¿Alguien sabe qué es el número?

Presentamos una visión global sobre los números naturales que puede ser útil para los profesores de matemáticas de educación primaria y secundaria. Usamos un episodio de clase, en el que un formador de profesores presenta la construcción logicista de los números naturales, como contexto de reflexión sobre los diversos significados de los números. Nuestro análisis está basado en la noción de significado personal e institucional de los objetos matemáticos, entendidos como sistemas de prácticas operativas y discursivas ligadas a la solución de situaciones–problemas

Population parameters of the trigonal tivela Tivela mactroides (Bivalvia: Veneridae) from Caicara Beach, Anzoátegui, Venezuela

Aspects of population dynamics of the trigonal tivela Tivela mactroides, an important fisheries resource, were studied from October 2002 to September 2003 at Caicara Beach, Anzoátegui, Venezuela. The substratum consists mainly of fine-grained sand (100–250 µm; 80.33%). The mean density of the clam on the shoreline was 53.83 ind m–2, but there were significant monthly differences. The population structure was bimodal, with a dominant group from October 2002 showing a high growth rate. Maximum population biomass was recorded in March 2003 (25.12 ± 4.44 gDW m–2) and minimum in July 2003 (2.37 ± 0.41 gDW m–2). The condition index was maximum in July 2003 (26.26 ± 4.64%) and minimum in March 2003 (16.48 ± 13.23%), with a monthly mean of 17.55 ± 3.02%. The mean dry tissue yield was 23.58 ± 5.05%. The growth parameters estimated, L∞ = 44.32 mm and K = 0.165 month–1, indicate an average longevity of 18 months. The strong monthly decline in density and biomass, with adult mortality of 0.20 month–1, reveals that the population is seriously affected by human extraction, and we therefore recommend reducing the intensity of exploitation

Biological performance of the penshell Atrina maura and mussel Mytella strigata under different water flow regimes

Three water flow rates (3.1, 7.3, and 12.1 cm s–1) were used during a 26-day period to determine their effects on shell dimensions, tissue mass, condition index, biochemical composition of tissues, and mortality of the penshell Atrina maura and mussel Mytella strigata kept in an open-flow flume. These parameters were measured at the beginning and end of the trial and significant differences were determined using non-parametric comparisons of multiple independent samples. Both species were able to withstand high seston loads and daily oscillations of temperature, which yielded negligible mortalities. Both species were affected differently by water flow. Atrina maura grew significantly larger above a flow of 7.3 cm s–1, which appears typical of species inhabiting tidal channels. At this flow, there was a significant increase in lipids and acylglycerols within the digestive gland, adductor muscle, and mantle tissue, accompanied by a significant decline in proteins and carbohydrates within the same organs. In contrast, the flow did not have a significant effect on M. strigata, except for increased lipid and acylglycerol reserves within body tissues at 3.1 and 7.3 cm s–1, and increased lipid and acylglycerol levels within the gonad at 7.3 and 12.1 cm s–1. Digestive gland proteins, carbohydrates, and glycogen increased at the highest flow. Data suggest that the penshell is a flow-conforming mollusk that prefers flows exceeding 7.3 cm s–1, and that the strigate mussel is a flow-regulating species in the range of flows that were studied.

Unificación de criterios para la determinación de la materia orgánica del suelo. Estudio interlaboratorio

Se compararon dos métodos (M1 y M2) para la determinación de la materia orgánica (MO) de los suelos, en los laboratorios de seis instituciones: UCV-Agronomía, INIA-Yaracuy, UNERG, EDAFOFINCA, INIA-Guárico y AGRI de Venezuela, con la finalidad de unificar criterios y bajar la dispersión en los resultados analíticos de la MO entre laboratorios. Los métodos están basados en la digestión húmeda de la muestra con ácido sulfúrico y dicromato de potasio, y la cuantificación del carbono orgánico por espectrofotometría UV-Visible. Las diferencias fundamentales comprendían: peso de muestra 0,1 g y 0,5 g; longitud de onda de 650 y 590 nm; y tiempo de reacción entre 3 y 24 horas para M2 y M1 respectivamente. Se utilizaron dos muestras con niveles de MO bajo y alto. Se evaluó la precisión de los métodos en términos de repetibilidad (r) y Reproducibilidad (R). Se encontró que al aplicar el método M1 se obtuvieron valores de MO más exactos (en relación al valor medio del límite de confianza establecido en las muestras de suelos) tanto para las muestras como para los laboratorios. Los valores de MO obtenidos por ambos métodos correlacionaron significativamente (r = 0,6591; p = 0,0005)  para la muestra de nivel alto en MO; y no presentaron diferencias significativas entre laboratorios. En comparación con el método M2, el método M1 oxida más carbono orgánico del suelo, con una mayor precisión y menor variabilidad entre laboratorios, por lo que se recomienda adoptar este último en el análisis de suelos con fines de fertilidad.  Palabras clave: Carbono orgánico de suelos; métodos de análisis; intercomparación de laboratorios.  AbstractTwo methods (M1 and M2) used to determine soil organic matter (SOM), were compared, in different laboratories from the following institutions: UCV-Agronomy, INIA-Yaracuy, UNERG, EDAFOFINCA, INIA-Guárico and AGRI of Venezuela. The aim was to unify criteria and to get the lower dispersion of the analytical results between laboratories. The methods are based on wet digestion  with sulfuric acid and potassium dichromate, and quantification of organic carbon by UV-Visible spectrophotometry. The fundamental differences among them, comprising: sample weight of 0.1 g and 0.5 g, the wavelength of 650 and 590 nm, and the reaction time between 3 and 24 hours for M2 and M1 respectively. Two control samples with low and high level of SOM were used. The accuracy of the methods in terms of repeatability (r) and Reproducibility (R) was evaluated. It was found that by applying the method M1, organic matter values ​​were more accurate (relative to the mean value confidence limits established in soils samples) for both samples and  laboratories. SMO values ​​obtained for both methods were significantly correlated (r = 0.6591, p = 0.0005) for the sample with high SMO concentration, and no significant differences between laboratories were found. Comparing both methods, M1 oxidized more soil organic carbon than M2, with the greatest precision and the lowest variability between laboratories, for this we recommend to adopt M1 in soil fertility analysis.   Key Word: Soil organic carbon; analysis methods; interlaboratory studies.&nbsp