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Leaf Internal Structure - Leaves contain cells that carry out photosynthesis, gas exchange, and evaporation. Modified Leaves - In some plants, leaf development has been modified to provide for a unique need.
Flower They basically arises from the floral primordium which may be attached by small stack with plants body. The plant body that develops after germination depends on the activities of meristematic tissues.
Meristematic tissues are lumps of small cells with dense cytoplasm and propor-tionately large nuclei that act like stem cells in animals. That is, one cell divides to give rise to two cells. One remains meristematic, while the other is free to differentiate and contribute to the plant body.
In this way, the population of meristem cells is continually renewed. Molecular genetic evidence supports the hypothesis that stem cells and meristem cells may also share some common molecular mechanisms. Elongation of both root and shoot takes place as a result of repeated cell divisions and subsequent elongation of the cells produced by the apical meristems. In some vascular plants, including shrubs and most trees, lateral meristems produce an increase in girth.
Apical Meristems Apical meristems are located at the tips of stems and at the tips of roots, just behind the root cap.
The plant tissues that result from primary growth are called primary tissues. During periods of growth, the cells of apical meristems divide and continually add more cells to the tips of a seedlings body. Thus, the seedling lengthens. Primary growth in plants is brought about by the apical meristems. The elongation of the root and stem forms what is known as the primary plant body, which is made up of primary tissues.
The primary plant body comprises the young, soft shoots and roots of a tree or shrub, or the entire plant body in some herbaceous plants. Both root and shoot apical meristems are composed of delicate cells that need protection. The root apical meristem is protected from the time it emerges by the root cap.
Root cap cells are produced by the root meristem and are sloughed off and replaced as the root moves through the soil.
A variety of adaptive mechanisms protect shoot apical meristem during germination figure The epicotyl or hypocotyl stemlike tissue above or below the cotyledons may bend as the seedling emerges to minimize the force on the shoot tip. In the monocots a late evolving group of angiosperms there is often a coleoptile sheath of tissue that forms a protective tube around the emerging shoot. Later in development, the leaf primordia cover the shoot apical meristem which is particularly susceptible to desiccation.
The apical meristem gives rise to three types of embryonic tissue systems called primary meristems. Cell division continues in these partly differentiated tissues as they develop into the primary tissues of the plant body. The three primary meristems are the protoderm, which forms the epidermis; the procambium, which produces primary vascular tissues primary xylem and primary phloem ; and the ground meristem, which differentiates further into ground tissue, which is composed of parenchyma cells.
In some plants, such as horsetails and corn, intercalary meristems arise in stem internodes, adding to the inter-node lengths. If you walk through a corn field when the corn is about knee high on a quiet summer night, you may hear a soft popping sound.
This is caused by the rapid growth of intercalary meristems. The amount of stem elongation that occurs in a very short time is quite surprising. Lateral Meristems Many herbaceous plants exhibit only primary growth, but others also 8.
Most trees, shrubs, and some herbs have active lateral meristems, which are cylinders of meristematic tissue within the stems and roots. Although secondary growth increases girth in many nonwoody plants, its effects are most dramatic in woody plants which have two lateral meristems.
Within the bark of a woody stem is the cork cambium, a lateral meristem that produces the cork cells of the outer bark. Just beneath the bark is the vascular cambium, a lateral meristem that produces secondary vascular tissue. The vascular cambium forms between the xylem and phloem in vascular bundles, adding secondary vascular tissue on opposite sides of the vascular cambium.
Secondary xylem is the main component of wood. Secondary phloem is very close to the outer surface of a woody stem. Removing the bark of a tree dam-ages the phloem and may eventually kill the tree. Tissues formed from lateral meristems, which comprise most of the trunk, branches, and older roots of trees and shrubs, are known as secondary tissues and are collectively called the secondary plant body.
Coordination of primary and secondary meristematic growth produces the body of the adult sporophyte plant. Plant bodies do not have a fixed size. Parts such as leaves, roots, branches, and flowers all vary in size and number from plant to planteven within a species. The development of the form and structure of plant parts may be relatively rigidly controlled, but some aspects of leaf, stem, and root development are quite flexible.
As a plant grows, the number, location, size, and even structure of leaves and roots are often influenced by the environment. A vascular plant consists of a root system and a shoot system. The root system anchors the plant and penetrates the soil, from which it absorbs water and ions crucial to the plants nutrition.
The shoot system consists of the stems and their leaves. The stem serves as a framework for positioning the leaves, the principal sites of photosynthesis. The arrangement, size, and other features of the leaves are of critical importance in the plants production of food.
Flowers, other reproductive organs, and, ultimately, fruits and seeds are also formed on the shoot. The reiterative unit of the vegetative shoot consists of the internode, node, leaf, and axillary buds.
Axillary buds are apical meristems derived from the primary apical meristem that allow the plant to branch or replace the main shoot if it is munched by an herbivore. A vegetative axillary bud has the capacity to re-iterate the development of the primary shoot. When the plant has transited to the reproductive phase of development, these axillaries may produce flowers or floral shoots.
Three basic types of tissues exist in plants: Each of the three basic tissues has its own distinctive, functionally related cell types. Some of these cell types In plants limited to primary growth, the dermal system is composed of the epidermis. This tissue is one cell thick in most plants, and forms the outer pro-tective covering of the plant. In young exposed parts of the plant, the epidermis is covered with a fatty cutin layer constituting the cuticle; in plants such as the desert succulents, a layer of wax may be added outside the cuticle.
In plants with secondary growth, the bark forms the outer protective layer and is considered a part of the dermal tissue system. Ground tissue consists primarily of thin-walled parenchyma cells that are initially but briefly more or less spherical. However, the cells, which have living protoplasts, push up against each other shortly after they are produced and assume other shapes, often ending up with 11 to 17 sides.
Parenchyma cells may live for many years; they function in storage, photosynthesis, and secretion. Vascular tissue includes two kinds of conducting tissues: The phloem also transports hormones, amino acids, and other substances that are necessary for plant growth. Xylem and phloem differ in structure as well as in function. Primary and secondary growth play important roles in establishing the basic body plan of the organism. Here we will look at how these meristems give rise to highly differentiated tissues that support the growing plant body.
In the earliest vascular plants, the vascular tissues produced by primary meristems played the same conducting roles as they do in contemporary vascular plants. There was no differentiation of the plant body into stems, leaves, and roots. The presence of these three kinds of organs is a property of most modern plants.
It reflects increasing specialization in relation to the demands of a terrestrial existence. With the evolution of secondary growth, vascular plants could develop thick trunks and become treelike.
This evolutionary advance in the sporophyte generation made possible the development of forests and the domination of the land by plants. Reproductive constraints would have made secondary growth and increased height nonadaptive if it had occurred in the gametophyte generation. Judging from the fossil record, secondary growth evolved independently in several groups of vascular plants by the middle of the Devonian period million years ago.
There were two types of conducting systems in the earliest plants systems that have become characteristic of vascular plants as a group. Sieve-tube members conduct carbohydrates away from areas where they are manufactured or stored.
Vessel members and tracheids are thick-walled cells that transport water and dissolved minerals up from the roots. Both kinds of cells are elongated and occur in linked Sieve-tube members are characteristic of phloem tissue; vessel members and tracheids are characteristic of xylem tissue.
In primary tissues, which result from primary growth, these two types of tissue are typically associated with each other in the same vascular strands. In secondary growth, the phloem is found on the periphery, while a very thick xylem core develops more centrally.
You will see that roots and shoots of many vascular plants have different patterns of vascular tissue and secondary growth. Keep in mind that water and nutrients travel between the most distant tip of a redwood root and the tip of the shoot. For the system to work, these tissues connect, which they do in the transition zone between the root and the shoot.
In the next section, we will consider the three tissue systems that are present in all plant organs, whether the plant has secondary growth or not. Xylem, the principal water-conducting tissues of plants, usually contains a combination of vessels, which are continuous tubes formed from dead, hollow, cylindrical cells vessel members arranged end to end, and tracheids, which are dead cells that taper at the ends and overlap one another. In some plants, such as gymnosperms, tracheids are the only water-conducting cells present; water passes in an unbroken stream through the xylem from the roots up through the shoot and into the leaves.
When the water reaches the leaves, much of it passes into a film of water on the outside of the parenchyma cells, and then it diffuses in the form of water vapor into the intercellular spaces and out of the leaves into the surrounding air, mainly through the stomata. This diffusion of water vapor from a plant is known as transpiration.
In addition to conducting water, dissolved minerals, and inorganic ions such as ni-trates and phosphates throughout the plant, xylem supplies support for the plant body. Primary xylem is derived from the procambium, which comes from the apical meristem. Secondary xylem is formed by the vascular cambium, a lateral meristem that develops later. Wood consists of accumulated secondary xylem.
Vessel members are found almost exclusively in an-giosperms. In primitive angiosperms, vessel members tend to resemble fibers and are relatively long. In more advanced angiosperms, vessel members tend to be shorter and wider, resembling microscopic, squat coffee cans with both ends removed. Both vessel members and tracheids have thick, lignified secondary walls and no living protoplasts at maturity.
Lignin is produced by the cell and secreted to strengthen When the continuous stream of water in a plant flows through tracheids, it passes through pits, which are small, mostly rounded-toelliptical areas where no secondary wall material has been deposited.
The pits of adjacent cells occur opposite one another. In contrast, vessel members, which are joined end to end, may be almost completely open or may have bars or strips of wall material across the open ends. Vessels appear to conduct water more efficiently than do the overlapping strands of tracheids. We know this partly because vessel members have evolved from tracheids independently in several groups of plants, suggesting that they are favored by natural selection.
It is also probable that some types of fibers have evolved from tracheids, becoming specialized for strengthening rather than conducting. Some ancient flowering plants have only tracheids, but virtually all modern angiosperms have vessels. Plants, with a muta-tion that prevents the differentiation of xylem, but does not affect tracheids, wilt soon after germination and are unable to transport water efficiently.
In addition to conducting cells, xylem typically includes fibers and parenchyma cells ground tissue cells. The parenchyma cells, which are usually produced in horizontal rows called rays by special ray initials of the vascular cambium, function in lateral conduction and food storage. An initial is another term for a meristematic cell. It divides to produce another initial and a cell that differentiates into a ray cell.
In cross-sections of woody stems and roots, the rays can be seen radiating out from the center of the xylem like the spokes of a wheel. Fibers are abundant in some kinds of wood, such as oak Quercus , and the wood is correspondingly dense and heavy. The arrangements of these and other kinds of cells in the xylem make it possible to identify most plant genera and many species from their wood alone.
These fibers are a major component in modern paper. Earlier paper was made from fibers in phloem. Phloem, which is located toward the outer part of roots and stems, is the principal food-conducting tissue in vascular plants. If a plant is girdled by removing a substantial strip of bark down to the vascular cambium , the plant eventually dies from starvation of the roots. Food conduction in phloem is carried out through two kinds of elongated cells: Seedless vascular plants and gymnosperms have only sieve cells; most angiosperms have sieve-tube members.
Both types of cells have clusters of pores known as sieve areas. Sieve areas are more abun-dant on the overlapping ends of the cells and connect the protoplasts of adjoining sieve cells and sieve-tube members. Both of these types of cells are living, but most sieve cells and all sieve-tube members lack a nucleus at maturity. This type of cell differentiation has parallels to the differentiation of human red blood cells which also lack a nucleus at maturity.
In sieve-tube members, some sieve areas have larger pores and are called sieve plates. Sieve-tube members occur end to end, forming longitudinal series called sieve tubes. Sieve cells are less specialized than sieve-tube members, and the pores in all of their sieve areas are roughly of the same diameter.
In an evolutionary sense, sieve-tube members are more advanced, more specialized, and, presumably, more efficient. Each sieve-tube member is associated with an adjacent specialized parenchyma cell known as a companion cell. Companion cells apparently carry out some of the metabolic functions that are needed to maintain the associated sieve-tube member.
In angiosperms, a common initial cell divides asymmetrically to produce a sieve-tube member cell and its companion cell. Companion cells have all of the Fibers and parenchyma cells are often abundant in phloem. The three tissue systems are found in the three kinds of vegetative organs in plants: Roots have a simpler pattern of organization and development than stems, and we will consider them first.
Four zones or regions are commonly recognized in developing roots. The zones are called the root cap, the zone of cell division, the zone of elongation, and the zone of matu-ration figure In three of the zones, the boundaries are not clearly defined. When apical initials divide, daughter cells that end up on the tip end of the root become root cap cells. Cells that divide in the opposite direction pass through the three other zones before they finish differentiating.
As you consider the different zones, visualize the tip of the root moving away from the soil surface by growth. This will counter the static image of a root that diagrams and photos convey. Show related SlideShares at end. WordPress Shortcode. Full Name Comment goes here. Are you sure you want to Yes No. Valarie Koch Your opinions matter! Mohit Raj amazing Show More. AnKit SRivastav , Student at cccc. No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide.
Biology project 1. A state of having sufficient biological defenses to avoid infection, disease,or other unwanted biological invasion. Capability of the body to resist harmful microbes from entering the body. Vikas Dixit Sir, duringacademic session as per the guidelines issuesby Central Board of Secondary Education. Vikas Dixit External Examiner P. Granulocytes Neutrophils: Neutrophils defend against bacterial or fungal infection , have a multilobed nucleus. The life span of a circulating human neutrophil is about 5.
Eosinophils primarily deal with parasitic infections. Eosinophils are also the predominant inflammatory cells in allergic reactions. They have a bi-lobed nucleus. Basophils are chiefly responsible for allergic and antigen response by releasing the chemical histamine causing vasodilation. Agranulocytes Lymphocytes: B cells, T cells and natural killer cells.
Monocytes eventually leave the bloodstream to become tissue macrophages.