We examined rock–soil–plant relationships of three endemic Cactaceae species [Cephalocereus apicicephalium E.Y. Dawson, C. nizandensis (Bravo & T. MacDoug.) Buxb., and C. totolapensis (Bravo & T. MacDoug.) Buxb.] from the tropical deciduous forest of the Tehuantepec River basin, Mexico. The goal was to explain the relationships between the species, rock, soil, and calcium oxalate. The x‐ray diffraction patterns from the rock and soil analyses showed that the species do not share either a rock or a soil type. Cephalocereus apicicephalium grew exclusively on outcrops of limestone with quartz on the summit to backslope of the hill and C. nizandensis grew in metalimestone outcrops on the summit and shoulder of the hill, while C. totolapensis preferred acid soils from andesite, growing also on siltstones or on mica schist on the shoulder to toeslope of the hill. Although only insignificant quantities of soluble Ca were found in the soil, weddellite is abundant in the plant tissue. This suggests that the plants take up the Ca they need and that weddellite is a genetically fixed characteristic related to deflection of excess radiation or an herbivore deterrent. Energy dispersive x‐ray analysis showed that calcium oxalate crystals contained other elements (Si, Mg, Na, K, Cl, and Fe) and that these impurities modified the crystal shape, as for bipyramids in C. totolapensis Biominerals did not change the mineral composition of the soil. The three species are distributed as edaphic or rocky islands, giving rise to their allopatric and patchy distribution across the landscape. Finally, the endemicity of the three Cephalocereus species is promoted by the parent material and the particular soil conditions in the area in which each one grows.
This research describes the trichomes produced in the old vegetative areoles of the fi ve species of Cephalocereus Pfeiff. Light and scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the crystals present in the stem vegetative areoles far from the apical meristem. We are reporting for the fi rst time for the Cactaceae family capitated trichomes with crystals. Crystals had diverse shapes (prismatic, styloid, sandy) and they all contain C as their principal element, as well as Na, Cl, Si, Mg, S, and Ca in different concentrations. Trichome and crystal characteristics allowed distinguishing the species of Cephalocereus. For example, capitate trichomes are shared by C. columna-trajani and C. senilis but differ because in C. columna-trajani there is one prismatic crystal per cell and sandy crystals in C. senilis. The other three species share non-capitated trichomes with prismatic crystals in the apical cell and styloids in the other cells. The species of Cephalocereus share the ontogenetic pathway of producing trichomes and spines in the areoles far from the apical meristem, suggesting that trichomes with crystals may be protecting the spines meristems. Key words: biominerals, Cephalocereus apicicephalium, Cephalocereus columna-trajani, Cephalocereus nizandensis, EDAX, scanning electron microscopy.Resumen: Esta investigación describe los tricomas de las areolas vegetativas de las cinco especies de Cephalocereus. La microscopía fotónica y electrónica de barrido, así como el análisis dispersivo de energía de rayos X, se utilizaron para caracterizar los cristales presentes en las areolas vegetativas de los tallos. Se reporta por primera vez para la familia Cactaceae tricomas capitados con cristales. Los cristales tuvieron diversas formas (prismas, estiloides, arena) y todos ellos contienen C como su elemento principal, así como Na, Cl, Si, Mg, S y Ca en diferentes concentraciones. Las características de los tricomas y de los cristales permitieron distinguir las especies de Cephalocereus. Por ejemplo, los tricomas capitados son compartidos por C. columna-trajani y C. senilis, pero se diferencian porque en C. columna-trajani hay un cristal prismático por célula y cristales de arena en C. senilis. Las otras tres especies comparten los tricomas no capitados con cristales prismáticos en la célula apical y estiloides en las otras células. Las especies de Cephalocereus comparten la vía ontogenética de producir tricomas y espinas en las areolas lejos del meristemo apical, lo que sugiere que los tricomas con cristales pueden estar protegiendo los meristemos.
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