The progamic phase, the period of pollen tube growth through the pistil, is a period of specific interactions between the male gametophyte and the pistil. Understanding of pollen germination and pollen tube growth are relevant for the study of pollen-pistil interactions and for understanding the function of components specifically accumulated in the transmitting tissue cell walls and intercellular matrix that may interact with pollen tubes.
Pollen germination and pollen tube growthPollen rehydration in angiosperms occurs when the pollen lands on the stigma. Uptake of water results in swelling of the pollen, restoration of its cytoplasmic organization and metabolism, and the build up of osmotic pressure which leads to exposure of the pollen grain apertures and the intine at those sites. Supposedly, at the site of pollen tube emergence, the intine is weakened by local enzyme activities (Mäkinen and Brewbaker 1967; Heslop-Harrison, 1969 1970) which, in combination with osmotic pressure, results in protrusion of the pollen tube. The total volume of the pollen grain varies among species, but in all species studied the volume is maximal at pollen tube emergence, and decreases afterwards (Heslop-Harrison 1987). During pollen imbi-