Fertilization pathway in flowering plants During pollination and fertilization, how does the male gamete nucleus actually reach the egg cell within the ovule?
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AIt rides on bee legs and directly sticks to the egg cell
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BThe pollen grain germinates a pollen tube; the male nucleus travels down this tube to the egg cell
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CIt is blown by wind directly into the embryo sac
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DIt diffuses through the stigma without any structure
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EIt is carried by nectar into the ovule
Answer
Correct Answer: The pollen grain germinates a pollen tube; the male nucleus travels down this tube to the egg cell
Explanation
Introduction / Context:Angiosperm fertilization involves precise cellular interactions from stigma to ovule. Understanding the role of the pollen tube clarifies how male gametes reach the female gametophyte.
Given Data / Assumptions:
- Pollination places pollen on the stigma.
- Fertilization requires male gamete nuclei to reach the egg cell in the embryo sac.
- The plant provides guided growth mechanisms.
Concept / Approach:Pollen hydration and germination on the stigma initiate pollen tube growth through the style to the ovary. Two sperm cells are delivered via this tube; one fuses with the egg (zygote), the other with the central cell (endosperm) in double fertilization.
Step-by-Step Solution:
Pollen adheres to stigma and absorbs water.Pollen tube emerges, guided by chemical cues along the style.Tube enters the ovule via micropyle and releases the two sperm cells; one nucleus fuses with the egg nucleus.Verification / Alternative check:Microscopy studies show tube growth and discharge in synergid cells, confirming delivery of sperm nuclei through the pollen tube.
Why Other Options Are Wrong:
- Bee legs/wind/nectar: assist pollination, not the gamete delivery into the embryo sac.
- Diffusion through stigma: gametes do not diffuse; a pollen tube is required.
Common Pitfalls:Equating pollination (transfer of pollen) with fertilization (fusion of gametes); they are distinct processes.
Final Answer:The pollen grain germinates a pollen tube; the male nucleus travels down this tube to the egg cell