Heterocarpy is a strategy in which reproductive output is partitioned between two or more groups of morphologically and ecologically dissimilar fruits. By partitioning offspring among two or more ecologically divergent groups, detrimental impacts of environmental heterogeneity on seedling establishment are reduced because the differing structural, dispersal, and dormancy traits among fruit morphs counterbalance contrasting spatial and temporal dispersal risks. We investigated heterocarpy in Grindelia ciliata (Asteraceae). G. ciliata capitula produce up to three structurally dissimilar achene morphs. Ray achenes are the heaviest, have a rounded shape, produce a non-functional pappus, and have a physical dormancy mechanism. Disc achenes are the lightest, have an elongate shape, produce a functional pappus, and germinate readily. Peripheral disc florets occasionally produce achenes that have the persistent pappus of disc achenes and the shape of ray achenes. Structural and functional characteristics were compared among G. ciliata achene morphs to evaluate how variation in these traits shapes the expression of heterocarpy. We identified differences in pericarp allocation and anatomical features that are ecologically important determinants of dormancy differences among G. ciliata achene morphs. Our results also indicate that plants modify patterns of achene morph production in response to environmental variation. Results of these studies are evaluated in a phylogenetic context to identify potential developmental and structural antecedents that may dictate the evolution of heterocarpy.

Key words: achenes, Asteraceae, dormancy, Grindelia ciliata, gumweed, heterocarpy