首頁 > 專欄

托福閱讀真題第298篇LeafShapes(答案文章最后)

更新時間:2023-12-04 12:01:45 閱讀：評論：0

2023年12月4日發(作者：采伐)

-

托福閱讀真題第298篇LeafShapes（答案文章最后）

Leaf Shapes

Plant leaves absorb sunlight and carbon dioxide for

photosynthesis, the process of converting sunlight, carbon

dioxide and water into a chemical ud 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 conrve 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 n of what we e. 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 ems to have little significance. espite 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° -10° higher than that of the surrounding air. This

increas 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. The 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;

the 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 compresd 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

urs of water and preferring moist soils. In drier areas still, such

as Mediterranean areas of low trees and shrubs where rainfall is

asonal, or northern regions where unfrozen water can be in

short supply, leaves become smaller still. Small leaves may also

be a respon 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 ems to have found a partial solution to the lf

internal shading. . The leaves have hard, T shaped stony cells

penetrating the leaf like drawing pins stuck into the surface. The 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. s 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.

leaves absorb sunlight and carbon dioxide for

photosynthesis, the process of converting sunlight, carbon

dioxide and water into a chemical ud for energy. Pores on the

leaf absorb the carbon dioxide, but they also allow water to

escape in a process called transpiration. The conflictingneeds to

intercept light and take up carbon dioxide on the one hand, and

to conrve water on the other, have resulted in a number of

different leaf shapes in different environments.

many factors govern leaf shape that it can be difficult to

make n of what we e. 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 ems to have little significance. espite this

wealth of variation due to genetics and growing conditions, a number of generalizations can be made about leaf shape.

many factors govern leaf shape that it can be difficult to

make n of what we e. 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 randomvariation within a single tree

or species that shape ems to have little significance. espite this

wealth of variation due to genetics and growing conditions, a

number of generalizations can be made about leaf shape.

, 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° -10° higher than that of the surrounding air. This

increas 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.

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. The 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;

the 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