A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). I. SHOOT APICAL MERISTEMS AT GERMINATION

At germination the shoot apical meristems of Echinocereus engelmannii were discs with a volume of 666,000 ,um3 and were composed of a unistratose tunica (volume: 260,000 ,um3) and a corpus which was two cell-layers thick (volume: 406,000 ,um3). Four days after germination the nucleus constituted 28.9% of the volume of the cell, and the vacuole constituted 24.5%. The mitochondria were 13.3% of the volume of the tunica cytoplasm, the chloroplasts 9.4%, and the dictyosomes only 1.2%. The endoplasmic reticulum was too sparse to be accurately measured. The organelles of the corpus were identical in size and shape to those of the tunica, but there were statistically significant differences in their cellular and cytoplasmic densities: the more distal corpus layer (C1) was less vacuolate (16.6% of the cell volume), and both corpus layers contained more chloroplasts, 12.0o of the cytoplasmic volume in Cl and 14.3% in the more proximal corpus layer (C2). During the first four days after germination there was a dramatic increase in the size of the central vacuole (e.g., from 15.4% to 24.5% in the tunica), and the mitochondria increased in density from 10.2% of the cytoplasmic volume to 13.3%. Chloroplast density also increased in all meristem layers, but the dictyosome density decreased, as much as a 30% loss in C2. There was also a highly significant reduction in the number of cisternae per dictyosome, from 5.47 to 4.77. Surprisingly, there was no change in heterochromatin: ca. 27% of the volume of the nuclei of all layers was heterochromatic at all stages studied. Thus, the organellar structure of corpus cells is distinctly different from that of tunica cells, and as the apical meristem becomes active after germination, the changes which occur are not uniform in the meristem but rather are zone-specific. WHILE general size, shape and zonation of shoot apical meristems have been studied intensively (Cutter, 1965; Gifford, 1954; Gifford and Corson, 1971; Halperin, 1978), relatively little is known of their physiology or ultrastructure. It would be valuable to know, precisely and accurately, the fine structure of meristematic cells, e.g., the actual or relative volume of the mitochondria, chloroplasts, or vacuoles of a cell. Stereological techniques have been developed that allow such quantitative analyses (Weibel, 1963, 1969, 1973, 1974), and by combining these techniques with comparative statistics, it is possible to describe more precisely the differences that exist among the various zones of the shoot apex, the changes that occur in a cell as it is pushed out of the apex, and the changes that occur as apical meristems alter their morphogenic activities. Germination, which involves rehydration and reactiI Received for publication 2 April 1979; revision accepted 25 June 1979. Support for this work was provided by Grant #PCM7621429 from the National Science Foundation. I would like to thank Beverly Humphrey for her extremely skillful assistance in all aspects of this research, and I would like to thank the Cell Research Institute of the University of Texas for the use of its facilities. vation, must be one of the most dramatic of all changes in metabolism. Variations in organellar structure should be more pronounced here than during other transitions. This paper presents a stereological study of the shoot apical meristems of seedlings of Echinocereus engelmannii (Parry) Riimpler (Fig. 1). This species was chosen because its meristems have been studied at the light microscope level (Mauseth, 1979), and because they are especially responsive to exogenous hormones (cytokinins induce the formation of the pith-rib meristem; gibberellic acid prevents the development of any zonation; Mauseth, 1979). The seedling meristems have a simple, easilystudied construction but are completely functional, producing leaf primordia and axis tissue. The objectives of this research were 1) to determine and compare the organellar structures of the tunica and corpus, 2) to evaluate the constancy of these structures so as to determine the precision with which organellar composition is controlled and 3) to determine the types and rates of changes that occur within the cells of a particular zone as the meristem goes from a state of quiescence to one of growth.