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.
Various dyes (e.g., a yellow dye from bearberry, a reddish dye from henna, and a pale blue dye from blue ash) can be extracted from leaves (see Appendix 3), although nearly all commercial dyes are now derived from coal tar. Many cordage fibers for ropes and twines come from leaves, with various species of Agave (century plants) accounting for about 80% of the world's production. Bowstring fibers are obtained from a relative of the common house plant Sansevieria (see Fig. 24.28), and Manila hemp fibers, which are used both in fine-quality cordage and in textiles, are obtained from the leaves of a close relative of the banana. Panama hats are made from the leaves of the panama hat palm, and palms and grasses are used in the tropics as thatching material for huts and other buildings.
In the high mountains of Chile and Peru, the leaves of the yareta plant are used for fuel. They produce a resin that causes the leaves to burn with an unusually hot flame. Leaves of many plants produce oils. Petitgrain oil, from a variety of orange tree leaves, and lavender, for example, are used for scenting soaps and colognes. Patchouli and lemon-grass oils are used in perfumes, as is citronella oil, which was once the leading mosquito repellent before synthetic repellents gained favor. Eucalyptus oil, camphor, cajeput, and pennyroyal (Fig. 7.26) are all used medicinally.
Leaves are an important source of drugs used in medicine and also of narcotics and poisons. Cocaine, obtained from plants native to South America, has been used medicinally and as a local anesthetic, but its use as a narcotic has, in recent years, become a major problem in western cultures. Andean natives chew coca leaves while working and are reported capable of performing exceptional feats of labor with little or no food while under the influence of
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Glass Cuts from Grass?
We all know that if we drop a glass bottle or dish, we need to pick up the pieces so that someone won't get hurt on the sharp edges of the broken glass. But did you know that something as soft as a blade of grass can also give you a nasty cut if you don't handle it carefully?
Many leaves, including those of grasses, are produced with tiny teeth along the margins. In some instances when a person brushes against the edge of the leaf, the teeth alone have enough rigidity to act like a miniature saw that can produce a cut on human skin. One of the reasons grasses are generally not palatable to humans is that they accumulate silica—the primary ingredient of glass. In some grasses, including rice cutgrass (Leersia oryzoides— Box Figure 7.1), the tiny teeth along the margins are covered with a film of glass that gives the downward-pointing teeth additional rigidity. This tiny saw instantly cuts if human skin is brushed against it.
If you have the opportunity to do so, examine the leaf margins of several grasses with a microscope and see if any of them have teeth encased in glass.
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