The growth responses of Portulaca oleracea plants from a central Michigan population are compared with representative populations of Portulaca grandiflora, a cultivar, and Portulaca pilosa, a plant of southern and relatively narrow distribution in this country. In comparison tests, and species show a significant interaction with photoperiod, light intensity, temperature, and soil types. Portulaca oleracea is a weed, in part because its process and pattern of growth give the plant quick response capability. In contrast to its companion species, P. oleracea uses a wide variety of photoperiods, and capsule numbers are positively correlated with amounts of light received. This weed is widely tolerant of light intensities, temperature regimes, and soil types, and the plants produce adequate levels of capsules over a wide range of these factors. In addition, individuals have the ability to produce large numbers of capsules rapidly when high levels of these factors become available.

A~str~ct. Portulaca ?lerace~ L. is a weed of open ag~cultural_ habitats. This paper evaluates the contnbutwn of the plant _s bre~dmg system and seed physiOlogy to Its success as a weed by comparing these aspects. of adaptation. with the same aspects in two other species of Portulaca, P. grandiflora Hook., a cultivar, and _P .. pliosa L., a. plant of comparatively limited distribution. Self-compatibility, and the se~d ch_aractenst~cs of longe~Ity, res_istance to digestive processes, the lack of after-ripening, and over~mtenng proficiency combme to give the plant quick-response capability when conditions surroundmg the seed are capable of supporting growth to maturity.Portulaca o_leracea belongs to the group of weeds whose seeds have specific germination require-ment~ for environmental fac~ors. When sufficient water is present, the majority of dormant seeds germmate up~n _exposure to hght and to temperatures >20°C. Portulaca grandijlora seeds germinate equa_lly ~ell m hght or dark and demonstrate a wide tolerance to temperature. Portulaca pilosa seeds reqmre hght and a temperature of 25°C for maximum germination percentages.These factors of seed production and seed physiology interact to produce large populations of P. oleracea plants that can quickly exploit open habitats that are continually supplied by man.