Passage of Pollen Tube
In cotton, the pollen produces a tube within an hour which grows on the surface of the stigmatic hairs, and then between the cells of the stigma at the bases of hairs and beyond. The cytoplasm of the stigmatic hair degenerates; no exudate is secreted. The tube continues growth through the intercellular spaces of the thin-walled cells of transmitting tissue. After reaching the thick-walled cells of the main strand, it actually grows through wall layer 3. It has been reported that pollen tube of Petunia grows within the compact matrix of the middle lamella of the transmitting tissue by enzymatically creating a pipe-like path in front. By the increased dictyosome activity the cells become thicker. Callose is deposited in the pit fields on the transmitting tissue after the passage of pollen tube.
The pollen tube passage probably changes the permeability of cells and callose is formed as a wound response and as a reaction against cell leakage. Once the pollen germinates and the pollen tube has penetrated the stigmatic tissue, the path of the pollen tube through the rest of the stigma and style appears to be determined by the nature and structure of the cell walls and the morphology and distribution of the transmitting tissue. The nutritive role of the transmitting tissue was recognized early. Pollen tubes of Lilium, Petunia and Oenothera are shown to draw nourishment (sugar and amino acids) from the stylar tissue. Growth of tubes through style causes an increased inflow of carbohydrates into the pistils. In Aegle marmelos the cells surrounding the stylar canals show an optimal concentration of starch just before pollination, subsequently as the starch is digested, the canal cells and the basal portions of the stigmatic papillae show reducing sugars which also disappear within 3 days after pollination. Disappearance of stylar starch has also been observed in Fritillaria, Zephyranthes and Pavonia.