B Microfilament Endoplasmic Vesicles containing reticulum cell wall precursors Callose plugs Vegetative Sperm nucleus cells Mitochondrion Exine wall Intine wall Golgi Vesicle fusing with cell membrane at tube tip Figure 19.29 (A)Schematic illustration of a pollen tube growing by tip extension.Callose plugs limit the volume occupied by the cytoplasm to regions close to the grow ing tip.(B)Fluorescence micograph of pollen tubes growing through a pistil The callose plugs that block off the pollen tube at regular intervals are visual- ized by staining with aniline blue