Thesis (M.A.)--Boston University / In spite of the rapid development of the study of plant viruses and virus diseases there is still no generally accepted definition of a virus. Cook (1946) divided the history of plant virus study into three periods. The first dated from 1576, with the first published description of a virus disease, the breaking of tulips by Carolus Clusius and ended in 1868 with a description of the variegation of Abutilon striatum Dicks..The second began in 1882 with Mayer's work on tobacco mosaic. The third started in 1906 when the study of plant viruses was really beginning. Smith (1948) suggested adding a fourth period commencing in 1935 with Stanley's crystallization of the tobacco mosaic virus as a definite entity. Since then the physicist, the biochemist, and the serologist have joined in the research. Estimates of losses due to plant virus diseases are difficult to make because of varying conditions. Bawden (1950) stated that the one thing common to all plant viruses is that they are nucleoproteins containing a ribose nucleic acid. [TRUNCATED]