托福阅读真题+题目+答案：Marsh Plant Strategies

托福阅读真题+题目+答案：Marsh Plant Strategies

A salt marsh is a coastal ecosystem in the zone between land and open salt water that is regularly flooded by the tides. Although salt marshes are highly productive and superficially lush, they are extremely stressful habitats for vascular plants (plants with internal systems for conducting liquid). The plants that dominate salt marshes are specialized to deal with the problems associated with waterlogged, saline soils (soils that are salty and full of water). Most vascular plants would die under typical salt marsh conditions.

1.The word “superficially” in the passage is closest in meaning to

A.generally

B.particularly

C.on the surface

D.on occasion

Tidal flooding of salt marshes creates waterlogged soils, particularly at low, frequently flooded elevations and in poorly-drained sediments. The physiological problem with waterlogged soils is that they become depleted of oxygen. Marsh plants produce oxygen as a byproduct of photosynthesis (the process by which plants obtain energy from sunlight) aboveground. Their belowground tissues, however, consume oxygen through metabolic activities faster than the soil oxygen levels can be replenished. Without oxygen, plant roots are unable to provide nutrients to aboveground plant structures.

Wetland plants have both architectural and metabolic adaptations to cope with waterlogged soils. Architecturally, plants that live in waterlogged soils usually have roots near the soil surface to facilitate the roots' uptake of oxygen. ▋Smooth cordgrass, which dominates frequently flooded salt marsh habitats on the Atlantic coast of North America, has surface roots in the top 2-3 centimeters of the marsh surface to oxygenate deeper root structures. ▋Mangroves, which dominate tropical wetlands, have aboveground roots that oxygenate belowground roots. ▋Plants that dominate waterlogged soils typically have well-developed aerenchyma tissue, a type of tissue that transports oxygen to belowground structures.▋ The aerenchyma tissue forms a system of air passages extending from the leaves to belowground plant parts that allows oxygen to passively diffuse (spread without the plant's expending energy) to belowground roots. Marsh plants that are common in waterlogged low marsh soils all have well-developed aerenchyma tissue, and many marsh plants respond to waterlogged soil by increasing the size of the tissue. Marsh plants that live in drier high marsh habitats have less developed aerenchyma tissue than those found in low marsh habitats. In addition to these structural modifications, many marsh plants also have well developed metabolic pathways that avoid using oxygen, thus reducing their dependence on root oxygenation.

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