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Temporal changes in the distribution range of plant communities and habitats should be considered for optimal conservation. However, this information is often lacking. In this research, we investigated the changes in the spatial distribution pattern of the plant communities characterised by the presence of three Mediterranean pines considered native or putative native to Sardinia (Italy), namely Pinus halepensis, P. pinaster, and P. pinea. We analysed historical and current aerial photographs to prepare maps of the past and current distribution of natural pine formations, complemented with the aid of pine-related toponymy maps and interviews with local people. We calculated how the surface of natural pine formations varied across time, and found a high rate of recovery during the last decades. This rate is doubled when compared to the average extent of other Sardinian woody formations in the same period. Among the three pine species, the area of P. halepensisand P. pinasterformations increased the most. We also found that about 90% of pine formations fall within protected areas. More than 128 ha of natural pine formations are included in the priority habitat 2270, while those included in habitat 9540 cover 1100 ha. Our study provides a complete survey of the distribution of natural Sardinian pine formations, along with the quantification of their increase in the last decades, thus highlighting the importance of diachronic analyses for monitoring spatial changes in plant communities. We suggest considering the trends in the extent of vegetation formations and habitats for conservation purposes.

The measurement of density by observing the attenuation of transmitted gamma rays was adapted to the measurement of sediment density. A dual probe, utilizing transmission techniques, was evaluated for use in measuring the density of sediments collected from runoff of plots or small watersheds. The equipment consists of a 7‐mc. cesium137source placed in one access tube and a probe containing a NaI scintillation crystal, photomultiplier tube, and preamplifier in the second tube. The high‐voltage power supply and scaler ratemeter for readout are supplied in a portable unit powered by silver‐cadmium recharge‐able dry cells. Cesium137gamma radiation is measured by employing electronic discrimination to eliminate all phostons of energies below about 0.65 Mev. Sediment densities were measured in silt boxes installed on a small watershed at the North Mississippi Branch Experiment Station, Holly Springs, Mississippi, following laboratory calibration tests. The dual probe employed measured densities of sediments to within 0.01 absolute density units. The vertical resolution of the apparatus is less than I inch so that density measurements of thin layers and of materials at interfaces can be made. In one field test sediment weights, of the order of 8,500 pounds, computed for a silt box on the basis of dual probe density determinations varied less than 1% from a gravimetric determination. Changes in sediment density with time, with additional increments of sediment, and with storms were effectively measured.

To the Editor:—Relative to the comments of Dr. Pendleton Tompkins, it may be stated that in the 23 complete cases of the 27 reported, 4 conceptions resulted from a single donor insemination on the day of prediction; 5 resulted from a single insemination with the husband's specimen on the day of prediction. Two of these had coitus later, but within eight hours of the insemination. In these 9 cases, no other intercourse occurred at any time during the cycle. Ten other conceptions resulted from coitus on the predicted day, as follows: 5 from isolated coitus which occurred once only, and at no other time in the cycle; 2 from coitus performed twice within ten hours and at no other time in the cycle, and 3 from coitus performed twice within the twenty-four hour period of the predicted day of ovulation. The 4 remaining persons had coitus on the predicted