The important factors that stimulates the pollen tube germination are:
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The essential requirement for the pollen germination is the high relative humidity.
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The stigma should possess a catching and retaining surface for the pollen.
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The stigma also produces certain exudates which are highly viscous and adhesive with high refractive index. These exudates will helps in the retaining the pollen on the stigma and also initiates in germination.