Extracts of many plants have been shown to contain germination and growth inhibitors (1, 2, 3, 4, 5). In some instances these inhibitors play a role in the distribution of species. Thus dried Encelia leaves contain a material which leaches slowly into the ground and inhibits the growth of species susceptible to it (2). This suggests that certain weeds may be aided in their competition with crop plants by similar inhibitors. Helgeson and Konzak (4), for example, found that aqueous extracts of field bindweed (Convolvulus arvensis) and Canada thistle (Cirsium arvense) inhibited the germination of seeds and growth of seedlings of certain crop species.