Since neatblue 3.3.2, now we add child theme named "Sweet Purple"
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.