INDUCTION OF CELL DIFFERENTIATION IN DISSOCIATED CELLS AND FRAGMENTS OF CALLITHAMNION ROSEUM

Branching in regenerates of Callithamnion roseunr is a phenomenon of cellular differentiation. Starting from single cells or filaments, a gradual morphological change in cell types occurs: from parallel cross-walled, branchless stalk cells to oblique cross-walled, branch-bearing main axis cells. The development of a plating technique and a synthetic sea water medium permits quantitative study of cell differentiation (or branching) in populations of regenerating cells or fragments. The distribution of regenerating fragments on a given plate is non-random as seen from the fact that it does not conform to the Poisson series. Clustering and attachment of neighboring fragments increases the likelihood of branch initiation. The change with age in the distribution of plated regenerating fragments is presented. Developmental potentialities of 5 fragment types were examined. Each fragment type could give rise to branching regenerates. Although apical fragments composed of more than 10 cells usually branched first in a plate, single, large, basal cells seem to have a greater regenerative capacity than do small apical fragments. WHEN THE MAIN morphological features of spiral (or more correctly, helical) branching had become known in several Floridean algae (Naigeli, 1861), the question arose as to what factors regulate the constant arrangement and orientation of lateral organs in these plants. The helix generated in Polysiphonia through the development of the laterals normally turns to the left. The first side branch of the trichoblasts (hairshaped lateral organs) develops on their right side seen from the outside. Axillary branches are displaced to the left, and the trichoblasts are "in the upper left angle of the first formed pericentral cell of the the joint" (Rosenvinge, 1903). To explain the development of various spiral arrangements in the Florideae, Schwendener (1880) suggested that young laterals remained in contact with the axis and thereby prevented the development of further laterals until separated by subsequent elongation of the axis. Subsequent investigators (Berthold, 1882; Rosenvinge, 1888; Falkenberg, 1901) could not observe any contact between young laterals and the axis in numerous Rhodomelaceae having regular spiral arrangements. However, Berthold Received for publication 1 1 June 1967. This work was supported by research grant GM 12377 from the National Institutes of Health to the author and a grant to Dr. Richard M. Klein from the National Science Foundation. The author thanks Dr. Ralph 0. Erickson for his advice and help with the statistics and Dr. Richard M. Klein for valuable help throughout this work. The photographic assistance of Mr. Walter Singer is gratefully acknowledged. I Present address: Osborn Laboratories of Marine Sciences-New York Aquarium, Brooklyn, N.Y. (1882) and Rosenvinge (1888), in agreement with Cramer (1864) and Kny (1873), showed an oblique orientation of the cross-walls of the presumptive branch-bearing segment cell, their most distal point marking the position of the future branch in numerous Ceramiaceae (including Callithamnion borreri Berthold, 1882, Table 22, Fig. 6) and Rhodomelaceae. Threefold differences between the length of opposite lateral walls were not uncommon (Rosenvinge, 1902). Helical successions of oblique cross-walls have also been described in moss rhizoid cells (MullerThurgau, 1874). Controversy also arose concerning whether the cross-walls of the presumptive branchbearing segment cells are originally oblique or whether they are horizontal and become raised unilaterally only through the development of lateral organs. In other words, is obliqueness a primary phenomenon and branching dependent on it, or vice versa? Schwendener (1883) took the latter view, while Berthold (1882) and Rosenvinge (1888, 1902) reported the development of oblique segment-cell walls preceding any laterals on the apex. It is significant that vegetatively "sterile" or non-branching segment cells are delimited by horizontal walls from the apical cell in Polysiphonia urceolata (Rosenvinge, 1902). Rosenvinge (1888) and Falkenberg (1901) further showed that segment cell nuclei of several Polysiphonia species occur on those sides of the axis where the development of future laterals may be predicted. Although nuclei of nonbranching segment cells are frequently much smaller than their sister apical cell nuclei, slight nuclear differences exist between branch-producing segment and apical cells (Rosenvinge, 1902).