MECHANICS OF VIRUS TRANSMISSION BY DODDER In 1940 Bennett discovered that a virus could be transmitted from an infected to a healthy plant through a bridging dodder that was parasitizing both plants. Since then numerous workers have shown that many viruses are transmissible. The subject has been reviewed by Mooi-Bok (1949), Sibilia (1951), Bawden (1956, 1964), and Fulton (1964). Transmission probably involves an association of dodder and host cells, and directional movement of nutrients. It has long been known that dodder haustoria establish cellular connections with the host plant. Thoday (1911) found that the protoplasts of dodder and host phloem cells are in close association but do not appear to fuse. Bennett (1944b) hypothesized that dodder acquires viruses with the food materials from the phloem of the host. His studies of dodder stems inoculated with beet curly-top virus or with cucumber mosaic virus indicated that after entering the dodder a virus moves more rapidly toward growing points than in the opposite direction. He suggested that yellows-type viruses move from the phloem of the dodder into the phloem of the healthy host by temporary reversals of phloem nutrient flow and that mosaic-type viruses move from the parenchyma of the haustorium to that of the host through plasmodesmatal strands or from possible naked protoplasmic connections of dodder and host, as well as through the phloem. Bennett (1956) proposed that the above idea of temporary-reversalof-flow fits well with the theory that materials in the phloem move ". . . by mass flow of liquid content of the phloem from the regions of carbohydrate supply toward regions of food utilization." Cochran (1946) found that consistent transmission of tobacco mosaic virus could be achieved only by pruning the dodder and shading the healthy host while keeping the infected host in bright light; Bennett (1956) suggested that this forced a temporary reversal of phloem nutrient flow. Canova (1955) observed that Cuscuta epithymum Murr. acquired beet-yellows virus but did not transmit it, and Bennett (1960) found the same to be true for C. californica Hooker &