Interior of fruiting capsule of balloon vine C. halicacabum. The upper portion of the capsule wall has been trimmed away and shows the central position of the seeds. The ‘fruit size’ variable we measured was the shortest distance from the fruit capsule perimeter to the seed coat for each seed. The second species in this study, Cardiospermum grandiflorum, has larger fruits of similar architecture (Image source: Wikimedia commons: Cardiospermum halicacabum).