Ontogeny and structure of the phloem of tobacco

Abstract

Abstract does not appear. First page follows. Introduction This study on phloem anatomy of tobacco was originally intended to serve as a basis for certain projected investigations on the host-virus relations in the curly-top disease. During the work, however, the discordant statements discovered in the literature concerning some problems of histogenesis and nomenclature prompted a more detailed developmental study than at first contemplated. The principal problems considered were as follows: the origin and development of external and internal phloem; the distinction between the primary and secondary phloem; the comparative structure of the mature and the degenerating phloem; and the origin of the so-called “pericyclic fibers” that occur on the periphery of the phloem in mature stems. The study of development and structure of the sieve tubes, though secondary, furnished information about the morphologic characteristics of these elements in phloem of different stages of development. The material for study was limited to petioles and stems of two species of tobacco: Nicotiana tabacum L. and Nicotiana clauca Graham. The first species was selected as an experimental plant because it is used in many virus studies; the second as a plant used, together with N. tabacum, in certain important curly-top investigations. Moreover, for studying secondary phloem, N. glauca, being a woody shrub, was more suitable than the herbaceous N. tabacum.