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Bean leaves experimentally inoculated with bean rust were subjected to heat treatment at 50° for 20 s, at different times after inoculation, and the modifications induced by heating in the pathogen and host cells were studied by electron microscopy, at different times after the treatment. This induced no alterations either in healthy or infected leaf cells. On the contrary, it caused severe ultrastructural alterations in the pathogen. These alterations were already visible in the haustoria and hyphae at the end of the treatment, became progressively more severe with time, and led to the death of the pathogen two days after the treatment. From this time onwards the haustoria appeared shrunken, extremely electron-dense, and encased in a mass of granular material, seemingly secreted by the host cell. No encasement was ever found around haustoria apparently dead by senescence in untreated leaves.

Summary The leaves of three species of plants (Medicago sativa L.,Ocymum basilicum L., andNicotiana tabacum L., cv.Samsun), previously infected with lucerne mosaic virus (LMV), were studied in the electron microscope. In the leaves ofM. sativa andO. basilicum the infection was systemic, while in the leaves ofN. tabacum the infection was local. In all the three species LMV particles were always detected only in the cytoplasm. Apart from the presence of virus particles in the cytoplasm, no ultrastructural alterations were found inM. sativa andN. tabacum. InO. basilicum, the green leaf areas did not show any prominent alterations, while the yellow leaf areas showed marked alterations of the chloroplasts. In these, the lamellar system was scarcely developed, and the few thylakoids tended to fragment, curl and disappear. In the chloroplast stroma, there was an abnormal development of filamentous structures, resembling the stromacentre.

RECENT progress achieved in the transmission of the virus diseases of the grape vine to herbaceous test-plants, induces us to disclose further results1.