RELATIONSHIPS AMONG SHOOT APICAL MERISTEM ONTOGENIC FEATURES IN TRICHOCEREUS PACHANOI AND MELOCACTUS MATANZANUS (CACTACEAE)

Various morphometric parameters of shoot apical meristems were measured during the ontogeny of Melocactus matanzanus and Trichocereus pachanoi. While some ontogenic features varied dramatically (i.e., apical surface curvature, dK/dl, varied by 0.15–0.17, while the apical volume increased from 10⁵ μm³ to 10⁷ or 10⁸ μm³), the percent volume of the central-mother-cell zone (CMCZ), pith-rib meristem (PRM), and the peripheral zone (PZ) showed remarkable intraspecific constancy: CMCZ = 9.2%, PRM = 10.4%, PZ = 80% for M. matanzanus, and CMCZ = 10%, PRM = 24.5%, PZ = 65% for T. pachanoi. Seedlings (2 plastochrons old) showed distinctly different percent volumes from older forms: CMCZ = 7.8%, PRM = 5.9%, PZ = 86% for M. matanzanus, and CMCZ = 8.9%, PRM = 13%, PZ = 77% for T. pachanoi. Geometric expressions of cell patterns within zones of the apex remained relatively constant throughout development. On the basis of intra- and interspecific comparisons of the percent volumes among 23 taxa and the chronology in the appearance of zones within the apex, it is concluded that while there are no apparent biological reasons for the actual percent volume values computed from our sample set, the constancy in zones reflects a high degree of homeostasis which is achieved early in ontogeny.