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Difficulty: MediumEvolutionary Trends and Adaptive Features in Organisms

During the evolutionary diversification of land plants, major structural and physiological modifications occurred to overcome terrestrial environmental stresses. Which of the following evolutionary trends allowed seed plants to become fully independent of external liquid water for sexual reproduction?

  1. The development of pollen tubes to transport non-motile male gametes directly to the ovuleAnswer
  2. B
    The production of flagellated sperm cells that swim through xylem vessels to reach the female gamete
  3. C
    The persistence of a dominant, independent gametophyte generation equipped with true vascular tissues
  4. D
    The formation of naked seeds enclosed entirely within mature ovary walls prior to pollination

Answer

The development of pollen tubes to transport non-motile male gametes directly to the ovule
The major evolutionary adaptation that freed seed-bearing plants from requiring external water for fertilization was siphonogamy. Microspores develop into pollen grains that transfer by wind or vectors and germinate a pollen tube to carry non-motile male nuclei directly to the egg in the ovule.

Step-by-Step Solution

1
Analyze primitive plant reproductive requirements
Lower land plants such as mosses (bryophytes) and ferns (pteridophytes) rely on swimming flagellated sperm, which mandate a film of environmental surface water for fertilization.
Identifying the evolutionary barrier presented by water-dependent fertilization.
2
Examine the evolutionary adaptation in spermatophytes (seed plants)
Spermatophytes evolved microspores that develop into pollen grains capable of wind or animal dispersal, followed by siphonogamy (pollen tube growth).
Determining how male gametes reach the female gametophyte without swimming in liquid water.
3
Select the correct adaptive trend
The pollen tube directly conveys non-motile male gametes to the female gametophyte within the ovule, achieving complete terrestrial independence for fertilization.
Matching the structural adaptation to the evolutionary trend of land survival.

Key Concept

Evolutionary trend in plant reproduction: Siphonogamy and water independence
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Topics

Evolutionary Trends and Adaptive Features in Organisms

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