Tannosomes are formed when the chloroplast membrane forms pockets filled with tannin. Slowly, the pockets break off as tiny vacuoles that carry tannin and to the large vacuole filled with acidic fluid. Tannins are then released into the vacuole and stored inside as tannin accretions.
^Brillouet, J. -M.; Romieu, C.; Schoefs, B.; Solymosi, K.; Cheynier, V.; Fulcrand, H.; Verdeil, J. -L.; Conejero, G. (2013). "The tannosome is an organelle forming condensed tannins in the chlorophyllous organs of Tracheophyta". Annals of Botany112 (6): 1003. doi:10.1093/aob/mct168.edit