Microfilament Endoplasmic Vesicles containing reticulum cell wall precursors Callose plu Vegetative Sp Mitochondri Exine wall Intine wall Golgi Vesicle fusing with cell mbrane at tube tip Figure 19.29 ( A) Schematic llustration of a pollen tube growing by tip extension. Callose plugs limit the volume occupied by the cytoplasm to regions close to the grow Igure ing tip. (B) Fluorescence micrograph 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 blueFigure 11-9