TOEFL Reading Mock Test 1

The transition of plants from aquatic environments to terrestrial landscapes approximately 450 million years ago represents one of the most pivotal events in the history of life on Earth. However, the terrestrial realm was a harsh, dry environment, and early land plants lacked the complex root systems necessary to extract water and essential minerals from the primitive, nutrient-poor soil. To overcome these formidable challenges, early plants formed close, mutually beneficial associations with soil fungi, a symbiotic relationship known as mycorrhiza (mycorrhiza: a cooperative association between a fungus and the roots of a plant). Fossilized remains of early land plants from the Devonian period reveal structures that are remarkably similar to the fungal associations observed in modern plant species. This evolutionary partnership was so successful that today, approximately eighty to ninety percent of all terrestrial plant families maintain some form of mycorrhizal relationship. Biologists classify mycorrhizal associations into two primary categories based on how the fungal cells interact with the physical structure of the plant root: ectomycorrhizae and endomycorrhizae. In ectomycorrhizal associations, which are most common among woody trees in temperate forests, the fungal cells form a dense sheath, or mantle, around the outer surface of the root. They also penetrate between the outer cells of the root itself, forming a network but never actually entering the individual plant cells. In contrast, endomycorrhizal associations, particularly the widespread arbuscular mycorrhizae, are characterized by the fungus actually penetrating the cell walls of the root cortex. Inside the plant cells, the fungi branch into delicate, tree-like structures called arbuscules. These specialized structures do not rupture the plant cell membrane; instead, they press against it, maximizing the surface area of contact between the two organisms without killing the host cell. The primary benefit of the mycorrhizal relationship for the host plant lies in the vast surface area provided by the fungal partner. [1] Fungi absorb mineral nutrients and water from the soil through their network of hyphae (hyphae: fine, branching tubes that make up the body of a multicellular fungus). [2] Consequently, the host plant gains access to pools of phosphorus, nitrogen, and essential micronutrients that would otherwise remain locked in the soil matrix. [3] In return for these nutrients, the plant transfers up to twenty percent of its photosynthetically produced carbon, in the form of simple sugars, to the fungus. [4] This exchange is highly regulated, with both partners adjusting their contributions based on the availability of resources; a plant growing in nutrient-rich soil may restrict carbon allocation to its fungal partner, while a fungus may withhold nutrients if the plant does not provide adequate sugars. Beyond simple nutrient exchange between a single plant and its fungal partner, mycorrhizal networks can extend through the soil to connect multiple plants of the same or different species. These underground systems, often referred to as common mycorrhizal networks, allow for the transfer of resources and information across entire plant communities. Research has demonstrated that mature canopy trees can funnel carbon and nutrients through these networks to shaded seedlings, significantly improving the seedlings' chances of survival in the dim forest understory. Furthermore, plants can use these networks to transmit chemical warning signals. When a plant is attacked by herbivores, it can send distress signals through the mycorrhizal network to neighboring plants, prompting them to synthesize defensive chemical compounds before the pests arrive. Despite the ecological resilience of mycorrhizal associations, modern human activities pose a substantial threat to these delicate underground systems. Intensive agricultural practices, particularly deep tillage—the mechanical turning of soil—disrupt and physically fragment the extensive networks of hyphae, reducing their ability to assist crop plants. Additionally, the heavy application of synthetic chemical fertilizers, particularly those rich in soluble phosphorus, can suppress mycorrhizal formation. Because plants in highly fertilized soils can obtain nutrients without fungal assistance, they often cease allocating carbon to their fungal partners, leading to the decline of the local mycorrhizal community. Recognizing this, sustainable agricultural initiatives are increasingly focusing on conservation tillage and the inoculation of soils with beneficial fungi to restore these natural symbiotic networks, thereby reducing reliance on chemical inputs and enhancing crop resilience.

Glaciers are massive, slow-moving rivers of ice that form in areas where the accumulation of snow exceeds its ablation (ablation: the loss of snow and ice through melting or evaporation) over many years. Though they appear static to the naked eye, glaciers are dynamic systems in constant motion, driven primarily by the relentless pull of gravity. The rate and style of this movement are not uniform; rather, they depend on temperature, ice thickness, and the topography of the underlying landscape. To understand how these colossal structures shape Earth's valleys and transport massive quantities of rock, geologists categorize glacial movement into two primary mechanisms: internal deformation and basal sliding. Internal deformation, also known as plastic flow, occurs throughout the entire body of the glacier, but it is most pronounced at depth where pressure is greatest. Deep within the glacier, the weight of the overlying ice exerts immense pressure on the ice crystals below. Under this pressure, the solid ice loses its rigidity and begins to behave like a highly viscous, plastic fluid. The individual ice crystals align parallel to the direction of the glacier's flow and slide past one another along internal planes of weakness. This process of microscopic slippage accumulates, causing the glacier to deform and creep slowly downslope. Because the friction against the bedrock slows the ice at the very bottom, and the lack of pressure keeps the top layer brittle, the fastest-moving portion of a glacier undergoing pure internal deformation is typically near the upper-middle section. In contrast, basal sliding involves the entire glacier sliding as a single, cohesive unit over the bedrock beneath it. This mechanism is highly dependent on the presence of liquid water at the glacier's base, which acts as a lubricant to reduce friction. Under the massive weight of a glacier, the melting point of ice at the base decreases—a phenomenon called pressure melting (pressure melting: the lowering of the melting temperature of ice under high pressure). This allows a thin film of water to form even when temperatures are slightly below freezing. This subglacial water fills the microscopic gaps between the ice and the rock, allowing the glacier to glide smoothly over the terrain. Basal sliding can account for up to 90 percent of a glacier's total movement, particularly in temperate regions where temperatures hover near the freezing point. The geological characteristics of the bedrock also play a critical role in determining the efficiency of basal sliding. When a glacier encounters a bedrock obstacle, such as a large ridge, the ice on the upstream side experiences localized pressure increases. This localized pressure triggers pressure melting, and the resulting water flows around the obstacle to the downstream side, where the pressure is lower. Once on the downstream side, the water refreezes because of the pressure drop, a cyclical process known as regulation. This constant melting and refreezing allows the glacier to flow past obstacles that would otherwise halt its progress. However, if the bedrock is highly permeable or fractured, the lubricating water may drain away into the ground, drastically reducing the sliding rate and forcing the glacier to rely more heavily on internal deformation. [1] In some regions, the volume of subglacial water can increase dramatically during summer melting cycles. [2] As surface water drains through deep crevasses, it pools at the glacier's bed. [3] This sudden lubrication can cause the entire ice mass to surge forward unexpectedly. [4] Such rapid advancements, known as glacial surges, can displace millions of tons of ice over just a few weeks. These events demonstrate that glacial movement is not always a steady, predictable process but can instead be highly episodic and sensitive to external climatic shifts.