolar Neurons :
- Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar neurons.
Tumors in nervous tissue include:
- Gliomatosis cerebri, Oligoastrocytoma, Choroid plexus papilloma, Ependymoma, Astrocytoma (Pilocytic astrocytoma, Glioblastoma multiforme), Dysembryoplastic neuroepithelial tumour, Oligodendroglioma, Medulloblastoma, Primitive neuroectodermal tumor
- Ganglioneuroma, Neuroblastoma, Atypical teratoid rhabdoid tumor, Retinoblastoma, Esthesioneuroblastoma
From Yahoo Answers
Answers:Sclerenchyma cells function in structural support of the stem and protection of the vascular bundles. Sclerenchyma cells are small and posses tough, thick cell walls. These walls are often impregnated with lignin, a highly branched polymer that makes cell walls more rigid. Mature sclerenchyma cells can not elongate and are found in regions of the plant that have stopped growing in length. At maturity sclerenchyma cells may actually be dead.
Answers:The vascular bundle is used for transport (of water, minerals and the products of photosynthesis) and for support. To transport the water and minerals it has long, hollow xylem vessels which connect the roots and the leaves. For the transport of sugars it has long cells called phloem sieve tubes To support the stem, the vascular bundle has many long fibres which are strengthened with a substance called lignin. The fibres have thick cell walls. John H
Answers:3. carry carbon dioxide to the bundle sheath cells. (just had this question on my bio quiz)
Answers:palisade cells are elongated cells in the palisade layer of a leaf. Their shape prevents them from fitting tightly together and leave holes between the cells to allow air from the outside travel through the leaf which then helps with photosynthesis. xylem cells are tubular, elongated cells that are dead at maturity. They are comprised of tracheids (long tapered water conducting cells) that disintegrates, leaving the cells thickened walls behind to form a nonliving conduit through which water can flow freely. Cilitated cells are cells with hair-like structures on them that usually occur in large numbers on a cell surface. Together, their numerous hairs wave back and forth to promote movement. Their thin shape allows it to be easily flexed back and forth. Nerve cells are bundles of fiber like extensions of neurons that allow electronic signals to be relayed quickly throughout the body of an organism. I hope this helps