全文获取类型
收费全文 | 315篇 |
免费 | 3篇 |
专业分类
318篇 |
出版年
2016年 | 1篇 |
2015年 | 2篇 |
2014年 | 3篇 |
2013年 | 4篇 |
2012年 | 12篇 |
2011年 | 21篇 |
2010年 | 26篇 |
2009年 | 27篇 |
2008年 | 26篇 |
2007年 | 24篇 |
2006年 | 22篇 |
2005年 | 14篇 |
2004年 | 10篇 |
2003年 | 5篇 |
2002年 | 2篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 9篇 |
1997年 | 15篇 |
1996年 | 9篇 |
1995年 | 2篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 4篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1966年 | 1篇 |
1956年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有318条查询结果,搜索用时 15 毫秒
1.
JONATHAN R. HENDRICKS 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2009,42(1):55-66
The paucity of sinistral (left-coiling) relative to dextral (right-coiling) species of gastropods in the marine realm is an enigma. In Conus , one of the most diverse marine animal genera, sinistral shell coiling has evolved as a species-wide character only once. Fossils of this species, Conus adversarius , are found in Upper Pliocene and lowermost Pleistocene deposits in the southeastern USA. Conus adversarius had nonplanktonic larval development; this may have been a critical factor for the early establishment of the species, as well as sinistral marine species in other clades. Notably, most specimens of aberrantly sinistral modern Conus are derived from typically dextral species that have nonplanktonic development. If C. adversarius was reproductively isolated from dextral conspecifics, then this species may provide an example of nearly instantaneous sympatric speciation in the fossil record. Furthermore, the common and widespread – while geologically short-lived – fossil shells of C. adversarius show large amounts of variability in form and this variation may be related, at least in part, to a pleiotropic effect associated with the reversed coiling direction of this species. 相似文献
2.
Abstract: Fossils of dyrosaurid crocodyliforms are limited in South America, with only three previously diagnosed taxa including the short‐snouted Cerrejonisuchus improcerus from the Paleocene Cerrejón Formation of north‐eastern Colombia. Here we describe a second dyrosaurid from the Cerrejón Formation, Acherontisuchus guajiraensis gen. et sp. nov., based on three partial mandibles, maxillary fragments, teeth, and referred postcrania. The mandible has a reduced seventh alveolus and laterally depressed retroarticular process, both diagnostic characteristics of Dyrosauridae. Acherontisuchus guajiraensis is distinct among known dyrosaurids in having a unique combination of craniomandibular characteristics, and postcranial morphology that suggests it may have occupied a more placid, fluvial habitat than most known Old‐World dyrosaurids. Results from a cladistic analysis of Dyrosauridae, using 82 primarily cranial and mandibular characters, support an unresolved relationship between A. guajiraensis and a combination of New‐ and Old‐World dyrosaurids including Hyposaurus rogersii, Congosaurus bequaerti, Atlantosuchus coupatezi, Guarinisuchus munizi, Rhabdognathus keiniensis and Rhabdognathus aslerensis. Our results are consistent with an African origin for Dyrosauridae with multiple dispersals into the New World during the Late Cretaceous and a transition from marine habitats in ancestral taxa to more fluvial habitats in more derived taxa. 相似文献
3.
4.
Mapping the Microenvironment for Seed Germination in the Field 总被引:1,自引:0,他引:1
Gaps in the canopy have a major influence upon the microenvironmentfor seed germination in natural vegetation. Two methods formeasuring gaps in grasslands are described. The first uses afield-portable instrument for mapping R/IR ratios under thecanopy, and the second employs infrared photography and imageanalysis to measure gaps from above. The methods are illustratedwith results from an experiment in which grazing sheep are theprinciple influence upon the structure of the grass canopy.Results of both methods show that the effects of current andpast grazing regimes may be detected in the canopy structureof the grassland. Seeds of Geranium dissection sown into gapsof different sizes showed greater establishment in small gaps(12 cm2) than under a closed canopy Canopy gap, germination, grassland, grazing, Geranium dissectum 相似文献
5.
Statistics for near independence in multivariate extreme values 总被引:17,自引:0,他引:17
6.
7.
SUSAN G LANGRETH JONATHAN D. BERMAN G. PATRICK RIORDAN LINDA S. LEE 《The Journal of eukaryotic microbiology》1983,30(3):555-561
The mechanism of action of antileishmanial compounds is poorly understood. Ultrastructural changes in Leishmania tropica within human macrophages exposed in vitro to Pentostam, pentamidine, amphotericin B, WR 6026, ketoconazole, and Formycin B were examined in these experiments. In Pentostam-treated cultures, some organisms exhibited diminished definition of mitochondrial and other membranes, while other organisms had completely disintegrated. Pentostam-exposed macrophages demonstrated loss of membrane definition in the absence of further alterations; it is therefore hypothesized that impaired macrophage membrane function may contribute towards the effect of this drug against macrophage-contained organisms. Leishmania parasites in pentamidine-treated cultures initially demonstrated swollen kinetoplasts and fragmentation of the kinetoplast DNA core. The initial observed effect of the other four drugs on the parasites was cytoplasmic condensation. These ultrastructural studies suggest that all five non-antimonial drugs may have different mechanisms of action than antimony (Pentostam) against Leishmania. 相似文献
8.
PER ALSTRÖM PETE DAVIDSON J. W. DUCKWORTH JONATHAN C. EAMES TRAI TRONG LE CU NGUYEN URBAN OLSSON CRAIG ROBSON & ROB TIMMINS 《Ibis》2010,152(1):145-168
A new species of Phylloscopus warbler, which we name Phylloscopus calciatilis Limestone Leaf Warbler, is described from central and northern Vietnam and central and northern Laos; it probably also breeds in southernmost China. In morphology, the new species is very similar to Sulphur-breasted Warbler Phylloscopus ricketti , but it is smaller with a proportionately larger bill and rounder wing. Its song and calls are diagnostic. Based on mitochondrial and nuclear DNA, the new species is most closely related to P. ricketti and Yellow-vented Warbler Phylloscopus cantator , and it is inferred to be sister to the latter. The mitochondrial divergences between these three species are at the low end of the variation found in other species of Phylloscopus and Seicercus warblers, but greater than in other taxa generally treated as subspecies. Possible introgressive hybridization between the new species and P. ricketti is discussed, but more data are needed to establish whether it does occur and, if it does, to what extent. The new species appears to have a restricted breeding range in limestone karst environments, where it is locally common and therefore not under any immediate threat. In view of the recognition of the new species, all previous records of P. ricketti sensu lato need to be re-evaluated. 相似文献
9.
10.
A detailed study was made of the content per leaf lamina ofseveral biochemical components during growth and the entirelifespan of the primary leaf of bean, Phaseolus vulgaris L.,var. Dwarf Horticultural Bush. Total DNA measured spectrofluorometricallywas retained even after abscission and withering of the lamina.Possible reasons for the lack of DNA degradation in very oldleaves are presented. Using improved extraction methods it was found that both putativecytoplasmic polyribosomes and the total organelle plus cytoplasmicribosomal RNAs rose to a peak during leaf growth, declined rapidly,and finally levelled off during maturity and senescence. Even37 d old senescent leaves retained polysomes as large as hexamers.Organelle rRNA peaked late in the growth phase and about 4 dafter the peak in cytoplasmic rRNA. During leaf maturity andsenescence, both of these rRNA types decreased in parallel.Total protein and chlorophyll peaked at different times duringleaf growth, declined at a steady rate during maturity, andthen declined faster in the senescence phase. The trends arecompared with each other and to the literature. 相似文献