Numerical estimations, based upon the seedlings appearing in soil samples from known areas, indicate the presence of many viable weed seed in soils from the permanent wheat and barley fields at Rothamsted and Woburn, both before and during fallowing. Comparatively few spp. germinate freely throughout the yr.; most spp. show periodicity, the majority of seedlings appearing during autumn or winter and relatively few in late spring or summer. Many weed seed show natural dormancy, while others exhibit induced dormancy for many yrs. due to burial in the soil under unsuitable germination conditions. The association between the weed flora and the type of long time manuring appears to be influenced by the nature of the soil and the repeated growing of a spring or autumn sown cereal crop. The marked preference of certain weeds for either wheat or barley appears to be caused by the method of cultivation rather than by the influence of the crop upon the weeds.