A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). V. COMPARISON WITH THE SHOOT APICAL MERISTEMS OF TRICHOCEREUS PACHANOI (CACTACEAE)

Relative and absolute volumes of organelles in shoot apical meristems of Trichocereus pachanoi are reported and compared with similar values published for three other species in two other families. Cellular ultrastructure varies from one meristem zone to another and from one species to another. It is concluded that there is no single, critical meristem ultrastructure, but rather meristem ultrastructure reflects meristem metabolism and is consequently unvarying within a species at a particular stage of development, but varies among species and as morphogenetic activity changes. Differences appear to correlate well with shoot growth rate and with phylogenetic relatedness. CONSIDERING THE ULTRASTRUCTURE of shoot apical meristems, several possibilities exist: there might be one optimum (or even essential) ultrastructure for all meristem cells regardless of the plant's habit, ecology, or phylogenetic position. This would suggest either that the meristems are not involved in the various morphogenetic metabolisms necessary to control the production of diverse types of leaves and axes, or that these various metabolisms may occur without any reflection in the cell's structure. A second possibility is that apical meristems display a variety of ultrastructures correlated with variation in phylogeny, development, morphogenetic activity, and/or environmental conditions. Stereological morphometric techniques are now available which allow meristems to be described with great precision and accuracy (Weibel, 1973). The meristem of Echinocereus engelmannii has been studied with these techniques (Mauseth, 1981a) and compared with seedling meristems (Mauseth, 1980a), mature tissues (Mauseth, 1980b, 1981b), and with leaf and spine primordia (Mauseth, 1982). This paper compares meristems of E. engelmanii, a slowly growing, perennial cactus, with meristems of Trichocereus pachanoi Br. and R., a quickly growing, perennial cactus (whose structure is described 'Received for publication 18 December 1980; revision accepted 16 March 1982. Support for this work was provided by NSF grants PCM76-21429 and PCM79-02626 and grant BRSG from the University Research Institute of the University of Texas. I thank the Cell Research Institute of the University of Texas for the use of its facilities. here), and with two quickly growing, annual non-cactus herbs, Pisum sativum (Lyndon and Robertson, 1976) and Sinapis alba (Havelange and Bernier, 1974; Havelange, Bernier and Jacqmard, 1974). The stereological measurements available for these four species show there are significant differences between them, and the differences appear to be correlated with growth rate and phylogenetic relatedness. MATERIALS AND METHODs-Plants of Trichocereus pachanoi were obtained from Mr. Steve Brack (905 Impala Dr., Belen, New Mexico 87002, USA). Shoot apical meristems were collected and processed for electron microscopy by the same procedures used for Echinocereus engelmannii (Mauseth, 1980a). Nine meristems were sampled. Relative volumes (Vv) were determined using a plastic overlay coherent-point grid (Weibel, 1973), with three sets of points spaced to optimize counting for objects of: 1) large diameter (nuclei, vacuoles, hyaloplasm); 2) medium diameter (mitochondria, chloroplasts); and 3) small diameter (dictyosomes, microbodies, lipid droplets). The formula Pt = (0.453)(1 Vv)/ [Vv x E2(Vv)] was used to determine the total number of points (Pt) necessary to obtain relative volume values with errors (E) of less than + 3.3% (Weibel, 1973). Cell volume was computed by combining shape determinations (Niklas and Mauseth, 1981) with measurements of cell length. Numbers of organelles per unit volume cytoplasm (Nv) was determined by counting organelle profiles per unit area of section (Na), then combining it with