A population of Porophyllum ruderale (Jacq.) Cass., a weed of the Compositae family, occurring in a fallow area near São Jose do RioPreto (State of São Paulo, Brazil) was studied in regard to: a) height, b) dry epigeous biomass, c) number of branches, d) number of heads per branch, e) number of heads per plant, f) number of achenes per head and g) output of achenes per plant. This output was relatively low (on an average of 2510) corresponding to 2008 viable achenes per plant, taking into account a germination percentage of 80%. The observed reproductive capacity is very higher than the number of mature individuals usually found in the field and does not explain, therefore, the widely scattered distribution of P. ruderale. Environmental factors, possibly soil conditions, interspecific competition and natural enemies, should be taken into account. The number of he ads and ache ne s per plant showed correlation with height and biomass, while the number of achenes per head (on an average of 53) showed no correlation with height, biomass and number of heads and achenes per plant, revealing to be a character little affected by vegetative vigours.
Weed Biology; Reproductive capacity; Porophyllum ruderale (Jacq.) Cass