全文获取类型
收费全文 | 131521篇 |
免费 | 2723篇 |
国内免费 | 817篇 |
出版年
2023年 | 247篇 |
2022年 | 220篇 |
2021年 | 513篇 |
2020年 | 465篇 |
2019年 | 456篇 |
2018年 | 12678篇 |
2017年 | 11354篇 |
2016年 | 8695篇 |
2015年 | 2506篇 |
2014年 | 2202篇 |
2013年 | 2879篇 |
2012年 | 7074篇 |
2011年 | 15203篇 |
2010年 | 13445篇 |
2009年 | 9342篇 |
2008年 | 11603篇 |
2007年 | 13143篇 |
2006年 | 2283篇 |
2005年 | 2171篇 |
2004年 | 2607篇 |
2003年 | 2499篇 |
2002年 | 2174篇 |
2001年 | 1276篇 |
2000年 | 1144篇 |
1999年 | 800篇 |
1998年 | 311篇 |
1997年 | 225篇 |
1996年 | 229篇 |
1995年 | 225篇 |
1994年 | 169篇 |
1993年 | 174篇 |
1992年 | 467篇 |
1991年 | 428篇 |
1990年 | 378篇 |
1989年 | 371篇 |
1988年 | 339篇 |
1987年 | 329篇 |
1986年 | 291篇 |
1985年 | 285篇 |
1984年 | 233篇 |
1983年 | 219篇 |
1982年 | 162篇 |
1979年 | 253篇 |
1974年 | 174篇 |
1973年 | 170篇 |
1972年 | 409篇 |
1971年 | 425篇 |
1970年 | 164篇 |
1969年 | 161篇 |
1968年 | 154篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
201.
Background
Chow and Liu showed that the maximum likelihood tree for multivariate discrete distributions may be found using a maximum weight spanning tree algorithm, for example Kruskal's algorithm. The efficiency of the algorithm makes it tractable for high-dimensional problems. 相似文献202.
203.
The peptidic sequence (Sarcosine-1-Isoleucine-8)-Angiotensin II has been demonstrated to be an in vitro specific and competitive antagonist of the Angiotensin II action. The present results show it to be a competitive antagonist also in vivo since pA2 values are similar, always reaches a 100% response on increasing the Angiotensin II dose, and when relating log (DR-1) and log dose of agonist, the slope is very near to one (0.925). 相似文献
204.
205.
The effects produced on bacteriorhodopsin by low concentrations of several detergents have been studied by absorption and fourth-derivative spectrophotometry. Sodium dodecyl sulfate induces the appearance of the blue form of bacteriorhodopsin (λmax = 600 nm) at pH values up to 7.0 in a reversible manner. The apparent pK of the purple-to-blue transition raised with increasing concentration of SDS. Of the other detergents tested, only sodium dodecyl-N-sarcosinate showed a slight red-shift of the absorption band to 580 nm, whereas sodium taurocholate, Triton X-100 and cetyltrimethylammonium bromide did not favour the appearance of the blue form. The effect of SDS was found to be consistent with a localized conformational change that moves away the counter-ion of the protonated Schiff base. 相似文献
206.
The effects of invasive macrophytes, water level fluctuations and predation on freshwater unionids Pyganodon grandis and Utterbackia imbecillis were studied in three small impoundments in Northeastern Texas in 2003–2005. Mussel density was sampled with quadrats. Mortality,
associated with the water level fluctuations and predation, was estimated by collecting dead shells on the shore at about
two month intervals. In two ponds, horizontal distribution of unionids was limited by dense beds of invasive and noxious macrophytes
(mainly Eurasian watermilfoil Myriophyllum spicatum and American lotus Nelumbo lutea): mussel densities were significantly lower in these macrophyte beds (P < 0.001). In the third pond with the lowest density of macrophytes (stonewort Chara sp.), unionids were distributed more evenly, and the average unionid biomass was the highest among all ponds studied. Vertical
distribution of unionids in all ponds was likely limited by low oxygen at depth >2 m. The total amount of shells found on
the shore per year varied from 0.1% to 28% of the total population in the pond and was negatively correlated with water level
(r = −0.72 to −0.81, P < 0.005). Mammalian predators consumed up to 19% of the total unionid population and predation was facilitated by water level
fluctuations.
Handling editor: K. Martens 相似文献
207.
208.
209.
The effects of dispersal limitation and topographic heterogeneity on beta diversity and phylobetadiversity in a subtropical forest 总被引:1,自引:0,他引:1
Yue Bin Zhigao Wang Zhangming Wang Wanhui Ye Honglin Cao Juyu Lian 《Plant Ecology》2010,209(2):237-256
We assessed the effects of topographic heterogeneity and stem density on species composition between grains of different sizes
(20 × 20, 50 × 50, and 100 × 100 m), based on partial Mantel tests. Similarity in species composition was measured by the
abundance-based Jaccard index (C_J) and by an index that incorporates phylogenetic information into C_J (pC_J). Plants were
divided into five groups, arbor, subarbor, and shrub according to life form and two other groups: species that produce dry
fruits (PDF) and that produce fleshy fruits (PFF). C_J and pC_J between any two grains at each grain size were calculated
separately for these groups and for all species combined. In order to examine what influences C_J and pC_J, we analyzed their
correlations with topographic heterogeneity variables and two dispersal limitation-related variables (stem and topographic
resistance). Our data indicate that at all three grain sizes, C_J and pC_J decrease with increasing distance for all plant
groups. Dispersal limitation and topographic heterogeneity were both important at 20 × 20 and 50 × 50 m grain sizes for C_J
and pC_J of all plant groups; and at 100 × 100 m grain size, topographic heterogeneity dominates over dispersal limitation
for some plant groups. C_J and pC_J of PDFs are less negatively correlated with stem resistance than those of PFFs. We conclude
that both beta diversity and phylobetadiversity are dependent on plant groups and grain sizes. 相似文献
210.
Jeffrey P Mower Pascal Touzet Julie S Gummow Lynda F Delph Jeffrey D Palmer 《BMC evolutionary biology》2007,7(1):135