Un site concis qui traite de la question, plus vidéo :
http://www.heliotown.com/Juniper_Pollen.html
Juniper Pollen : Exine Rupture Video
Pollen grains of Juniperus communalis are spherical and range from 21 to 25 micrometers in diameter.
The exine (outer pollen wall) is relatively thin, while the intine (inner pollen wall) is thick. In contact with water or aqueous environments, the intine will swell and often ruptures through the exine.
Here then is a short video of a Juniper pollen grain in water under a cover glass swelling and then rupturing as described above.