When seed and seedlings of H. annuus were irradiated and allowed to grow to maturity the average time of blossoming was shortened, even with doses producing no immediate visible effects. A checking action, proportional to the dose, is evident for 3 weeks after seed have been irradiated with medium doses. When medium or heavy doses are given, the leaves on unfolding present a peculiar pitted or mosaic appearance due to an unequal distribution of chlorophyll. Heavy dosage of soaked seed stops development as the cotyledons push through the soil, the root tip becoming brown and no stem tip appearing. Fasciations of stems, leaves, and flowers are induced by medium doses. Stem fasciations commonly take the form of flattening and dichotomous splitting of the main stem, while fasciation of the flower head is represented by fusion in the involucral region and by forking of the stalk below the involucre. Many leaf abnormalities and incipient fasciations marking earlier periods of growth disappear before maturity. Catalase activity is depressed in young seedlings from medium doses; older seedlings show recovery with but slight reduction in activity. Depressed respiration accompanies the marked inhibition of growth associated with heavy irradiation. Oxidase activity remains unchanged. Histological observations showed an enlargement and vacuolation of the cells in the meristematic region of the root tip of seedlings whose seed had been irradiated. Xylem is increased at the expense of the pith, and there is a greater suberin development in the hypocotyl regions of mature plants.