Altitudinal variation of resin ducts in Pinus taiwanensis Hayata (Pinaceae) needles
The variation of resin ducts in needles of Pinus taiwanensis along an altitude gradient (700 m- 3,100 m) in central Taiwan is revealed for the first time in this study, including position type, number, size, arrangement pattern and anatomical structural variation. Three types of resin ducts-medial, external and septal-were found according to their relative positions in the mesophyll. All needles sampled showed combinations of these three resin duct types. Mostly, 3-7 resin ducts were found in the mesophyll per needle, and the number of resin ducts was significantly greater at mid altitudes than at high and low altitudes. Septal ducts decreased in number with an increase in altitude; however, needles from mid altitudes had more external ducts. Eight patterns (basic and patterns I-VII) of resin ducts in the mesophyll of needles were delineated to present the arrangement variations. In addition to the primary ducts found in the mesophyll, we found the rare bundle sheath ducts in some needles, and their frequencies decreased obviously with an increase in altitude. The consistent combination feature of medial, external, and septal ducts in needles of P. taiwanensis from Taiwan could be easily distinguished from those with medial ducts from mainland China. However, this difference has unrecognized before, and it should be considered in future taxonomic work.
A reassessment of Pinus Subgen. Pinus in China
A recent study of Pinus subgen. Pinus for the Flora of China project recognized 10 native species with four new combinations, all at subspecies rank, and determined one new homonym, and six new synonyms. A neotype is designated for P. kesiya. Doubtful or little-known species are listed.
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths