An investigation of the phylogenetic and ontogenetic variability of shoot apical meristems in the Cactaceae.

The sizes, shapes and zonations of the shoot apical meristems of 22 species of cacti were examined. This family was chosen because of its great diversity of habits; the more primitive members are nonsucculent, leafy trees and more advanced members are highly specialized "leaf-less" stem-succulents. By combining these measurements with those already in the literature, a sample of almost 70 species was obtained. Apical meristems range in size from only 80 ,tm in diam in some species to as much as 1,500 ,um in diam in others. The shape ranges from being flat to almost hemispherical. Despite the great range in size and shape of the apical meristems, or the range in the morphologies of the leaves and stems which are produced by the meristems, all apices had the usual zonation: tunica, central mother cells, peripheral zone, and pith-rib meristem. The sizes of each of the zones, expressed either as the number of cells per zone or expressed as a percentage of the whole apex, were highly variable. The variation in apical dimensions and zone sizes occurred both phylogenetically and ontogenetically, and this has been interpreted to indicate that the morphogenetic mechanisms which control apical size and zonation are easily modified, both during the development of individual plants and during the evolution of new species. THE SHOOT APICAL meristems of many seed plants are composed of zones of cells, each zone having unique characteristics. These zones, usually referred to as the tunica (T), central mother cells (CMC), peripheral zone (PZ), and pith-rib meristem (PRM), have been the objects of intense research (for reviews see Cutter, 1965; Gifford, 1954; Gifford and Corson, 1971). Despite the great amount known about shoot apical meristems, certain questions remain. One of these concerns the role of these zones in the function of the shoot apical meristem, i.e., is zonation necessary in certain species for leaf primordium initiation and/or shoot axis production. This may seem trivial considering the fact that cryptogams function without zonation, but zonation did evolve, presumably due to some selective advantage that it confers upon the plants. A zonate apex obviously has a metabolism that is different from that of a nonzonate apex and it is not unreasonable to postulate that this change in metabolism might affect the leaf initiating mechanism. Another question involves the nature of the morphogenetic mechanisms responsible for establishing and maintaining the zonation: are the various zones dependent on or independent 1 Received for publication 11 July 1977; revision accepted 29 November 1977. Support for this work was provided by Grant PCM76-21492 from the National Science Foundation and by Grant 14-74990225-URI from The University Research Institute of The University of Texas. of each other. Also, which plant growth substances or physical factors are responsible for zonation? To provide information relating to these questions, an investigation was begun of the shoot apical meristems in the Cactaceae. This family, the members of which have zonate apices, is large and encompasses a great deal of morphological variation (Buxbaum, 1950; Ganong, 1898), and thus, apex variation can be compared with a variety of leaf and axis forms. In addition, through the investigations of Boke (all references), Freeman (1970), Mauseth (1976, 1977, 1978), and Mauseth and Halperin (1975) an extensive set of measurements of apical meristem sizes, shapes, and zonations is available for cacti. The first phase of this research concerned the development of zonation in the shoot apical meristems of cactus seedlings. The results (Mauseth, 1978) indicated that zonation was not necessary for either leaf primordium initiation or axis production. It also suggested that the various zones are independent of each other. The second phase of this research, reported here, considered the variation in apical meristem size, type of zonation present, and the variability of the sizes of the zones. These results likewise suggest that the zones are independent of each other and of other morphogenetic processes, and also indicate that the control mechanisms are quite easily modified.