A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). III. SUBAPICAL AND MATURE TISSUES

A stereological, morphometric study of the ultrastructure of subapical cells, xylem parenchyma, and cortical chlorenchyma of Echinocereius engelmannii shows that each of these cell types has a unique organellar composition. The cells of any of these tissues are unique not only in vacuolation (which is visible at the light microscope level), but also in the nucleus/cytoplasm ratio and the concentrations of mitochondria, chloroplasts, and dictyosomes. Furthermore, the differences between each of these cell types were large and statistically significant. It had previously been found that the cells of each zone of the shoot apical meristems of E. engelmannii are different from those of the other zones, but the present study suggests that, considering the large ranges of structure possible in the nonapical cells of this species, the apical meristem variation should be considered as only a minor difference and that the meristem zones are really quite similar to each other. THE PREVIOUS PAPER of this series (Mauseth, 1981) reported that each zone of the shoot apical meristems of Echinocereus engelmannii was composed of cells with unique ultrastructures, such that central mother cells (CMCs) could be distinguished from either peripheral zone (PZ) cells or pith-rib meristem (PRM) cells. This demonstration that the zones are not identical ultrastructurally is important, but knowing the biological significance of those differences is similarly important. The CMCs are richer in mitochondria (7.52% of protoplasmic volume) than the PRM cells (4.58% of protoplasmic volume; Mauseth, 1981), but does this reflect a major difference in cellular architecture and metabolism or does a difference of this size indicate exceptional similarity, such that only differences as large as perhaps 4.58% vs. 20% or 50% have functional meaning with regard to meristem metabolism? It is not possible to judge this without knowing the range of cellular structures that could possibly exist in this species. Morphometric studies of animal cells suggest that specialized metabolisms are associated with highly characteristic organellar relationships (David, 1977). Therefore, the ultrastructures of several types of I Received for publication 31 March 1980; revision accepted 25 October 1980. Support for this work was provided by NSF grants #PCM76-21429 and #PCM79-02626 and from Grant #BRSG from the University Research Institute of The University of Texas. I thank Beverly Humphrey for her extremely skillful assistance in all aspects of this research, and I thank the Cell Research Institute of The University of Texas for the use of its facilities. mature, specialized tissues were studied, and the results are reported below. Also, it should be possible to identify some of the properties characteristic of meristematic cells by noting which characters are lost as cells are transferred from the base of the apical meristem into the subjacent, maturing tissues below. Therefore, cells of the subapical meristem below the lower boundary of the apex were also examined. MATERIALS AND METHODs-The mater-ials for this study were taken from the same plants as in the previous study (Mauseth, 1981) and were processed simultaneously with the apical meristems in the same fixing, dehydrating, and embedding solutions. Because of the randomization which occurred during processing, it was not possible to determine which apical block was associated with which set of subapical tissues; therefore, paired-comparison statistical techniques were not possible. Comparisons were made between means. All morphometric techniques were identical to those used previously (Mauseth, 1980, 1981) except for the determination of the volume of the nuclei, vacuoles, and cytoplasm of the large cells of the cortical parenchyma. For these, the mean nuclear diameter was determined at the light-microscope level, then nuclear volume was computed. Cellular volume and surface area were also determined from light-microscope measurements of mean length, width, and height. Cytoplasmic volume was computed by multiplying the surface area of the cell by the mean thickness of the cytoplasm (de-