The transmission of viruses by seed although limited is found in plant species belonging to various families, - for example, Leguminoseae, Solanacease, Compositae, Rosaceae, Cucurbitaceae, Gramineae and others. The percent of virus infected seed on individual plants is very uneven and it is not impossible that this is conditioned by the genotype of the plant. The time of infection of the plant has a strong effect on the virus infestation of seeds. It is thought that the earlier a plant is subject to infection the greater will be the percent of seed subject to virus infrection. An unfavorable weather period which delays the growth and development of plants also increases the amount of virus infection in seeds. Viruses which are transmitted by seed infect the generative cells and some of them are transmitted by pollen to healthy plants. Viruses transmitted by seed are located and maintain themselves mostly in the germ and less frequently in the seed coat. Immature seeds on infected plants are usually 100% infected. During the period of their maturation the virus 'perishes' and the rate of infected seed decreases to a few percent of the total. The placement of virus infected seed on the plant is scattered and shows no regularity. In some cases virus infected seed is poorly formed and shows other morphological changes (e.g., in yellow lupine) which makes it possible to eliminate it from seed materials by sorting. In mature seed viruses are very durable and survive for many years.