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托福阅读真题第298篇Leaf Shapes Leaf Shapes Plant leaves absorb sunlight and carbon dioxide for photosynthesis, the process of converting sunlight, carbon dioxide and water into a chemical used for energy. Pores on the leaf absorb the carbon dioxide, but they also allow water to escape in a process called transpiration. The conflicting needs to intercept light and take up carbon dioxide on the one hand, and to conserve water on the other, have resulted in a number of different leaf shapes in different environments. So many factors govern leaf shape that it can be difficult to make sense of what we see. Leaf size and shape vary according to the conditions under which they grow, including day length, temperature, moisture, and nutrition. Leaves growing later in the summer can be progressively more deeply lobed or longer and narrower. It is perhaps not surprising, therefore, that it is sometimes very difficult to identify a tree from a leaf. Sometimes there is so much apparently random variation within a single tree or species that shape seems to have little significance. Despite this wealth of variation due to genetics and growing conditions, a number of generalizations can be made about leaf shape. Large, flat leaves are obviously good at catching light. Moreover, they hold a thick, still layer of air over the leaf called a boundary layer, which thermally insulates the leaf leading to a temperature 3°C -10°C higher than that of the surrounding air. This increases photosynthesis but also water loss and can lead to leaf burning in bright light. Large leaves tend therefore to be found in shaded areas in wet and humid places where water loss is not so crucial and there is less risk of tearing, exactly the situation in many tropical forests. With small leaves (or leaflets of compound leaves), the boundary layer is thinner, air moves more easily, and the leaf is kept cool by convection (the flow of air) rather than evaporation of water. These leaves are therefore more common in areas where water is more precious, such as the less humid conditions of temperate areas. Lobes and teeth on a leaf create turbulence to destroy the boundary layer, making the leaf effectively smaller sill; these are common in clearings in tropical forests where high light intensity and lower humidity require efficient cooling, and on larger-leaved temperate trees. This is further accentuated in trees such as poplars, where the petioles (the small stalks that attach the leaf to the stem) are compressed at the side, causing the leaf to shake in the gentlest of breezes, further removing the boundary layer. This is akin to shaking your hands to dry them, which may account for poplars being some of the most excessive users of water and preferring moist soils. In drier areas still, such as Mediterranean areas of low trees and shrubs where rainfall is seasonal, or northern regions where unfrozen water can be in short supply, leaves become smaller still. Small leaves may also be a response to poor or wet soils, which limit root growth and therefore ability to take up water. Plants of dry, sunny areas also tend to have thick leaves. Thick leaves are less efficient at photosynthesis. The chloroplast layer (the part of the leaf that conducts photosynthesis) is thicker, and thick leaves shade each other and compete for carbon dioxide, but they produce more food without extra transpiration costs. The olive tree seems to have found a partial solution to the self internal shading. . The leaves have hard, T shaped stony cells penetrating the leaf like drawing pins stuck into the surface. These prongs appear to have the function of transmitting light into the leaf. Paradoxically, thick leaves are also found in just the opposite conditions: rain forests. Here the tough, glossy leaves are designed to reduce the removal of minerals by the abundant rain sloshing over the leaves. Rain is encouraged to run off by the glossy surface and the elongation of the leaf tip into a“drip tip." This prevents water resting on the leaf and the removal of minerals, and the growth of light-robbing organisms on the surface. As might be anticipated, tall rain forest trees that emerge from the canopy above others have thick leaves but without drip tips: they are dried rapidly by the sun. 1.Plant leaves absorb sunlight and carbon dioxide for photosynthesis, the process of converting sunlight, carbon dioxide and water into a chemical used for energy. Pores on the leaf absorb the carbon dioxide, but they also allow water to escape in a process called transpiration. The conflicting needs to intercept light and take up carbon dioxide on the one hand, and to conserve water on the other, have resulted in a number of different leaf shapes in different environments. 2.So many factors govern leaf shape that it can be difficult to make sense of what we see. Leaf size and shape vary according to the conditions under which they grow, including day length, temperature, moisture, and nutrition. Leaves growing later in the summer can be progressively more deeply lobed or longer and narrower. It is perhaps not surprising, therefore, that it is sometimes very difficult to identify a tree from a leaf. Sometimes there is so much apparently random variation within a single tree or species that shape seems to have little significance. Despite this wealth of variation due to genetics and growing conditions, a number of generalizations can be made about leaf shape. 3.So many factors govern leaf shape that it can be difficult to make sense of what we see. Leaf size and shape vary according to the conditions under which they grow, including day length, temperature, moisture, and nutrition. Leaves growing later in the summer can be progressively more deeply lobed or longer and narrower. It is perhaps not surprising, therefore, that it is sometimes very difficult to identify a tree from a leaf. Sometimes there is so much apparently random variation within a single tree or species that shape seems to have little significance. Despite this wealth of variation due to genetics and growing conditions, a number of generalizations can be made about leaf shape. 4.Large, flat leaves are obviously good at catching light. Moreover, they hold a thick, still layer of air over the leaf called a boundary layer, which thermally insulates the leaf leading to a temperature 3°C -10°C higher than that of the surrounding air. This increases photosynthesis but also water loss and can lead to leaf burning in bright light. Large leaves tend therefore to be found in shaded areas in wet and humid places where water loss is not so crucial and there is less risk of tearing, exactly the situation in many tropical forests. 5.With small leaves (or leaflets of compound leaves), the boundary layer is thinner, air moves more easily, and the leaf is kept cool by convection (the flow of air) rather than evaporation of water. These leaves are therefore more common in areas where water is more precious, such as the less humid conditions of temperate areas. Lobes and teeth on a leaf create turbulence to destroy the boundary layer, making the leaf effectively smaller sill; these are common in clearings in tropical forests where high light intensity and lower humidity require efficient cooling, and on larger-leaved temperate trees. This is further accentuated in trees such as poplars, where the petioles (the small stalks that attach the leaf to the stem) are compressed at the side, causing the leaf to shake in the gentlest of breezes, further removing the boundary layer. This is akin to shaking your hands to dry them, which may account for poplars being some of the most excessive users of water and preferring moist soils. In drier areas still, such as Mediterranean areas of low trees and shrubs where rainfall is seasonal, or northern regions where unfrozen water can be in short supply, leaves become smaller still. Small leaves may also be a response to poor or wet soils, which limit root growth and therefore ability to take up water. 6.With small leaves (or leaflets of compound leaves), the boundary layer is thinner, air moves more easily, and the leaf is kept cool by convection (the flow of air) rather than evaporation of water. These leaves are therefore more common in areas where water is more precious, such as the less humid conditions of temperate areas. Lobes and teeth on a leaf create turbulence to destroy the boundary layer, making the leaf effectively smaller sill; these are common in clearings in tropical forests where high light intensity and lower humidity require efficient cooling, and on larger-leaved temperate trees. This is further accentuated in trees such as poplars, where the petioles (the small stalks that attach the leaf to the stem) are compressed at the side, causing the leaf to shake in the gentlest of breezes, further removing the boundary layer. This is akin to shaking your hands to dry them, which may account for poplars being some of the most excessive users of water and preferring moist soils. In drier areas still, such as Mediterranean areas of low trees and shrubs where rainfall is seasonal, or northern regions where unfrozen water can be in short supply, leaves become smaller still. Small leaves may also be a response to poor or wet soils, which limit root growth and therefore ability to take up water. 7.Plants of dry, sunny areas also tend to have thick leaves. Thick leaves are less efficient at photosynthesis. The chloroplast layer (the part of the leaf that conducts photosynthesis) is thicker, and thick leaves shade each other and compete for carbon dioxide, but they produce more food without extra transpiration costs. The olive tree seems to have found a partial solution to the self internal shading. . The leaves have hard, T shaped stony cells penetrating the leaf like drawing pins stuck into the surface. These prongs appear to have the function of transmitting light into the leaf. 8.Paradoxically, thick leaves are also found in just the opposite conditions: rain forests. Here the tough, glossy leaves are designed to reduce the removal of minerals by the abundant rain sloshing over the leaves. Rain is encouraged to run off by the glossy surface and the elongation of the leaf tip into a“drip tip." This prevents water resting on the leaf and the removal of minerals, and the growth of light-robbing organisms on the surface. As might be anticipated, tall rain forest trees that emerge from the canopy above others have thick leaves but without drip tips: they are dried rapidly by the sun. 9.Paradoxically, thick leaves are also found in just the opposite conditions: rain forests. ⬛ Here the tough, glossy leaves are designed to reduce the removal of minerals by the abundant rain sloshing over the leaves. ⬛ Rain is encouraged to run off by the glossy surface and the elongation of the leaf tip into a“drip tip." ⬛ This prevents water resting on the leaf and the removal of minerals, and the growth of light-robbing organisms on the surface. As might be anticipated, tall rain forest trees that emerge from the canopy above others have thick leaves but without drip tips: they are dried rapidly by the sun. ⬛ 10.
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