全文获取类型
收费全文 | 93010篇 |
免费 | 8453篇 |
国内免费 | 45篇 |
出版年
2023年 | 254篇 |
2022年 | 663篇 |
2021年 | 1618篇 |
2020年 | 936篇 |
2019年 | 1190篇 |
2018年 | 1520篇 |
2017年 | 1324篇 |
2016年 | 2247篇 |
2015年 | 3848篇 |
2014年 | 4271篇 |
2013年 | 5065篇 |
2012年 | 6886篇 |
2011年 | 6853篇 |
2010年 | 4376篇 |
2009年 | 3979篇 |
2008年 | 5735篇 |
2007年 | 5770篇 |
2006年 | 5546篇 |
2005年 | 5326篇 |
2004年 | 5192篇 |
2003年 | 4950篇 |
2002年 | 4699篇 |
2001年 | 1050篇 |
2000年 | 818篇 |
1999年 | 1116篇 |
1998年 | 1327篇 |
1997年 | 909篇 |
1996年 | 827篇 |
1995年 | 735篇 |
1994年 | 694篇 |
1993年 | 743篇 |
1992年 | 663篇 |
1991年 | 655篇 |
1990年 | 573篇 |
1989年 | 515篇 |
1988年 | 548篇 |
1987年 | 438篇 |
1986年 | 409篇 |
1985年 | 498篇 |
1984年 | 655篇 |
1983年 | 491篇 |
1982年 | 595篇 |
1981年 | 574篇 |
1980年 | 483篇 |
1979年 | 384篇 |
1978年 | 401篇 |
1977年 | 353篇 |
1976年 | 333篇 |
1975年 | 262篇 |
1974年 | 329篇 |
排序方式: 共有10000条查询结果,搜索用时 771 毫秒
41.
Fire has historically been an important ecological component of forests in the Intermountain Region of the northwestern United States. This study is set in a small biogeographically disjunct mountain range. Our research objectives were to (1) investigate the historical frequency, severity, size, and spatial pattern of fire; (2) determine if and how fire regimes have changed since Euro-American settlement; and (3) compare how fire regimes of a small isolated range compare to nearby, but considerably larger, mountain agglomerations. Our findings suggest that this mountain range has historically supported fires typified by small size and high frequency, resulting in a high degree of spatial pattern complexity compared to mountain agglomerations. We also found disparity in size and burn severity solely within the study area based on the bisecting Continental Divide. Since the advent of Euro-American settlement in the 1870s, fire frequency and sizes of individual fires in the West Big Hole Range have significantly decreased resulting in an estimated 87% reduction in area burned. We discuss potential relationships of mountain range isolation and fire regimes in the Intermountain Region. Furthermore, we suggest that the relative small size of this mountain range predisposes it to greater anthropogenic effects upon fire occurrence. 相似文献
42.
Bruce M. Taylor Ronald W. Sarver Gregory Fici Roger A. Poorman Barry S. Lutzke Antonio Molinari Thomas Kawabe Karl Kappenman Allen E. Buhl Dennis E. Epps 《The protein journal》2003,22(1):31-40
The time dependency of the spontaneous aggregation of the fibrillogenic β-Amyloid peptide, Aβ1–40, was measured by turbidity, circular dichroism, HPLC, and fluorescence polarization. The results by all methods were comparable and they were most consistent with a kinetic model where the peptide first slowly forms an activated monomeric derivative (AM), which is the only species able to initiate, by tetramerization, the formation of linear aggregates. The anti-Aβ antibody 6E10, raised against residues 1–17, at concentrations of 200–300 nM delayed significantly the aggregation of 50 μM amyloid peptide. The anti–Aβ antibody 4G8, raised against residues 17–24, was much less active in that respect, while the antibody A162, raised against the C-terminal residues 39–43 of the full-length Aβ was totally inactive at those concentrations. Concomitant with the aggregation experiments, we also measured the time dependency of the Aβ1–40–induced toxicity toward SH-EP1 cells and hippocampal neurons, evaluated by SYTOX Green fluorescence, lactate dehydrogenase release, and activation of caspases. The extent of cell damage measured by all methods reached a maximum at the same time and this maximum coincided with that of the concentration of AM. According to the kinetic scheme, the latter is the only transient peptide species whose concentration passes through a maximum. Thus, it appears that the toxic species of Aβ1–40 is most likely the same transient activated monomer that is responsible for the nucleation of fibril formation. These conclusions should provide a structural basis for understanding the toxicity of Aβ1–40 in vitro and possibly in vivo. 相似文献
43.
R. Michael Roberts Fuller W. Bazer 《BioEssays : news and reviews in molecular, cellular and developmental biology》1984,1(1):8-11
Uteroferrin, a purple-colored, iron-containing acid phosphatase, with many of the properties of a lysosomal hydrolase, transports iron from the mother to the conceptus in pregnant pigs. Uteroferrin, however, is but one member of what may be a broad class of iron-containing phosphatases with unusual spectral properties which result from a novel type of di-iron active site. The biological function of uteroferrin is unknown. We argue here that the in vivo function of uteroferrin, despite its undoubted ability to act as a potent acid phosphatase, is that of a transplacental iron transporter. 相似文献
44.
Background
Phylogenies capture the evolutionary ancestry linking extant species. Correlations and similarities among a set of species are mediated by and need to be understood in terms of the phylogenic tree. In a similar way it has been argued that biological networks also induce correlations among sets of interacting genes or their protein products. 相似文献45.
Three acetotrophicMethanosarcina species, which included marine, nonmarine, and thermophilic strains, were grown on acetate in a 10-liter pH auxostat. Specific growth rates and molar growth yields were constant throughout growth. Cell yields were up to 18-fold greater than previously reported. These properties of the pH auxostat indicate that it is a preferred culture method for the biochemical study of methanogenesis from acetate. 相似文献
46.
Linda L. Laack Michael E. Tewes Aaron M. Haines John H. Rappole 《Acta theriologica》2005,50(4):505-514
The ocelotLeopardus pardalis Linnaeus, 1758 is an endangered felid in the United States currently restricted to southern Texas. The objectives of our
study were to obtain data on ocelot parturition dates, fecundity, sex ratios, den characteristics, and first year survival,
all of which are critical in development of population viability models. Sixteen parturition events were recorded ranging
from mid-April to late December for 12 wild ocelots. Cumulatively, litters consisted of 1 or 2 kittens (ˉ = 1.2 ± 0.44 SD).
Cumulative sex ratio was 1∶2.5 (male:female); however, there was no significant difference between the observed sex ratio
and a 1∶1 sex ratio. Ten den sites were in close proximity (≤ 10 m) to dense thornshrub. Adult female ocelots used 2 to 4
den sites for each litter with distance between consecutively occupied dens ranging from 110 to 280 m (ˉ = 158 m ± 93 SD).
An estimated annual survival for ocelots 0 to 1 year of age was 0.68. Evidence suggests that ocelots in the wild may breed
more frequently than had been previously hypothesized. 相似文献
47.
48.
49.
50.
Hydrobiologia - To observe effects on the phosphorus retention mechanisms of a lake after re-colonisation by macrophytes, Potamogeton crispus L. and Elodea canadensis Michx. were planted in lab... 相似文献