Ecology of achene dimorphism in Heterotheca latifolia. I. Achene structure, germination and dispersal

(1) Information is presented on structural differences between achene types of Heterotheca latifolia (Compositae), when they are produced, where on the plant they are produced, laboratory germination, seedling growth and dispersal biology. (2) While weights of ray and disc achenes do not differ significantly (c. 730 μg), their structures do. Disc achenes have a wind-dispersed pappus while ray achenes have none. Disc achenes have a thinner and anatomically different pericarp (half as much biomass) compared with ray achenes and disc embryos are 60% heavier than ray embryos. (3) Plants produce smaller heads with relatively more ray achenes as their reproductive season progresses. Also, larger plants produce larger heads with relatively more disc achenes up to a threshold plant size beyond which head size and the ray to disc quotient is constant. (4) Fresh disc achenes germinate quickly and completely while ray achenes have delayed and fractional germination which can be enhanced by excision of the embryo. Ray-achene germination increases with achene age and is inhibited by darkness and burial while disc-achene germination is less sensitive to these factors. (5) While growth rate does not differ between ray and disc seedlings, disc seedlings at a given age are larger due to the initial difference in embryo size. (6) Dispersal differences are found between ray (lacking a pappus) and disc achenes (with pappus) of the same mass. Wind dispersal is greater for disc than ray achenes. Plant height enhances disc achene dispersal more than ray achene dispersal.