VARIATION IN STRUCTURE AMONG FRUITS OF MALOIDEAE (ROSACEAE)

Fruits of Rosaceae subfamily Maloideae are known as pomes, a fruit type unique to the Maloideae. Using light microscopy we examined the pomes of 173 species, broadly representing the taxonomic diversity of the subfamily. Significant variation occurs among pomes in retention, orientation, and texture of calyx lobes on the distal end of the fruit; density and distribution of sclereids in the flesh; heterogeneity among flesh parenchyma cells; carpel number and number of ovules per carpel; degree of connation among the carpels and adnation between ovary and hypanthium; and texture of the core. Phenetic clustering closely links congeneric fruits. Sorbus is a notable exception; fruits of the compound-leaved species form a cluster unrelated to fruits of the simple-leaved species. Fruits of Malus are also heteromorphic. Division of the Maloideae into two tribes, Crataegeae and Sorbeae, based largely on texture of the core is not substantiated. We hypothesize that the ancestral pome had five carpels, minimal connation of carpels, minimal adnation of ovaries to the hypanthium, two ovules per carpel, and a leathery core. Various adaptations for frugivore dispersal of the seeds partially explain trends in pome evolution.