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Humans use shade trees and shrubs in landscaping for cooling as well as for aesthetic effects. The leaves of shade plants planted next to a dwelling can make a significant difference in energy costs to the homeowner. Humans also use for food the leaves of cabbage, parsley, lettuce, spinach, chard, and the petioles of celery and rhubarb, to mention a few. Many spices and flavorings are derived from leaves, including thyme, marjoram, oregano, tarragon, peppermint, spearmint, wintergreen, basil, dill, sage, cilantro, and savory.
The leaves of many oaks and several other plants generally turn some shade of brown or tan when their cells break down and die, due to a reaction between leaf proteins and tannins stored in the cell vacuoles. This is similar to the formation of leather when tannins react with animal hides. Leaves of many other deciduous plants, however, exhibit a variety of colors and drop before turning brown.
If the leaves of all plants could function normally under any environmental condition, various leaf modifications would provide no special benefits to a plant. But the form and structure of tropical rain-forest plants do not adapt them to thrive in a desert, and cacti soon die if planted in a creek because their structure, form, and life cycles are attuned to specific combinations of environmental factors, such as temperature, humidity, light, water, and soil conditions. The modifications of leaves occupying any single ecological niche may be very diverse, resulting in such a rich variety of leaf forms and specializations throughout the Plant Kingdom that only a few may be mentioned here.
Highly specialized insect-trapping leaves have intrigued humans for hundreds of years. Almost 200 species of flowering plants are known to have these leaves. Insectivorous plants grow mostly in swampy areas and bogs of tropical and temperate regions. In such environments, certain needed elements, particularly nitrogen, may be deficient in the soil, or they may be in a form not readily available to the plants.
Most photosynthesis takes place in the mesophyll between the two epidermal layers, with two regions often being distinguishable. The uppermost mesophyll consists of compactly stacked, barrel-shaped, or post-shaped parenchyma cells that are commonly in two rows. This region is called the palisade mesophyll and may contain more than 80% of the leaf’s chloroplasts. The lower region, consisting of loosely arranged parenchyma cells with abundant air spaces between them, is called the spongy mesophyll. Its cells also have numerous chloroplasts.
The lower epidermis of most plants generally resembles the upper epidermis, but the lower is perforated by numerous tiny pores called stomata. Some plants (e.g., alfalfa, corn) have these pores in both leaf surfaces, while others (e.g., water lilies) have them exclusively on the upper epidermis; they are absent altogether from the submerged leaves of aquatic plants.
If a typical leaf is cut transversely and examined with the aid of a microscope, three regions stand out: epidermis, mesophyll, and veins (referred to as vascular bundles in our discussion of roots and stems). The epidermis is a single layer of cells covering the entire surface of the leaf. The epidermis on the lower surface of the blade can sometimes be distinguished from the upper epidermis by the presence of tiny pores called stomata, which are discussed in the section that follows.
Many of the roughly 275,000 different species of plants that produce leaves can be distinguished from one another by their leaves alone. The variety of shapes, sizes, and textures of leaves seems to be almost infinite. The leaves of some of the smaller duckweeds are less than 1 millimeter (0.04 inch) wide. The mature leaves of the Seychelles Island palm can be 6 meters (20 feet) long, and the floating leaves of a giant water lily, which reach 2 meters (6.5 feet) in diameter, can support, without sinking, weights of more than 45 kilograms (100 pounds) distributed over their surface.
The earliest records of glass being used by humans date back to about 2600 B.C., when the ancient Egyptians and Babylonians made beads from the material. The use of glass panes for windows, however, did not begin until the Roman Imperial period a little over 2,000 years ago. Since then, the use of glass windows for admitting light to buildings of all sizes and shapes has become almost universal.
Do you know the properties of wood ? Are you sure?
Hereby the complete list the properties of wood, explained in depth.
The use of wood by humans for fuel, shelter, weapons, and other purposes dates back into antiquity, and present uses are so numerous that it would be impossible to list in a work of this type more than the most important ones. Before discussing the economic importance of wood, let’s take a brief look at its properties.
Although most higher plants have an erect shoot system, many species have specialized stems that are modified for various functions. The overall appearance of specialized stems may differ markedly from that of the stems discussed so far, but all stems have nodes, internodes, and axillary buds; these features distinguish them from roots and leaves, which do not have them. The leaves at the nodes of these specialized stems are often small and scalelike. They are seldom green, but full-sized functioning leaves may also be produced.
Most monocots (e.g., grasses, lilies) are herbaceous plants that do not attain great size. The stems have neither a vascular cambium nor a cork cambium and thus produce no secondary vascular tissues or cork. As in herbaceous dicots, the surfaces of the stems are covered by an epidermis, but the xylem and phloem tissues produced by the procambium appear in cross section as discrete vascular bundles scattered throughout the stem instead of being arranged in a ring.
In the early stages of development, the primary tissues of stems of young herbaceous dicots, woody dicots, and conebearing trees are all arranged in a similar fashion. In woody plants, however, obvious differences begin to appear as soon as the vascular cambium and the cork cambium develop.
In general, plants that die after going from seed to maturity within one growing season (annuals) have green, herbaceous (nonwoody) stems. Most monocots are annuals, but many dicots are also annuals.
Primary xylem, primary phloem, and the pith, if present, make up a central cylinder called the stele in most younger and a few older stems and roots. The simplest form of stele, called a protostele, consists of a solid core of conducting tissues in which the phloem usually surrounds the xylem. Protosteles were common in primitive seed plants that are now extinct and are also found in whisk ferns, club mosses , and other relatives of ferns. Siphonosteles, which are tubular with pith in the center, are common in ferns.
There is an apical meristem (tissue in which cells actively divide) at the tip of each stem, and it is this meristem that contributes to an increase in the length of the stem. The apical meristem is dormant before the growing season begins.
A woody twig consists of an axis with attached leaves . If the leaves are attached to the twig alternately or in a spiral around the stem, they are said to be alternate, or alternately arranged. If the leaves are attached in pairs, they are said to be opposite, or oppositely arranged, or if they are in whorls (groups of three or more), their arrangement is whorled. The area, or region (not structure), of a stem where a leaf or leaves are attached is called a node, and a stem region between nodes is called an internode. A leaf usually has a flattened blade, and in most cases is attached to the twig by a stalk called the petiole.
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Neat Blue 3.3.1
- [Fixed] Font-size issue
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- [Improved] Transload & Upload progress bar style
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The pH (acidity or alkalinity) of a soil affects both the soil and the plants growing in it in various ways. Cranberries, for example, thrive under acidic conditions, but a soil that is unusually acid or alkaline may be toxic to the roots of other plants, and mycorrhizae do not survive in soils having pH extremes. These conditions, however, do not normally directly affect plants nearly as much as they affect nutrient availability.
Water in the soil occurs in three forms. Hygroscopic water is physically bound to the soil particles and is unavailable to plants. Gravitational water drains out of the pore spaces after a rain. If drainage is poor, it is this water that interferes with normal plant growth. Plants are mainly dependent on the third type, capillary water, which is water held against the force of gravity, in pores of the soil. The structure and organic matter of the soil—which enable the soil to hold water against the force of gravity—the density and type of vegetational cover, and the location of underground water tables largely determine the amount of capillary water available to the plant.
Years ago, prospectors looking for promising sites to mine silver or gold often noticed that certain plants grew on old mine tailings. They reasoned that these plants might be indicators of the precious metals entrapped in the soil. It turns out that these prospectors had it right.
Soil texture refers to the relative proportions of sand, silt, and clay in a given soil. Sands are usually composed of many small particles bound together chemically or by a cementing matrix material. Silt consists of particles that are mostly too small to be seen without a lens or a microscope.
Roots have been important sources of food for humans since prehistoric times, and some, such as the carrot, have been in cultivation in Europe for at least 2,000 years. A number of cultivated root crops involve biennials (i.e., plants that complete their life cycles from seed to flowering and back to seed in two seasons). Such plants store food in a swollen taproot during the first year of growth, and then the stored food is used in the production of flowers in the second season.
Although almost 80% of our atmosphere consists of nitrogen gas, plants cannot convert the nitrogen gas to usable forms. A few species of bacteria, however, produce enzymes with which they can convert nitrogen into nitrates and other nitrogenous substances readily absorbed by roots. Members of the Legume Family (Fabaceae), which includes peas, beans, alfalfa, and a few other plants such as alders, form associations with certain soil bacteria that result in the production of numerous small swellings called root nodules that are clearly visible when such plants are uprooted.
More than three-quarters of all seed plant species have various fungi associated with their roots. The association is mutualistic; that is, both the fungus and the root benefit from it and are dependent upon the association for normal development.
