In pollen types, the tube is formed by a wall that apears continuous with intine. for example in Avocado, pollen tube at germiniation may be surrounded only by the plasma membrane. Wall deposition doesnt occur until the pollen tubes have entered the stigma

by ( Sedgly.1979)

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http://www.jstor.org/stable/10.1086/314129?seq=1#page_scan_tab_contents

Germination occurs in an inaperturate pollen grain as a result of exine distension (harmomegathy, hydration). The exine breaks.

If pseudocolpi are present, mixed with colpi as in Melastomataceae, then the endoaperture is the most breakable point for pollen grain germination.

Dear Dr. Barth,

thank you for your comment. As a matter of fact, I still wonder why the term pseudocolpi is coined if pollen tube emergence can take place via exine rupture in stead. Another fact, "pseudocolpi" are thinning in the ektexine and this coincide well with the strict definition of aperture form irrespective to function.

Thank you

In some inaperturate pollen grains e.g. some members of Euphorbiaceae, there are some vulnerable areas of exine from where pollen tube might be germinated and again which is genetically controlled. In Canaceae (also inaperturate) the deposition of sporopollenin over intine is in lumps. So germ tube emerges from any where in the non-deposited area of sporopollenin. Such apertures are also known as omni-aperturate types.

Inaperturate pollen grains e.g. as in monocots have been genetically endowed with the mechanism of germination. As explained by Prakash with the deposition of Sporopollenin, the exine becomes vulnerable for absorption of sugary deposite that enable germination.