sentence: Interestingly , a comparison of pollen tube pathways in the pistil in ancient angiosperm species with one to three ovules has shown a difference in pollen tube growth in their open carpels , sealed with secretion rather than tissue ; In Amborella ( Amorellaceae ) only a few pollen tubes enter the open mouth of the stylar canal ( Williams 2009 ) , while in Brasenia and Cobomba ( Cabombaceae ) pollen tubes do not enter through the open mouth but instead reach the canal by growing directly through the carpel wall allowing more pollen tubes to enter ( Taylor and Williams 2009 ) .