Preventing Water Loss
The early land plants split into two very different lineages: the bryophytes-mosses, liverworts, and hornworts-and the vascular plants, which include everything else. Most bryophytes not only lack internal water storage but also lack any system for internal transport. They have no roots and absorb water and minerals directly through their exposed surfaces, from either raindrops or the soil. Water seeps from lower, wetter parts of the plant to upper, drier parts, defying gravity thanks to a physical process called capillary action.
Most of the plants we see around us today are vascular plants, with comprehensive systems of internal transport. Typically larger and more complex than bryophytes, they include ferns, cone- bearing trees and shrubs, and flowering plants. Simple external water transport and tolerance of desiccation, or drying out, are insufficient for the survival of these larger-sized plants. With an internal system of very narrow tubes, however, capillary action and evaporation can maintain an existing stream of water in a tall tree. Evaporation from the leaves exerts a powerful pull on the collective chains of water molecules extending all the way down to the roots. Within the narrow tubes, water molecules cling tenaciously to one another and continuously move upward. However, such a massive internal stream cannot be restored if it should run dry.
In a dehydrated tree or even a much smaller shrub or herb, the connection between the evaporative force at the top and the water supply at the bottom is broken. A renewed water supply in the roots can rise only so high through capillary action before the drag of gravity brings it to a halt. So, except for some relatively small forms, vascular plants cannot recover from dehydration. Vascular plants thus shifted from a strategy of desiccation tolerance to one of desiccation prevention, and they are characterized by adaptations for improved absorption, internal transport, storage, and control of water loss.
Vascular plants protect themselves first of all with a hard, waxy cuticle, a protective covering that is thicker and less permeable than the modest cuticles found in bryophytes. The cuticle is produced by the tightly packed outer layer of cells called the epidermis. Since vascular plants have roots, which do most of the absorption of water, the leaves and other surfaces can be more thoroughly waterproofed than those of nonvascular plants. A thick cuticle can seal a plant off almost completely from the external environment, and that is pretty much what desert cacti do under dry conditions. Cacti store considerable quantities of water in their tissues, and they replenish that store through their roots whenever the soil is moistened by the rain. They can remain alive for months or even years between rare desert storms, but they cannot grow much during that time.
Growth requires an input of carbon dioxide in order to make carbohydrates, so all vascular plants, even cacti, must open themselves up sometimes via tiny pores called stomata. This allows water vapor to diffuse out at the same time carbon dioxide is diffusing in, seeming to defeat the original purpose of the cuticle! But stomata can be closed when the threat of dehydration is highest or when absorption of carbon dioxide is not needed. Most plants close their stomata at night when photosynthesis-the process by which plants convert light energy into chemical energy-is halted. Cacti, however, reverse that, opening to admit carbon dioxide at night, and closing during the heat of the day. Carbon stored at night is then released for use during the daytime. Cacti are highly adapted for desert existence, but the loss of water through stomata is a compromise faced to some degree by all vascular plants.
The unavoidable loss of water through plant leaves via the process of transpiration is clearly a constant threat to plants, but that threat is balanced by some benefits that are not obvious at first. Evaporation of water from the leaves has a cooling effect, like the sweating of animals. Evaporative cooling in leaves exposed to bright sunlight prevents overheating and damage to the photosynthetic system. In addition, as transpiration pulls water upward in the plant, dissolved minerals are pulled up with it, and so this may be essential for keeping the foliage well nourished.
1
The early land plants split into two very different lineages: the bryophytes-mosses, liverworts, and hornworts-and the vascular plants, which include everything else. Most bryophytes not only lack internal water storage but also lack any system for internal transport. They have no roots and absorb water and minerals directly through their exposed surfaces, from either raindrops or the soil. Water seeps from lower, wetter parts of the plant to upper, drier parts, defying gravity thanks to a physical process called capillary action.
According to paragraph 1, all of the following are true about bryophytes EXCEPT:
AThey include three different types of plants.
BThey are typically unable to store water.
CThey take in water through their unprotected surfaces.
DThey have internal transport systems similar to those of vascular plants.
2
Most of the plants we see around us today are vascular plants, with comprehensive systems of internal transport. Typically larger and more complex than bryophytes, they include ferns, cone- bearing trees and shrubs, and flowering plants. Simple external water transport and tolerance of desiccation, or drying out, are insufficient for the survival of these larger-sized plants. With an internal system of very narrow tubes, however, capillary action and evaporation can maintain an existing stream of water in a tall tree. Evaporation from the leaves exerts a powerful pull on the collective chains of water molecules extending all the way down to the roots. Within the narrow tubes, water molecules cling tenaciously to one another and continuously move upward. However, such a massive internal stream cannot be restored if it should run dry.
The word “comprehensive” in the passage is closest in meaning to
ASimple
BThorough
CTypical
Deffective
3
Most of the plants we see around us today are vascular plants, with comprehensive systems of internal transport. Typically larger and more complex than bryophytes, they include ferns, cone- bearing trees and shrubs, and flowering plants. Simple external water transport and tolerance of desiccation, or drying out, are insufficient for the survival of these larger-sized plants. With an internal system of very narrow tubes, however, capillary action and evaporation can maintain an existing stream of water in a tall tree. Evaporation from the leaves exerts a powerful pull on the collective chains of water molecules extending all the way down to the roots. Within the narrow tubes, water molecules cling tenaciously to one another and continuously move upward. However, such a massive internal stream cannot be restored if it should run dry.
According to paragraph 2, which of the following is a characteristic or a tall tree that allows it to avoid drying out?
AIts very deep system of roots
BIts external system of pulling water up from the roots
CIts ability to evaporate water through its leaves
DIts ability to expand the width of its internal tubes during dry conditions
4
In a dehydrated tree or even a much smaller shrub or herb, the connection between the evaporative force at the top and the water supply at the bottom is broken. A renewed water supply in the roots can rise only so high through capillary action before the drag of gravity brings it to a halt. So, except for some relatively small forms, vascular plants cannot recover from dehydration. Vascular plants thus shifted from a strategy of desiccation tolerance to one of desiccation prevention, and they are characterized by adaptations for improved absorption, internal transport, storage, and control of water loss.
According to paragraph 3, improved absorption, internal transport, storage, and control of water loss are all examples of adaptations that help certain plants
Astay alive even during desiccation
Bkeep from drying out
Crecover more quickly after drying out
Destablish a connection to a new water supply
5
Vascular plants protect themselves first of all with a hard, waxy cuticle, a protective covering that is thicker and less permeable than the modest cuticles found in bryophytes. The cuticle is produced by the tightly packed outer layer of cells called the epidermis. Since vascular plants have roots, which do most of the absorption of water, the leaves and other surfaces can be more thoroughly waterproofed than those of nonvascular plants. A thick cuticle can seal a plant off almost completely from the external environment, and that is pretty much what desert cacti do under dry conditions. Cacti store considerable quantities of water in their tissues, and they replenish that store through their roots whenever the soil is moistened by the rain. They can remain alive for months or even years between rare desert storms, but they cannot grow much during that time.
According to paragraph 4, why are cacti able to remain alive for so long between storms?
AThey have thick, protective coverings that help them store large amounts of water.
BThey increase the thickness of their cuticles during storms and decrease it under dry conditions.
CThey grow additional roots during dry seasons.
DThey absorb water through their cuticles when they are moistened by rain.
6
Vascular plants protect themselves first of all with a hard, waxy cuticle, a protective covering that is thicker and less permeable than the modest cuticles found in bryophytes. The cuticle is produced by the tightly packed outer layer of cells called the epidermis. Since vascular plants have roots, which do most of the absorption of water, the leaves and other surfaces can be more thoroughly waterproofed than those of nonvascular plants. A thick cuticle can seal a plant off almost completely from the external environment, and that is pretty much what desert cacti do under dry conditions. Cacti store considerable quantities of water in their tissues, and they replenish that store through their roots whenever the soil is moistened by the rain. They can remain alive for months or even years between rare desert storms, but they cannot grow much during that time.
Growth requires an input of carbon dioxide in order to make carbohydrates, so all vascular plants, even cacti, must open themselves up sometimes via tiny pores called stomata. This allows water vapor to diffuse out at the same time carbon dioxide is diffusing in, seeming to defeat the original purpose of the cuticle! But stomata can be closed when the threat of dehydration is highest or when absorption of carbon dioxide is not needed. Most plants close their stomata at night when photosynthesis-the process by which plants convert light energy into chemical energy-is halted. Cacti, however, reverse that, opening to admit carbon dioxide at night, and closing during the heat of the day. Carbon stored at night is then released for use during the daytime. Cacti are highly adapted for desert existence, but the loss of water through stomata is a compromise faced to some degree by all vascular plants.
In paragraphs 4 and 5, what is the author’s purpose in discussing desert plants called cacti?
ATo help explain why vascular plants cannot recover from complete desiccation
BTo emphasize that drying out is not the only threat faced by vascular plants
CTo provide evidence that growth is almost impossible for vascular plants facing dry conditions.
DTo show how the adaptations of vascular plants can prevent desiccation in even the driest environments
7
Growth requires an input of carbon dioxide in order to make carbohydrates, so all vascular plants, even cacti, must open themselves up sometimes via tiny pores called stomata. This allows water vapor to diffuse out at the same time carbon dioxide is diffusing in, seeming to defeat the original purpose of the cuticle! But stomata can be closed when the threat of dehydration is highest or when absorption of carbon dioxide is not needed. Most plants close their stomata at night when photosynthesis-the process by which plants convert light energy into chemical energy-is halted. Cacti, however, reverse that, opening to admit carbon dioxide at night, and closing during the heat of the day. Carbon stored at night is then released for use during the daytime. Cacti are highly adapted for desert existence, but the loss of water through stomata is a compromise faced to some degree by all vascular plants.
Paragraph 5 suggests that cacti only open their stomata at night because
Amore carbon dioxide is available at night
Bdoing so limits the amount of water they lose through evaporation
Cwater vapor can diffuse into their stomata at night
Dthey need to release carbon dioxide at night, when they are not photosynthesizing
8
The unavoidable loss of water through plant leaves via the process of transpiration is clearly a constant threat to plants, but that threat is balanced by some benefits that are not obvious at first. Evaporation of water from the leaves has a cooling effect, like the sweating of animals. Evaporative cooling in leaves exposed to bright sunlight prevents overheating and damage to the photosynthetic system. In addition, as transpiration pulls water upward in the plant, dissolved minerals are pulled up with it, and so this may be essential for keeping the foliage well nourished.
According to paragraph 6, transpiration helps plants in all of the following ways EXCEPT
Aby keeping plants cool
Bby helping the parts of a plant involved in photosynthesis avoid damage
Cby removing substances that could harm plants
Dby bringing nutrients up to the leaves of plants
9
Vascular plants protect themselves first of all with a hard, waxy cuticle, a protective covering that is thicker and less permeable than the modest cuticles found in bryophytes. [■] The cuticle is produced by the tightly packed outer layer of cells called the epidermis. [■] Since vascular plants have roots, which do most of the absorption of water, the leaves and other surfaces can be more thoroughly waterproofed than those of nonvascular plants. [■]A thick cuticle can seal a plant off almost completely from the external environment, and that is pretty much what desert cacti do under dry conditions. [■]Cacti store considerable quantities of water in their tissues, and they replenish that store through their roots whenever the soil is moistened by the rain. They can remain alive for months or even years between rare desert storms, but they cannot grow much during that time.
Look at the four squaresthat indicate where the following sentence could be added to the passage
This is accomplished by the cuticle
Where would the sentence best fit？Click on a square sentence to the passage.
10
There are two main types of land plants, bryophytes and vascular ,plants.
AVascular plants are good at absorbing water through their external surfaces to prevent themselves from drying out.
BThe most important difference between the two plant lineages is that bryophytes do not use capillary action to absorb water but vascular plants do
CMost vascular plants close their pores during the day to prevent water loss and open them at night when temperatures are cooler.
DVascular plants are able to maintain a chain of water molecules from roots to leaves, but dehydration can permanently disrupt the chain
EThe thick cuticles of vascular plants help them to limit the amount of water lost through evaporation, although evaporation has some benefits.
FWater loss in vascular plants cannot be entirely prevented because they must open their pores to take in carbon dioxide for growth.
