Bacterium malvacearum, the causative agent of ‘black-arm’, or angular leaf-spot disease of the cotton plant, has been described in certain recent papers as capable of producing a systemic infection of the plant without, however, causing any manifest symptoms of disease until external conditions become suitable for the development of local lesions. Further, it has been claimed that this infection may extend to the developing ovules, so that internally infected seed results, which on germination may give rise to infected plants, the degree of such infection being conditioned in large measure by the soil temperature prevailing at the time of germination.