Pollen Tube Growth
There are significant differences in the fine structure of the tips of pollen tubes in compatible and incompatible pistils such as Lilium. Tubes growing in compatible pistils show deep embayment but no compartments (tubes in incompatible pistil have a compartmented cap). The compatible tubes undergo a transition from autotrophic nutrition (characterized by compartmented cap of golgi derived vesicles) to heterotrophic condition in which the secretion product from the stylar canals enter the pollen tubes through the deep embayments (tubes growing in incompatible pistils are unable to make this transition and thus are unable to continue growth due to the exhaustion of endogenous food reserves).
The pollen tube wall is composed mainly of polysaccharides. Since the pollen grain carries limited food reserves and a relatively large amount of new pollen wall material is synthesized during growth, it has been assumed that at least a part of the substance needed for pollen tube wall is contributed from the polysaccharides present in the pistil tissues. It has been shown that the polysaccharide component of the stigmatic exudate of L. longiflorum is incorporated into the cytoplasm of the growing pollen tubes and later a specific fraction of the incorporated exudate is extensively metabolised before being utilized for pollen tube wall biosynthesis.