全文获取类型
收费全文 | 630614篇 |
免费 | 70199篇 |
国内免费 | 400篇 |
出版年
2018年 | 5294篇 |
2016年 | 7199篇 |
2015年 | 9755篇 |
2014年 | 11216篇 |
2013年 | 17021篇 |
2012年 | 18596篇 |
2011年 | 18892篇 |
2010年 | 12669篇 |
2009年 | 11733篇 |
2008年 | 17149篇 |
2007年 | 17429篇 |
2006年 | 16771篇 |
2005年 | 15952篇 |
2004年 | 15739篇 |
2003年 | 15265篇 |
2002年 | 14840篇 |
2001年 | 27605篇 |
2000年 | 27762篇 |
1999年 | 22343篇 |
1998年 | 8082篇 |
1997年 | 8529篇 |
1996年 | 8234篇 |
1995年 | 7455篇 |
1994年 | 7562篇 |
1993年 | 7461篇 |
1992年 | 18918篇 |
1991年 | 18498篇 |
1990年 | 18118篇 |
1989年 | 17988篇 |
1988年 | 16538篇 |
1987年 | 15844篇 |
1986年 | 14581篇 |
1985年 | 14779篇 |
1984年 | 12091篇 |
1983年 | 10431篇 |
1982年 | 8076篇 |
1981年 | 7235篇 |
1980年 | 6970篇 |
1979年 | 11735篇 |
1978年 | 9079篇 |
1977年 | 8420篇 |
1976年 | 7990篇 |
1975年 | 8717篇 |
1974年 | 9332篇 |
1973年 | 9180篇 |
1972年 | 8279篇 |
1971年 | 7472篇 |
1970年 | 6674篇 |
1969年 | 6329篇 |
1968年 | 5712篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
Abstract. The structure of dune ponds hygrophytic vegetation has a spatial organisation in belts around the pond basin, closely related to water level and flooding regime. Doñana National Park has an important representation of temporal dune ponds, which are subjected to rainfall fluctuations and may be suffering the impact of water extraction from the neighbouring tourist resort. Permanent transects in a vegetation complex of five dune ponds have been monitored during a eight year period (1990-1997). This period was characterised by a number of dry years (annual rainfall around 300 mm), located between two wet cycles (800-900 mm). Transects were analysed in 1990 (wet period), 1994 (dry) and 1997 (wet) by hierarchical agglomera-tive clustering. During the dry period hygrophytic species showed regression, with a high mortality of some species like Ulex minor, while the xerophytic species advanced to lower areas. Seedlings of some xerophytic species colonised the dry surface of the pond basin. The lowering of the water table varied in the different ponds, ranging from 312 to 190 cm depending on topography and the distance to the pumping area. The new period of flooding during 1995-96 and 1996-97 cycles provided the opportunity for hygrophytic spe cies to re-establish themselves in their original places. This study suggest that changes in vegetation are caused by the interaction between weather conditions and human disturbance (water extractions). In our example man-made disturbance is more marked during the dry periods while wet periods tend to obscure the effects of water extractions. From a management perspective, long-term monitoring of water table and vegetation structure is revealed as a key procedure to the management of land-water ecotones on pressured areas and threatened habitats. 相似文献
173.
To control the environmentally detrimental impact of acid rock drainage, two different countermeasures, layers of acid-buffering
materials and sodium dodecyl sulphate addition, were tested for their efficiency in laboratory percolation experiments. In
the experiment with a layer of calcium bentonite, only the iron output was reduced. The experiments with layers of concrete
grains demonstrated a decrease of the microbial activity as well as a precipitation of heavy-metal ions, whereas the cell
numbers did not decrease. Furthermore, finely grained concrete (1–5 mm) formed a water-tight hardpan (self-sealing layer).
In the experiment with 1 mM sodium dodecyl sulphate, all the microorganisms were killed and hence metal sulphide dissolution
was stopped. With 0.1 mM sodium dodecyl sulphate only a short, transient inhibition of leaching was achieved. The bacteria
remained alive.
Received: 16 February 1998 / Accepted: 23 February 1998 相似文献
174.
T. P. Malyshkina 《Paleontological Journal》2012,46(4):392-399
Two new species of carcharhinid sharks from the Late Eocene deposits (Tavda Formation) of the southern Trans-Urals, Abdounia vassilyevae sp. nov., previously determined as A. aff. beaugei, and A. lata sp. nov., are described. To date, up to five Priabonian species of Abdounia have been recorded. New finds supplement the data on diversity, evolution, and paleobiogeography of Abdounia. 相似文献
175.
Monoclonal antibodies were used to investigate the immunochemistry of human erythrocyte acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7). A series of experiments on the sedimentation velocity and Stokes radius of acetylcholinesterase and its immune complexes indicated that each antibody recognized a single high-affinity binding site (epitope) on the monomeric enzyme. Further analysis suggested that the antibody-binding sites were replicated on multimeric enzyme forms but were subject to steric hindrance between nearby IgG molecules or adjacent enzyme subunits. The cellular localization of the epitopes was studied by measuring the binding of monoclonal antibodies to the cholinesterase of intact erythrocytes. The results implied that most of the epitopes are exposed to the external media. However, one antibody failed to bind to intact cells, despite a relatively high affinity for detergent-solubilized antigen, possibly because its epitope is buried in the lipid bilayer. 相似文献
176.
Summary A murine hybridoma (455) was cultured in four different serum free media formulations, and a newborn calf serum supplemented medium was used as a basis of comparison. The serum supplemented medium supports a higher cell growth rate and results in a higher IgG titer. However, the antibody secretion rate on a per cell basis is higher in the serum free media, indicating that serum could be inhibitory to antibody secretion. The results identify the possibility of a least eliminating serum during the monoclonal antibody production phase. 相似文献
177.
N. P. Todaria 《Biologia Plantarum》1986,28(4):280-287
A comparative study of dark respiration and effects of high and low temperature on dark respiration in five greenhouse cultivated tropical and sub-tropical species was made. The respiration rates determined manometrically were low in all species. Respiration rates increased with increasing temperature. Low temperature treatment accelerated the rate of respiration at 25 °G except in two species. An attempt has been made to determine the critical and optimum temperature for these species. The respiratory behaviour of these species has been discussed with respect to their original habitats and prevailing environmental conditions. 相似文献
178.
179.
Involvement of the galactosyl-1-phosphate transferase encoded by the Salmonella enterica rfbP gene in O-antigen subunit processing. 总被引:11,自引:5,他引:6 下载免费PDF全文
rfbT of Salmonella enterica LT2 was previously thought, together with rfaL, to be involved in the ligation of polymerized O antigen to core-lipid A, and three mutants were known. We report the mapping of the mutations to rfbP, the galactosyl-1-phosphate transferase gene, which is now shown to encode a bifunctional protein. The mutations which have the former rfbT phenotype are referred to as rfbP(T). We also show that rfbP(T) mutants are not blocked in the ligation step as previously believed but in an earlier step, possibly in flipping the O-antigen subunit on undecaprenyl pyrophosphate from the cytoplasmic to periplasmic face of the cytoplasmic membrane. 相似文献