For millions of years, land plants have lived in close association with soil fungi, forming a relationship known as mycorrhizae (mycorrhizae: symbiotic associations between fungus mycelium and plant roots). This ancient partnership is believed to have played a critical role in the colonization of land by early plants, which initially lacked complex root systems. In this mutualistic exchange, plants provide fungi with carbon compounds, such as sugars, manufactured through photosynthesis. In return, the fungi, with their extensive underground networks, absorb water and essential minerals, particularly phosphorus and nitrogen, from the soil and deliver them to the plant. This exchange is highly efficient; fungal threads can penetrate tiny soil pores that are completely inaccessible to even the finest plant roots, vastly expanding the plant's nutrient-gathering capacity. Beyond simple resource exchange between two partners, these fungal networks often connect multiple plants of different species, forming what ecologists call common mycorrhizal networks (CMNs). Through these shared underground webs, trees and plants in a forest are physically linked. Biologist Tomas Silva has conducted extensive field research demonstrating that these networks allow for the subterranean transport of resources from one plant to another. For instance, in mixed forests, mature trees with abundant access to sunlight can transfer carbon to shaded seedlings that are struggling to survive. This resource sharing is not restricted to members of the same species; paper birch trees have been shown to transfer carbon to Douglas fir trees, and vice-versa, depending on which species is experiencing a seasonal shortage. In addition to sharing physical resources, plants utilize CMNs as rapid communication channels. When a plant is attacked by herbivores, such as insects, it synthesizes defensive chemicals to protect itself. Simultaneously, it transmits chemical warning signals through the fungal network to neighboring plants. Upon receiving these signals, the neighboring plants immediately begin producing their own defensive compounds, even before any herbivores have reached them. This preemptive defense mechanism significantly reduces their vulnerability to infestation. This form of communication is much faster and more targeted than airborne chemical signaling, which is easily disrupted by wind and weather conditions. This cooperative sharing raises evolutionary questions regarding natural selection, which typically favors individuals that maximize their own fitness at the expense of competitors. [1] Traditional evolutionary models suggest that a plant should not expend precious resources to sustain a neighboring plant, especially one of a different species. [2] However, evolutionary biologist Tomas Silva proposes that maintaining a diverse and stable forest community ultimately benefits the individual host. [3] For example, a dying plant could become a breeding ground for pests that might then infect surrounding trees. [4] Furthermore, a diverse forest with a continuous canopy is more resilient to environmental disturbances, such as droughts or high winds, which benefits all resident plants. The fungal partners themselves are not merely passive conduits in this exchange; they are active regulators. Fungi construct these networks using hyphae (hyphae: branching, threadlike filaments that make up the body of a fungus). Research indicates that these hyphae can detect which plants are contributing the most carbon and will preferentially allocate nutrients to those generous partners. Conversely, plants that fail to provide carbon may have their nutrient supply restricted by the fungi. This active bargaining suggests that mycorrhizal networks are not purely altruistic systems, but rather complex biological markets where resources are traded based on supply, demand, and mutual benefit.
TOEFL · Bacaan · 20 soalan
Ujian Latihan TOEFL Bacaan 8
Ujian percubaan pendengaran dan bacaan yang asli, berdasarkan format TOEFL iBT yang diterbitkan.
Ini ialah ujian latihan asli yang ditulis oleh Loopy berdasarkan format yang diterbitkan. Ia bukan ujian rasmi ETS.
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Passage 1
Soalan 1
According to paragraph 1, what did early land plants lack that made mycorrhizae particularly critical for them?
Soalan 2
The word "vastly" in paragraph 1 is closest in meaning to
Soalan 3
According to paragraph 2, resource sharing through common mycorrhizal networks (CMNs) is notable because it
Soalan 4
The word "abundant" in paragraph 2 is closest in meaning to
Soalan 5
What can be inferred from paragraph 3 about plants that are NOT connected to a common mycorrhizal network?
Soalan 6
According to paragraph 3, what advantage does signaling through mycorrhizal networks have over airborne signaling?
Soalan 7
Why does the author mention that "a dying plant could become a breeding ground for pests" in paragraph 4?
Soalan 8
The word "allocate" in paragraph 5 is closest in meaning to
Soalan 9
Look at the four positions [1] [2] [3] [4] in paragraph 4. Where would the following sentence best fit? "Under this perspective, keeping nearby plants healthy serves as a form of indirect self-defense."
Soalan 10 (pilih semua yang berkenaan)
Mycorrhizal networks represent complex underground systems that link land plants and soil fungi in mutually beneficial relationships. （從六個選項中選出三個最能概括全文主要概念的選項；三對得 2 分、兩對得 1 分。）
Passage 2
The Isthmus of Panama, the narrow strip of land connecting North and South America, is one of the most geologically and climatologically significant formations in Earth’s history. Before its creation roughly three million years ago, a deep ocean channel known as the Central American Seaway separated the two continents. This passage permitted the unrestricted movement of water between the Atlantic and Pacific Oceans, mixing their temperatures, salinities, and marine life. The closing of this gateway, driven by tectonic forces, did not merely establish a land bridge for terrestrial animals; it also fundamentally altered global ocean currents, transforming the global climate and setting the stage for the modern epoch of cyclical ice ages. The formation of the isthmus was a gradual process spanning millions of years, initiated by the collision of major tectonic plates. As the Pacific plate pushed eastward, it underwent subduction (subduction: the process where one tectonic plate sinks beneath another) under the lighter Caribbean plate. The intense pressure and heat generated by this sinking plate melted rock into magma, which rose to the surface to form an arc of underwater volcanoes. Over time, these volcanoes grew tall enough to breach the ocean surface, forming chain-like islands. Continuous tectonic uplift and the deposition of sediment (sediment: mineral and organic matter settled at the bottom of water) washed from the emerging lands slowly filled the shallow channels between the islands, eventually forming a continuous land barrier. The closing of the Central American Seaway had immediate and dramatic consequences for the world's oceans. Prior to the complete closure of the seaway, warm equatorial waters moved freely from the Atlantic to the Pacific. [1] However, as the volcanic land bridge emerged, it blocked this westward flow. [2] Deprived of an outlet to the west, the warm water was forced to turn northward along the coast of North America. [3] This diversion of ocean currents fundamentally reorganized the global conveyor belt of marine circulation. [4] Ultimately, this northward flow became the modern Gulf Stream, carrying vast amounts of tropical heat toward northwestern Europe and the Arctic. The newly intensified Gulf Stream brought warm, highly saline water to the high latitudes of the North Atlantic. While it might seem counterintuitive that warmer water would lead to cooling, this warmth was the precise mechanism that triggered the Northern Hemisphere Glaciation. As the warm water reached the cold northern latitudes, it evaporated rapidly, transferring immense amounts of moisture into the atmosphere. Prevailing winds carried this moisture-rich air over northern Canada and Greenland, where it fell as heavy snow. Because these regions were cold enough to prevent summer melt, the accumulating snow compacted into glaciers. Over thousands of years, these glaciers expanded into massive ice sheets, inaugurating a period of global cooling. In addition to altering temperatures, the emergence of the isthmus drastically changed the salinity profile of the Atlantic Ocean. With the Pacific connection severed, the Atlantic became significantly saltier than the Pacific. Warm, dry winds blowing across the narrow isthmus evaporated water from the Atlantic and deposited the freshwater rain into the Pacific, leaving concentrated salt behind in the Atlantic basin. This high salinity made the northward-flowing Atlantic waters exceptionally dense as they cooled near Greenland. This dense, salty water sank rapidly to the ocean floor, driving a deep-ocean circulation system known as the thermohaline circulation (thermohaline circulation: a deep-ocean current system driven by differences in water temperature and salinity). This process sustained a stable global climate system, illustrating how a localized geological event can have far-reaching planetary repercussions.
Soalan 11
According to paragraph 1, what was the state of the Atlantic and Pacific Oceans before the formation of the Isthmus of Panama?
Soalan 12
The word "breach" in paragraph 2 is closest in meaning to
Soalan 13
According to paragraph 2, how did the physical gap between the volcanic islands eventually close?
Soalan 14
The word "it" in paragraph 4 refers to
Soalan 15
What can be inferred from paragraph 4 about the Northern Hemisphere Glaciation?
Soalan 16
According to paragraph 5, all of the following contributed to the high salinity of the Atlantic Ocean EXCEPT:
Soalan 17
The word "severed" in paragraph 5 is closest in meaning to
Soalan 18
Which of the following best expresses the essential information in the highlighted sentence? "The closing of this gateway, driven by tectonic forces, did not merely establish a land bridge for terrestrial animals; it also fundamentally altered global ocean currents, transforming the global climate and setting the stage for the modern epoch of cyclical ice ages."
Soalan 19
Look at the four positions [1] [2] [3] [4] in the passage. Where would the following sentence best fit? "This diversion of ocean currents fundamentally reorganized the global conveyor belt of marine circulation."
Soalan 20 (pilih semua yang berkenaan)
The tectonic formation of the Isthmus of Panama about three million years ago had profound, far-reaching effects on the Earth's oceans and climate. （從六個選項中選出三個最能概括全文主要概念的選項；三對得 2 分、兩對得 1 分。）