TF阅读真题第907篇Mycorrhizae

Mycorrhizae

 

 

Fungi are organisms, including mushrooms, yeasts, and molds, that absorb and decompose organic material. Most land plants have specialized roots that contain certain kinds of fungi. These plants’ specialized roots and the fungi that inhabit them are together known as mycorrhizae, or fungal roots. The fungi may consume as much as 20 to 30 percent of the plant’s carbon and energy. What does the plant get in return? The answer lies in the unique structure of the fungi, which are composed of slender, threadlike tubes called hyphae. These either form a closely woven mass around the root hairs or penetrate the cells of the root. Because the hyphae threads are much finer than the finest root hairs, they provide access to tiny spaces in the soil that could not otherwise be penetrated. Hyphae are especially helpful to plants in acquiring phosphorus, potassium, copper, zinc, and other nutrients that do not move freely with the flow of water being taken up by roots. The finest of root hairs has a diameter of 20 to 30 micrometers, while the diameter of a strand of mycorrhizal hyphae is only 1 to 2 micrometers.

A plant’s capacity to exploit a patch of soil expands tremendously with the prolific growth of its underground fungal partners. If you took a cubic centimeter of soil from the root zone of a mycorrhizal plant and spread all of the bits and pieces of root and root hairs end to end, the total length might measure a few inches. In that same volume, the length of mycorrhizal hyphae, if completely unraveled, might range from 20 to 40 meters! It is primarily the superiority of mycorrhizae at mining the soil for water and nutrients that makes the association worth the cost to the plant. Some analysts have speculated that in some mycorrhizal associations the roots are doing little more for the plant than serving as a vehicle to transport the attached fungi to deeper soil layers.

In some ecosystems, the mycorrhizal fungi function not just as passive absorbers of nutrients but also as active decomposers. Like many other fungi involved in decomposition, they are capable of releasing powerful enzymes that can externally digest wood and other organic matter. Before the liberated nutrients have a chance to float away in the soil environment, they are immediately taken up by the fungi and directly transmitted to the plant hosts. This shortcut in the nutrient cycle is particularly valuable to plants in tropical ecosystems, where heavy rains often wash free-floating soil nutrients below the root zone before they can be absorbed.

Evidence has been accumulating over the past several decades that, in addition to enhancing the function of the individual roots, mycorrhizae also often serve as a living underground connection between plants of different species, through which water, nutrients, and possibly other substances can be transferred. Because mycorrhizal fungi are not specific to a particular host plant, they often spread from plant to plant and species to species. The fungi do this for purely selfish reasons, of course, not with the objective of creating a pipeline between plants. For the fungi, it is to their advantage to attach to any plant that will have them, as a means of maximizing their intake of energy produced by photosynthesis (the process plants use to obtain energy from sunlight). The movement of nutrients from plant to plant through mycorrhizae was first clearly demonstrated in a field experiment conducted in the mid-1960s. Researchers applied radioactively labeled calcium and phosphorus to the cut stump of a maple tree and then tracked the movement of the calcium and phosphorus into attached mycorrhizae and eventually into adjacent plants. Since then, the movement of calcium, phosphorus, carbon, and nitrogen from plant to plant has been demonstrated in many plant species and in many ecosystems.

The importance of the underground plant-to-plant mycorrhizal conduits remains debatable, but many are convinced that in some ecosystems the sharing of resources through such networks is so great that the plant communities function as a unit and the distinction between individual plants becomes blurred. How large are these underground networks? No one knows for sure. It is quite possible that plants and mycorrhizae of many species are loosely linked together over tracts of land measuring thousands of square meters.

 

 

 

 

 

 

1

Fungi are organisms, including mushrooms, yeasts, and molds, that absorb and decompose organic material. Most land plants have specialized roots that contain certain kinds of fungi. These plants’ specialized roots and the fungi that inhabit them are together known as mycorrhizae, or fungal roots. The fungi may consume as much as 20 to 30 percent of the plant’s carbon and energy. What does the plant get in return? The answer lies in the unique structure of the fungi, which are composed of slender, threadlike tubes called hyphae. These either form a closely woven mass around the root hairs or penetrate the cells of the root. Because the hyphae threads are much finer than the finest root hairs, they provide access to tiny spaces in the soil that could not otherwise be penetrated. Hyphae are especially helpful to plants in acquiring phosphorus, potassium, copper, zinc, and other nutrients that do not move freely with the flow of water being taken up by roots. The finest of root hairs has a diameter of 20 to 30 micrometers, while the diameter of a strand of mycorrhizal hyphae is only 1 to 2 micrometers.

According to paragraph 1,all of the following are true of mycorrhizal fungi EXCEPT:

Negative Factual Information Questions否定事实信息题

AThey are associated with the roots of many kinds of plants.

BThey can use up a large part of their host plant’s ertergy supply.

CTheir hyphae have a diameter of 20 to 30 micrometers.

DThey are made of threadlike tubes called hyphae.

 

2

Fungi are organisms, including mushrooms, yeasts, and molds, that absorb and decompose organic material. Most land plants have specialized roots that contain certain kinds of fungi. These plants’ specialized roots and the fungi that inhabit them are together known as mycorrhizae, or fungal roots. The fungi may consume as much as 20 to 30 percent of the plant’s carbon and energy. What does the plant get in return? The answer lies in the unique structure of the fungi, which are composed of slender, threadlike tubes called hyphae. These either form a closely woven mass around the root hairs or penetrate the cells of the root. Because the hyphae threads are much finer than the finest root hairs, they provide access to tiny spaces in the soil that could not otherwise be penetrated. Hyphae are especially helpful to plants in acquiring phosphorus, potassium, copper, zinc, and other nutrients that do not move freely with the flow of water being taken up by roots. The finest of root hairs has a diameter of 20 to 30 micrometers, while the diameter of a strand of mycorrhizal hyphae is only 1 to 2 micrometers.

Paragraph 1 supports the idea that hyphae’s ability to enter very small spaces in the soil helps plants by

Factual Information Questions事实信息题

Areducing the amount of energy and nutrients that plants need to survive

Bgiving plants access to nutrients that plant roots cannot easily absorb with water

Cpreventing unwanted materials from entering plants’ root hairs

Dcreating spaces large enough for the finest root hairs to enter

 

3

A plant’s capacity to exploit a patch of soil expands tremendously with the prolific growth of its underground fungal partners. If you took a cubic centimeter of soil from the root zone of a mycorrhizal plant and spread all of the bits and pieces of root and root hairs end to end, the total length might measure a few inches. In that same volume, the length of mycorrhizal hyphae, if completely unraveled, might range from 20 to 40 meters! It is primarily the superiority of mycorrhizae at mining the soil for water and nutrients that makes the association worth the cost to the plant. Some analysts have speculated that in some mycorrhizal associations the roots are doing little more for the plant than serving as a vehicle to transport the attached fungi to deeper soil layers.

The word “prolific” in the passage is closest in meaning to

Vocabulary Questions词汇题

Asteady

Busual

Cabundant

Dlater

 

4

A plant’s capacity to exploit a patch of soil expands tremendously with the prolific growth of its underground fungal partners. If you took a cubic centimeter of soil from the root zone of a mycorrhizal plant and spread all of the bits and pieces of root and root hairs end to end, the total length might measure a few inches. In that same volume, the length of mycorrhizal hyphae, if completely unraveled, might range from 20 to 40 meters! It is primarily the superiority of mycorrhizae at mining the soil for water and nutrients that makes the association worth the cost to the plant. Some analysts have speculated that in some mycorrhizal associations the roots are doing little more for the plant than serving as a vehicle to transport the attached fungi to deeper soil layers.

Why does the author include the speculation that “in some mycorrhizal associations the roots are doing little more for the plant than serving as a vehicle to transport the attached fungi to deeper soil layers”?

Rhetorical Purpose Questions修辞目的题

ATo identify a situation in which mycorrhizal associations are costly for plants

BTo emphasize the great importance of mycorrhizal associations for plants’ability to obtain water and nutrients

CTo show that mycorrhizae are more effective in deeper soils than in surface soils

DTo explain why the length of mycorrhizae needs to be greater in deeper soils

 

5

In some ecosystems, the mycorrhizal fungi function not just as passive absorbers of nutrients but also as active decomposers. Like many other fungi involved in decomposition, they are capable of releasing powerful enzymes that can externally digest wood and other organic matter. Before the liberated nutrients have a chance to float away in the soil environment, they are immediately taken up by the fungi and directly transmitted to the plant hosts. This shortcut in the nutrient cycle is particularly valuable to plants in tropical ecosystems, where heavy rains often wash free-floating soil nutrients below the root zone before they can be absorbed.

According to paragraph 3,why are mycorrhizae particularly useful to plants in tropical environments?

Factual Information Questions事实信息题

AThey help hold their host plants in place during heavy rains.

BThey release enzymes that protect their host plants from external damage.

CThey remove organic material that can prevent roots from reaching deeper soil layers.

DThey absorb soil nutrients before the nutrients can be washed away.

 

6

Evidence has been accumulating over the past several decades that, in addition to enhancing the function of the individual roots, mycorrhizae also often serve as a living underground connection between plants of different species, through which water, nutrients, and possibly other substances can be transferred. Because mycorrhizal fungi are not specific to a particular host plant, they often spread from plant to plant and species to species. The fungi do this for purely selfish reasons, of course, not with the objective of creating a pipeline between plants. For the fungi, it is to their advantage to attach to any plant that will have them, as a means of maximizing their intake of energy produced by photosynthesis (the process plants use to obtain energy from sunlight). The movement of nutrients from plant to plant through mycorrhizae was first clearly demonstrated in a field experiment conducted in the mid-1960s. Researchers applied radioactively labeled calcium and phosphorus to the cut stump of a maple tree and then tracked the movement of the calcium and phosphorus into attached mycorrhizae and eventually into adjacent plants. Since then, the movement of calcium, phosphorus, carbon, and nitrogen from plant to plant has been demonstrated in many plant species and in many ecosystems.

According to paragraph 4， why do mycorrhizal fungi spread from plant to plant and species to species?

Factual Information Questions事实信息题

ATo create a nutrient pipeline between plants

BTo obtain as much energy as possible

CTo create stronger connections between plants

DTo find plants that are more likely to host them

 

7

Evidence has been accumulating over the past several decades that, in addition to enhancing the function of the individual roots, mycorrhizae also often serve as a living underground connection between plants of different species, through which water, nutrients, and possibly other substances can be transferred. Because mycorrhizal fungi are not specific to a particular host plant, they often spread from plant to plant and species to species. The fungi do this for purely selfish reasons, of course, not with the objective of creating a pipeline between plants. For the fungi, it is to their advantage to attach to any plant that will have them, as a means of maximizing their intake of energy produced by photosynthesis (the process plants use to obtain energy from sunlight). The movement of nutrients from plant to plant through mycorrhizae was first clearly demonstrated in a field experiment conducted in the mid-1960s. Researchers applied radioactively labeled calcium and phosphorus to the cut stump of a maple tree and then tracked the movement of the calcium and phosphorus into attached mycorrhizae and eventually into adjacent plants. Since then, the movement of calcium, phosphorus, carbon, and nitrogen from plant to plant has been demonstrated in many plant species and in many ecosystems.

According to paragraph 4,a mid-1960s field experiment provided the earliest evidence that

Factual Information Questions事实信息题

Amycorrhizae are not harmed by radioactive calcium and phosphorus

Bnutrients can be transferred from one plant to another through their mycorrhizae

Cmycorrhizal fungi can attach themselves to a wide variety of plants

Dcalcium, phosphorous, carbon, and nitrogen are the most common nutrients taken in by mycorrhizae

 

8

The importance of the underground plant-to-plant mycorrhizal conduits remains debatable, but many are convinced that in some ecosystems the sharing of resources through such networks is so great that the plant communities function as a unit and the distinction between individual plants becomes blurred. How large are these underground networks? No one knows for sure. It is quite possible that plants and mycorrhizae of many species are loosely linked together over tracts of land measuring thousands of square meters.

According to paragraph 5,many believe which of the following about ecosystems characterized by signficant sharing of resources through mycorrhizal networks?

Factual Information Questions事实信息题

AThese ecosystems cover a smaller geographic area than other ecosystems do.

BThese ecosystems contain a wider variety of plant species than other ecosystems do.

CThe plants in such ecosystems act more like a single whole than like separate plants.

DThe plants in such ecosystems require larger amounts of nutrients than plants in other ecosystems do.

 

9

Fungi are organisms, including mushrooms, yeasts, and molds, that absorb and decompose organic material. Most land plants have specialized roots that contain certain kinds of fungi. These plants’ specialized roots and the fungi that inhabit them are together known as mycorrhizae, or fungal roots. The fungi may consume as much as 20 to 30 percent of the plant’s carbon and energy. What does the plant get in return? The answer lies in the unique structure of the fungi, which are composed of slender, threadlike tubes called hyphae.  [■] These either form a closely woven mass around the root hairs or penetrate the cells of the root.  [■] Because the hyphae threads are much finer than the finest root hairs, they provide access to tiny spaces in the soil that could not otherwise be penetrated.  [■] Hyphae are especially helpful to plants in acquiring phosphorus, potassium, copper, zinc, and other nutrients that do not move freely with the flow of water being taken up by roots.  [■] The finest of root hairs has a diameter of 20 to 30 micrometers, while the diameter of a strand of mycorrhizal hyphae is only 1 to 2 micrometers.

Look at the four squaresthat indicate where the following sentence could be added to the passage

The second method of hyphae attachment is much more common than the first.Insert Text Questions句子插入题

Where would the sentence best fit？Click on a square  sentence to the passage.

10

Plants often have specialized roots that combine with fungi to become mycorrhizae, or fungal roots.

Prose Summary Questions概要小结题

Select 3 answers

ASome plants are not associated with mycorrhizal fungi, but those that are tend to be healthier, stronger, and more widespread.

BSome mycorrhizal fungi release chemicals that help decompose organic matter, making available nutrients that the hyphae gather up quickly.

CMycorrhizae allow resources to be shared among plants of different species, creating networks that may be quite large and important.

DAlthough living in association with mycorrhizal fungi has some disadvantages for plants, such fungi absorb hard-to-reach nutrients and pass them on to the host plant.

EMycorrhizae are particularly useful in environments where the soil is so densely packed that plant roots have trouble penetrating it.

FResearch suggests that many mycorrhizal associations are temporary, as fungi frequently move on to new plants in search of more resources.

答案：