Abstract
Two distinct achene types were visually identified in three composite West African weeds of arable land (Emilio sonchifolia, Syndrella nodiflora, Tridax procumbens) and morphological features measured. The achene types of T. procumbens differed in most measured parameters, particularly in the size of pappus. S. nodiflora produces achenes of two distinct shapes while E. sonchifolia achenes vary only in colouration. Germination of the dimorphic achenes was examined especially with respect to their response to light. No differences were observed in T. procumbens but those of E. sonchifolia differed in response to leaf shade, and achenes of S. nodiflora in response to both darkness and leaf shade. The effect of dimorphic achenes in terms of the advantages they endow to the species and to the agricultural problems they raise for control are discussed.
