Abstract
Carduus pycnocephalus and C. tenuiflorus are two important weed species in mediterranean-type ecosystems. They were studied in their native habitat in southern France from April to December 1981. These species both produce two types of seed (achenes) in each capitulum. Those in the center are not dormant and are easily dispersed, while the outer ones can be dormant and have no apparent means of dispersal. Our observations on pollen transfer and self fertilization show that the dispersable, non dormant seeds have a low probability of inbreeding, while the potentially dormant, non dispersed seeds have a high probability of inbreeding. The significance of this dimorphism is discussed.