LONDON. Association of Economic Biologists, October 13.—Prof. E. B. Poulton, president, in the chair.—E. J. Butler: Virus diseases in plants. The first demonstration that disease can be caused by a filtrable virus was by Iwanowski, in 1892, in mosaic disease of tobacco. Mosaic is now known in nearly 100 species of plants. Diseases like peach-yellows and others characterised by phloem-necrosis are probably caused by similar agents though the filtered juice is not infective. All hitherto tested can be transmitted by grafting, most of them by insects (the chief method in Nature), and many by inoculating with sap. Contact will not cause infection. Infection may be hereditary in the insect transmitter and in the plant. “Carriers”are known. The causal agents are believed to be living organisms. Several investigators have recently found large amoebiform corpuscles or smaller granules in infected cells. The former have been compared with cytoryctes or neuroryctes and the latter with Chlamvdozoa but a resemblance to Rickettsia is suggested. The causal organisms appear to be obligate parasites.—J. A. Arkwright: Virus diseases in animals and man. The chief points of interest common to plant and animal “virus diseases” are (1) the nature and properties of the virus (2) the means of transmission, e.g. “carriers”and insect vectors; (3) measures for prevention, e.g. breeding or selection, and isolation or destruction, (4) perhaps the concentration of the virus in certain special tissue cells. About fifty animal virus diseases have been described which may be roughly classified as follows: (1) visible, not filtrable, not cultivated, e.g. Rickettsia; (2) probably visible, filtrable, cultivated, e.g. pleuro-pneumonia of cattle, poliomyelitis; (3) filtrable, not cultivated, some (?) visible in the tissues, e.g. foot and mouth disease, vaccinia; (4) filtrable, very resistant, e.g. infectious anaemia of horses, fowl-pox. In general properties most do not differ much from bacteria, though some are very resistant to drying, glycerine, and heat. The smallest clearly visible and the largest filtrable particles are of the same order of size, i.e. about 0.2 micron. Living organisms may conceivably be much smaller than this. The differentiation of colloidal particles of about 0.02 micron in size by means of the microscope requires attention especially to their arrangement and their range of size and shape, rather than to the appearance of individual particles. Theoretically it is possible that an enzyme may be the cause of an infectious disease on the analogy of Twort's lytic substance and the bacteriophage of d'Herelle.