Structure and Function

The aperture usually acts as the (exclusive) germination site. Pollen tubes in inaperturate angiosperm pollen are produced without a preformed exit zone. In taxoid pollen the exine ruptures during hydration at a specialized region, the tenuitas, ulcus, or papilla in the center of a circular leptoma and is subsequently shed.

The intine including the protoplast is released and a pollen tube can be formed anywhere (resembling functionally an inaperturate pollen grain). Furthermore some angiosperm taxa shed the exine before pollen tube formation, e.g., in some Annonaceae.

During germination, usually a single pollen tube is formed. However, sometimes tube-like structures ("instant pollen tubes") are simultaneously formed in the anther or very quickly in shed pollen immediately after water contact. Their production is interpreted as a pre-germinative process (BLACKMORE and CANNON 1983).

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