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ACT Reading: Natural Science (Drill 4)

Drill 4 · Reading · Natural Science

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About This Drill

ACT Reading: Natural Science (Drill 4) is a Reading practice drill covering Natural Science. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.

Natural Science passages cover topics in biology, chemistry, physics, earth science, and ecology. As you read, focus on understanding the central phenomenon being described, the evidence or mechanisms the author presents, and any cause-and-effect relationships in the passage.

Passage

NATURAL SCIENCE: This passage is adapted from the article "The Language Beneath the Soil: How Trees Communicate Through Fungal Networks" by Ingrid Halvorsen (©2019, Forest Ecology Today). Beneath the forest floor, a network of extraordinary complexity connects tree to tree through a web of fungal filaments called mycorrhizae. These threadlike structures, thinner than a human hair, form symbiotic relationships with the roots of most tree species: the fungus receives sugars produced through photosynthesis, and in return extends the tree's effective root surface area by orders of magnitude, dramatically improving its access to water and soil nutrients. What researchers have more recently discovered is that this network does more than facilitate individual nutrition. Trees in a forest can transfer carbon, nitrogen, and water to one another through mycorrhizal connections, and the transfers are not random. Studies in temperate forests have found that large, established trees, sometimes called "hub trees" or "mother trees", send disproportionate quantities of carbon to smaller, younger trees, particularly seedlings growing in low-light conditions where photosynthesis alone would be insufficient for survival. The mechanisms behind these targeted transfers are not fully understood. Some researchers propose that the transfers are a passive consequence of concentration gradients, nutrients move toward where they are most needed simply because of differential pressure. Others suggest that trees may actively regulate flows through biochemical signaling, though evidence for deliberate "communication" remains contested. What is clear is that the mycorrhizal network functions as infrastructure: a forest is not a collection of independent organisms competing for resources but an integrated system in which the fates of individual trees are materially linked. The practical implications for forestry are significant. Logging practices that remove hub trees disrupt not only the canopy but the underground network, potentially impairing regeneration across a much wider area than the cleared patch itself.

Questions & Explanations

Question 1. The main purpose of the passage is to:

  • A) argue that trees are conscious organisms capable of deliberate decision-making.
  • B) describe the mycorrhizal network, explain how it enables resource transfers between trees, and note its implications for forestry. ✓
  • C) compare two competing theories about how mycorrhizal networks formed over evolutionary time.
  • D) challenge the prevailing view that fungi are beneficial to forest ecosystems

Explanation: B is correct. The passage introduces mycorrhizae (paragraph 1), explains nutrient transfers including the hub tree phenomenon (paragraphs 2–3), and closes with forestry implications (paragraph 4), a clear descriptive and analytical arc. A overstates the passage; the author explicitly says "deliberate communication remains contested." C is wrong, the two competing theories appear briefly in paragraph 3 but are not the passage's main focus. D directly contradicts the passage, which presents fungi as essential to forest health.

Question 2. According to the passage, the fungus benefits from its relationship with tree roots by:

  • F) receiving sugars produced through photosynthesis. ✓
  • G) gaining access to more sunlight through the forest canopy.
  • H) obtaining carbon and nitrogen directly from the atmosphere.
  • J) using tree root systems as protection from soil disturbance.

Explanation: F is correct. The passage states directly: "the fungus receives sugars produced through photosynthesis." This is a straightforward detail question with a direct textual answer. G, H, and J are not mentioned in the passage as benefits to the fungus.

Question 3. The passage most strongly suggests that removing hub trees from a forest is problematic because:

  • A) hub trees produce more oxygen than younger trees and their removal worsens air quality.
  • B) their deep root systems are what prevent soil erosion across the surrounding area
  • C) they are key nodes in the underground network, and their removal can impair regeneration well beyond the cleared area. ✓
  • D) younger trees have not yet developed mycorrhizal connections and cannot survive without hub tree shade according to the study.

Explanation: C is correct. The final paragraph states that removing hub trees "disrupt not only the canopy but the underground network, potentially impairing regeneration across a much wider area than the cleared patch itself." A introduces oxygen production, which is not mentioned. B introduces soil erosion, also not mentioned. D misreads the passage, young trees do have mycorrhizal connections; the problem is loss of the carbon source, not shade.

Question 4. As it is used in the passage, "concentration gradients" refers to:

  • F) the tendency for nutrients to move from areas of higher concentration to areas of lower concentration. ✓
  • G) the biochemical signals trees use to direct nutrients to specific locations.
  • H) the density of mycorrhizal filaments per square meter of forest floor.
  • J) the rate at which young seedlings absorb carbon directly from the surrounding soil

Explanation: F is correct. The passage explains: "nutrients move toward where they are most needed simply because of differential pressure"; this is the passive concentration gradient explanation. Nutrients move from high-concentration areas (established trees) to low-concentration areas (seedlings) without any active direction. G describes the competing biochemical signaling hypothesis, not concentration gradients. H and J are not mentioned.

Question 5. The author's statement that "a forest is not a collection of independent organisms competing for resources but an integrated system" is best understood as:

  • A) a summary of the passage's central insight about how forests actually function. ✓
  • B) an admission that competition between trees does not occur in any form.
  • C) a claim that trees are biologically more similar to animals than plants, in the passage's account
  • D) a concession that the mycorrhizal network theory remains unproven.

Explanation: A is correct. This statement closes the third paragraph and crystallizes the entire passage's argument: the mycorrhizal network transforms our understanding of a forest from a competitive arena to a cooperative, interconnected system. B is too strong, the passage doesn't say competition never occurs, only that the network creates material interdependence. C introduces animal comparisons not present in the passage. D is wrong, the author is making a confident claim here, not expressing doubt.