Fruit evolution has been labile among major clades of Hydrangeaceae. Most members of the family have capsular fruits, but these display a range of dehiscence patterns. The plesiomorphic condition for the family is a septicidal, basipetal dehiscence of the ovary wall proceeding from near the fruit apex (at the style base). In the basal clades, this form of dehiscence extends over most of the ovary length in Fendlera and Jamesia and over the entire ovary length in Carpenteria, Fendlerella, and Whipplea. Fruit dehiscence in Philadelphus is similar to that of Fendlera and Jamesia, except that it is loculicidal rather than septicidal. Restricted septicidal, basipetal dehiscence appears to have evolved separately in Kirengeshoma, Deinanthe, and Platycrater. Although nested among clades characterized by basipetal dehiscence, Deutzia differs markedly from this pattern. Fruits of Deutzia have two regions of dehiscence: (1) over the ovarian septa acropetally from the base of the ovary to near the insertion of the calyx, which leads to valves (corresponding to individual carpels) that spread outward from the base; and (2) between the separate styles, creating a gap at the apex of the fruit. Hydrangea and Cardiandra are similar to Deutzia in having a dehiscence zone between the separate styles to create an opening at the fruit apex; however, cladograms based on the chloroplast genes rbcL and matK indicate that dehiscence in the interstylar region may have evolved independently three times. A clade consisting of Decumaria, Pileostegia, and Schizophragma, which is nested among species of Hydrangea, has shifted to an "intercostal" dehiscence characterized by the separation of plates of intercostal tissue of the ovary wall from the main vascular ribs. Broussaisia and Dichroa, two genera restricted largely to the tropics, have shifted to indehiscent fleshy fruits.

Key words: anatomy, cladograms, dehiscence, fruit, Hydrangeaceae, phylogeny