THE ROLE OF AUXIN IN DIFFERENTIATION OF XYLEM AROUND A WOUND

Abstract

THE DEVELOPMENT of living organisms is usually classified into two maj or groups of phenomena: those connected with growth and those connected with differentiation. This paper is an attempt to answer experimentally a few of the questions concerning the hormonal nature of some of the limiting factors in cell differentiation which are raised by accounts of plant anatomists. The selection of a cell-type for this investigation was guided by the following considerations: (1) the typical developmental history of the cell-type should be relatively well established; (2) the pattern of differentiation of the cell-tiype should preferably show some features which distinguish it from the pattern followed by other cell-types and which make it more amenable to experimentation; (3) the cell-type, when fully differentiated, should be easily recognizable by the simplest histological techniques. These characteristics are well shown by xylem cells. For the purposes of experiment, attention was focused on the regeneration of xylem strands after the vascular strands in a stem internode had been severed by a wound. The most striking peculiarity of such regeneration, as described by Simon (1908), Kaan Albest (1934) and Sinnott and Bloch (1945), is that the new xylem cells are differentiated in a strictly basipetal direction, by the transformation of already existing parenchyma cells, the new cells always being in continuity with the previously differentiated ones. What could be limiting the differentiation of these xylem cells? A clue is perhaps given by an experiment of Kaan Albest: she found that excising the distal leaves and buds (whether axillary or apical) markedly reduces the speed and amount of xylem-regeneration around a wound. No additional reduction was obtained when isolated internode sections devoid of all leaves and buds were used.