2023年12月4日发(作者:采伐)
托福阅读真题第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
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.
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.
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.
xically, 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.
xically, 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.⬛
10.
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