全文获取类型
收费全文 | 4685篇 |
免费 | 474篇 |
国内免费 | 6篇 |
出版年
2023年 | 31篇 |
2022年 | 44篇 |
2021年 | 134篇 |
2020年 | 67篇 |
2019年 | 78篇 |
2018年 | 92篇 |
2017年 | 86篇 |
2016年 | 160篇 |
2015年 | 221篇 |
2014年 | 226篇 |
2013年 | 254篇 |
2012年 | 318篇 |
2011年 | 323篇 |
2010年 | 229篇 |
2009年 | 177篇 |
2008年 | 294篇 |
2007年 | 280篇 |
2006年 | 283篇 |
2005年 | 238篇 |
2004年 | 218篇 |
2003年 | 206篇 |
2002年 | 190篇 |
2001年 | 71篇 |
2000年 | 51篇 |
1999年 | 54篇 |
1998年 | 36篇 |
1997年 | 25篇 |
1996年 | 33篇 |
1995年 | 39篇 |
1994年 | 31篇 |
1993年 | 25篇 |
1992年 | 37篇 |
1991年 | 39篇 |
1990年 | 34篇 |
1989年 | 28篇 |
1988年 | 37篇 |
1987年 | 20篇 |
1986年 | 40篇 |
1985年 | 26篇 |
1984年 | 31篇 |
1983年 | 16篇 |
1982年 | 22篇 |
1981年 | 19篇 |
1980年 | 27篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 14篇 |
1974年 | 20篇 |
1973年 | 16篇 |
1971年 | 14篇 |
排序方式: 共有5165条查询结果,搜索用时 15 毫秒
111.
The enzyme tyrosinase (E.G. 1.14.18.1) has been immobilized in a polyacrylamide gel and intermittently assayed for enzyme activity over a period of 19 days using phenol as the substrate. The results of these studies indicate that the immobilized enzyme could be incorporated into a system to detect phenol and related compounds that are found in industrial effluents and as surface water contaminants. 相似文献
112.
It has been shown that plants can accumulate K+ through an energy-dependent process. The effect of alkylguanidines, in particular octylguanidine on the uptake of 86Rb+ by excised barley roots (Hordeum vulgare var. Apizaco LV-72), has been studied. 86Rb+ was used as tracer of K+. The uptake of 86Rb+ which is linear with time and shows saturation kinetics is inhibited by octylguanidine. Half-maximal inhibition of 86Rb+ uptake is attained at 50 μM octylguanidine. Octylguanidine induces a decrease in the Vmax of the process and increases the Km of the system for Rb+. When the effects of various alkylguanidines were studied, the following order of effectiveness was encountered; octylguanidine = hexilguanidine > butylguanidine > ethylguanidine > guanidine. This suggests that guanidines inhibit Rb+ uptake by interacting through its positively charged guanidinium group with a Rb+ carrier while the alkyl chain interacts with the hydrophobic milieu of the membrane. 相似文献
113.
114.
Tritoniid sea slugs (Mollusca, Gastropoda, Opisthobranchia) are reported for the first time from Bouvet Island. Tritonia dantarti sp. n. shows morphological and anatomical differences with regard to the two previously known tritoniids reported from Antarctica
and Sub-Antarctica, Tritonia vorax (Odhner 1926) and Tritonia challengeriana Bergh, 1884. Regarding the external morphology, T. dantarti sp. n. is characterized by a very bright orange coloration in the dorsum, white dorsal crests, highly ramified dendritic
gills (the largest vertically orientated and the rest laterally orientated), and a quadrangular cross section. The radula
presents very long, thin lateral teeth and the jaws present mainly unicuspidate, striated denticles. The seminal receptacle
is large, pear-shaped and its grey pigmentation differs from the rest of the genital system. The Antarctic species T. challengeriana was also found in waters off Bouvet Island, while the Subantarctic species T. vorax was not found. 相似文献
115.
Maintaining up-to-date annotation on reference genomes is becoming more important, not less, as the ability to rapidly and
cheaply resequence genomes expands. 相似文献
116.
117.
118.
Since actinomycetes are the source of the majority of antibiotics and DNA cloning systems have recently been established for these organisms1–3 we are now in a position to devise approaches for increasing yields or combining the beneficial properties of two (or more) antibiotics in one molecule. Here we assess the capabilities of the available technology to improve the production of aminoglycoside antibiotics. 相似文献
119.
120.
Aim The highly endemic fishes of the arid Southwest USA have been heavily impacted by human activities resulting in one of the most threatened fish faunas in the world. The aim of this study was to examine the patterns and drivers of taxonomic and functional beta diversity of freshwater fish in the Lower Colorado River Basin across the 20th century. Location Lower Colorado River Basin (LCRB). Methods The taxonomic and functional similarities of watersheds were quantified to identify patterns of biotic homogenization or differentiation over the period 1900–1999. Path analysis was used to identify the relative influence of dam density, urban land use, precipitation regimes and non‐native species richness on observed changes in fish faunal composition. Results The fish fauna of the LCRB has become increasingly homogenized, both taxonomically (1.1% based on βsim index) and functionally (6.2% based on Bray–Curtis index), over the 20th century. The rate of homogenization varied substantially; range declines of native species initially caused taxonomic differentiation (?7.9% in the 1960s), followed by marginal homogenization (observed in the 1990s) in response to an influx of non‐native species introductions. By contrast, functional homogenization of the basin was evident considerably earlier (in the 1950s) because of the widespread introduction of non‐native species sharing similar suites of biological traits. Path analysis revealed that both taxonomic and functional homogenization were positively related to the direct and indirect (facilitation by dams and urbanization) effects of non‐native species richness. Main conclusions Our study simultaneously examines rates of change in multiple dimensions of the homogenization process. For the endemic fish fauna of the LCRB, we found that the processes of taxonomic and functional homogenization are highly dynamic over time, varying both in terms of the magnitude and rate of change over the 20th century. 相似文献