ORIGIN OF BUDS IN TOBACCO STEM SEGMENTS CULTURED IN VITRO

Abstract

DESPITE THE great wealth of botanical literature concerned with the initiation of adventitious buds (Priestley and Swingle, 1929; Buvat, 1944-45; Peeters, 1947), there have been relatively few anatomical studies on bud initiation in tissues cultured in synthetic media. Buvat (1944-45) has summarized these and has, himself, added considerably to an understanding of the problem of dedifferentiation2 of plant cells. However, his investigations concern principally the cytological aspects of primordium formation. Although Camus (1949) has studied the changes in subjacent tissues during bud development, he has not considered the details of initiation of the shoot primordia. Previous communications (Skoog, 1944; Skoog and Tsui, 1948, 1951; Sterling, 1950) have shown a marked correlation between relatively high levels of adenine, supplied through a culture medium, and bud initiation in tobacco stem segments. Obviously, the establishment of such a relationship makes possible the chemical control of bud formation. It is, therefore, of interest to place on record the successive steps in this controlled initiation of shoot primordia in vitro. This paper presents an anatomical description of such development and considers the latter in relation to the general problems of meristematic activity and dedifferentiation. MATERIALS AND METHODS.-The materials and methods used are those reported earlier (Sterling, 1950), with the limitation that this account deals only with the tobacco stem segments which received adenine sulfate (40 mg./l.) in the basal medium. The material is representative of the samples used in physiological studies on growth and tissue composition. Segments grown on control media were described in the earlier paper. SITES OF BUD ORIGIN.-In these cultures, there are three maj or regions of bud formation, Viz., cambium-external phloem, internal phloem, and callus. The region of the cambium and its young phloic derivatives produces the maj ority of bud primordia. In most instances, no sharp distinction can be made between these two classes of cells in the rapidly dividing cambial zone; therefore, this source of buds was previously reported as cambial (Sterling, 1950). However, in some cases bud primordia are found exclusively in the external phloem, with differentiated sieve tubes lying be-