Oecologia 110, 449 (1997)]. The analysis and ecological interpretation of this issue are complex, but low levels of cover in the unmanipulated reference plots at some of our sites (sometimes 50% at the annual-dominated Portuguese field site) provide evidence against the automatic exclusion of plots with low cover.

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Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature.-Single-walled carbon nanotubes with a large mean diameter of about 1.85 nm are prepared by a semicontineous hydrogen arc discharge method. These nanotubes achieve a H 2 storage capacity of 4.2 wt% (or a H:C atomic ratio of 0.52) at room temperature under modestly high pressure (about 10 MPa). -(LIU, C.; FAN, Y. Y.; LIU, M.; CONG, H. T.; CHENG, H. M.; DRESSELHAUS, M. S.; Science (Washington, D.

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Pure vapor-grown carbon nanofibers (VGCNF's) with controllable diameters of 10-200 nm were prepared by an improved floating catalyst method. Through transmission electron microscopy (TEM) observation, it was found that VGCNF's have a duplex structure, a hollow and high-crystallinity graphite filament called primary carbon fiber surrounded by a pyrocarbon layer with low graphite crystallinity. It was observed using high-resolution TEM that VGCNF's have excellent graphitic crystallinity with graphite layers stacked neatly parallel to fiber axis. Moreover, x-ray diffraction results showed that the graphitic crystallinity of carbon fibers became higher with decreasing diameter of carbon fibers.