Unformatted text preview: -up (may lead to catastrophic fire) • Increase in fire: never achieves stable/climax community, succession is suspended, increase/decrease in biodiversity (with explanation) © 2008 The College Board. All rights reserved. Visit the College Board on the Web: www.collegeboard.com. AP® BIOLOGY 2008 SCORING GUIDELINES
4. Flowering plants have evolved various strategies for fertilization. (a) Describe the process of fertilization in flowering plants. (3 points maximum) Double fertilization (2 points maximum) • Sperm + egg → zygote (2n)/fertilized egg • Sperm (n) + 2 polar nuclei (n+n) → endosperm food source (3n) Pollen tube formation (1 point maximum) • Pollen grain adheres to stigma, absorbs water and germinates; growth of pollen tube (tube nucleus) • Generative nucleus divides into two sperm nuclei (or pollen grain has two sperm nuclei) • Development of embryo sac (female gametophyte) (b) Discuss TWO mechanisms of pollen transfer and the adaptations that facilitate each mechanism. (4 points maximum) Mechanism (1 point for each mechanism discussed with action verb; 2 points maximum) Wind (e.g., blows, carries) Adaptations (1 point for each adaptation appropriate to the mechanism(s) discussed; 2 points maximum) Pollen shape (pits) Lightweight pollen Feather-like, sticky stigma High pollen:ovule ratio Male flowers elevated/exposed anther Stem/stamen modification for pollen release Barbs, spikes on pollen (attaches) Nectar/fragrance/color/UV patterns Coevolution of animals (spec...
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