Explanation:
In the usual porogamous route, the pollen tube grows through the stigma and style, enters the ovary, reaches the ovule through its funicle, approaches the micropyle, and then enters a synergid.
2. Which feature of the female gametophyte is most directly associated with the final guidance of a pollen tube toward the micropyle?
A.
The antipodal cells at the chalazal end
B.
The central cell surrounding the polar nuclei
C.
The synergids located at the micropylar end
D.
The integumentary cells at the chalazal end
Correct Answer:
The synergids located at the micropylar end
Explanation:
Synergids occupy the micropylar end of the embryo sac and help attract and guide the approaching pollen tube, thereby directing its final growth toward the micropyle.
3. A mutation eliminates the receptive synergid but leaves the micropyle and pollen tube structurally normal. Which effect is most directly expected?
A.
The pollen tube will be unable to germinate on the stigma
B.
The pollen tube may reach the micropyle but fail in effective final guidance or entry
C.
The pollen tube will necessarily reverse its growth toward the chalaza
D.
The pollen tube will enter the ovule through the integument far from the micropyle
Correct Answer:
The pollen tube may reach the micropyle but fail in effective final guidance or entry
Explanation:
The receptive synergid provides important guidance and reception cues near the micropyle. Its absence can impair the final micropylar approach even when earlier pollen-tube growth is normal.
4. If the micropyle of an otherwise viable ovule is experimentally sealed before pollen-tube arrival, which outcome best reflects the role of micropyle-directed growth?
A.
The pollen tube is likely to approach the ovule but cannot use the normal entry route
B.
The pollen tube will automatically enter through the chalaza in every species
C.
The pollen tube will bypass the ovule and fertilize the embryo sac through the funicle
D.
The pollen tube will be redirected into the antipodal cells before reaching the ovule
Correct Answer:
The pollen tube is likely to approach the ovule but cannot use the normal entry route
Explanation:
Micropyle-directed growth normally culminates in entry through the micropyle. Blocking this opening prevents the usual porogamous access route to the micropylar synergids.
5. Which observation would provide the strongest evidence that a chemical signal from the micropylar region guides pollen-tube growth?
A.
Pollen tubes elongate at the same rate in all directions in a uniform medium
B.
Pollen tubes bend specifically toward an isolated micropylar synergid or its secreted material
C.
Pollen grains increase in size after absorbing water on the stigma
D.
Pollen tubes become thicker as they pass through the style
Correct Answer:
Pollen tubes bend specifically toward an isolated micropylar synergid or its secreted material
Explanation:
Directional bending toward a micropylar synergid or its diffusible products indicates chemotropic guidance associated with the micropylar region.
6. A pollen tube grows normally through the style but shows random orientation after entering the ovary. Which process is most specifically defective?
A.
Pollen hydration on the stigma
B.
Pollen mitosis within the anther
C.
Ovular guidance toward the micropyle
D.
Formation of the pollen exine
Correct Answer:
Ovular guidance toward the micropyle
Explanation:
Normal growth through the style shows that early pollen-tube development is intact. Random orientation in the ovary points to failure of cues that guide the tube toward the micropyle.
7. Why is the micropylar position of the synergids functionally significant for pollen-tube growth?
A.
It places the synergids near the normal entry point and allows them to provide terminal guidance cues
B.
It prevents the pollen tube from entering the ovule until the chalaza is reached
C.
It enables the synergids to transport pollen grains through the funicle
D.
It causes the pollen tube to grow away from the embryo sac before entering the ovule
Correct Answer:
It places the synergids near the normal entry point and allows them to provide terminal guidance cues
Explanation:
Because synergids lie adjacent to the micropyle, they are strategically positioned to attract and receive the pollen tube at the end of its directed growth.
8. In a normally oriented anatropous ovule, the most appropriate interpretation of pollen-tube entry through the micropyle is:
A.
Chalazogamy, because the tube enters near the chalaza
B.
Porogamy, because the tube enters through the micropylar opening
C.
Mesogamy, because the tube passes through the integument at any point
D.
Apogamy, because the tube avoids the embryo sac
Correct Answer:
Porogamy, because the tube enters through the micropylar opening
Explanation:
Entry through the micropyle is termed porogamy. It represents the usual micropyle-directed route of pollen-tube access to the embryo sac.
9. If the micropylar synergid secretes a pollen-tube attractant, which experimental result would most strongly support its role in directional growth?
A.
Heat-inactivated secretion attracts pollen tubes as efficiently as active secretion
B.
Pollen tubes accumulate near active secretion but not near an equivalent control medium
C.
Pollen tubes grow only when the ovule is completely removed
D.
Pollen tubes respond only to a signal placed at the chalazal end
Correct Answer:
Pollen tubes accumulate near active secretion but not near an equivalent control medium
Explanation:
Preferential accumulation near active micropylar synergid secretion demonstrates that a localized signal can orient pollen-tube growth toward the micropyle.
10. Which combination would most directly disrupt micropyle-directed pollen-tube growth while leaving pollen germination itself initially unaffected?
A.
Removal of the stigma surface and inhibition of pollen hydration
B.
Blocking the micropyle and disabling guidance signaling from the micropylar synergid
C.
Increasing the number of antipodal cells and enlarging the chalaza
D.
Thickening the pollen exine and preventing pollen dispersal
Correct Answer:
Blocking the micropyle and disabling guidance signaling from the micropylar synergid
Explanation:
The micropyle is the normal entry opening, while the micropylar synergid contributes terminal guidance. Disrupting both directly interferes with the tube's approach and entry without necessarily preventing initial germination.
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Micropyle-Directed Growth
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