THE meeting of the American Association for the X Advancement of Science, which was held at Columbus, Ohio, in December, was the occasion for the presentation of several interesting papers which make fundamental contributions to our knowledge of plant virus diseases. The first categorical indication of structure of the virus particle can be obtained from a comparison of the work of John W. Gowen, of Iowa State College, with earlier findings. His work on X-ray inactivation and size of various organisms, including insects, bacteria and viruses, shows that functional correlation of these two factors “must be between the size of some vital substances within the cell rather than the cell as a whole”. The ‘repro-ductive’ part of a virus particle has a molecular weight of 15,000,000, which compares with about 7,000,000 suggested by other workers for the whole virus particle. The portion of a virus susceptible to inactivation by X-rays is apparently denser than the rest of the particle.