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does not appear. First page follows. Introduction The phloem of plants affected by the curly-top disease undergoes profound histologic and cytologic changes. Among these, the degeneration and death of certain cells and the hyperplastic increase in the number of sieve-tube elements are the most striking symptoms (Esau, 1933), (1935a), (1935b).3 The inception of phloem degeneration in a developing organ depends on the presence of mature sieve tubes, while the first internal symptoms are localized in areas directly connected by phloem with the inoculated leaf (Esau, 1935a), (1935b). The interpretation of the nature of phloem degeneration in curly-top plants has been based almost entirely upon studies of a single host, the sugar beet (Esau, 1933), (1935a), (1935b); (Artschwager and Starrett, 1936); a limited consideration of the anatomy of curly-top tobacco showed comparable pathologic changes in this host (Bennett and Esau, 1936). The present paper is a result of rather comprehensive studies on the phloem of the Turkish variety of Nicotiana Tabacum affected with curly top. It also briefly considers phloem degeneration in tomato affected by the same disease and compares the effects of curly top and of tobacco mosaic upon the phloem of tobacco. Nicotiana Tabacum was selected for study because this species has served repeatedly in investigations concerning translocation and tissue relations of the curly-top virus (Bennett, 1934), (1937); (Bennett and Esau, 1936). At the same time, the choice of tobacco offered an opportunity to examine the course of phloem degeneration in a plant that differed considerably from the sugar beet in its development, its morphology, and its reaction to the curly-top virus. Tobacco, unlike the beet, produces a stem with secondary vascular tissues during the first year of growth, has internal phloem and no specialized underground storage organ nor anomalous secondary growth. With regard to curly-top infection, tobacco commonly recovers from the disease (Bennett, 1934); (Wallace, 1939), a tendency not characteristic of the sugar beet.

Phloem anatomy of tobacco affected with curly top and mosaic