全文获取类型
收费全文 | 12422篇 |
免费 | 1191篇 |
国内免费 | 11篇 |
专业分类
13624篇 |
出版年
2021年 | 143篇 |
2019年 | 105篇 |
2018年 | 103篇 |
2017年 | 112篇 |
2016年 | 173篇 |
2015年 | 345篇 |
2014年 | 376篇 |
2013年 | 508篇 |
2012年 | 682篇 |
2011年 | 666篇 |
2010年 | 474篇 |
2009年 | 423篇 |
2008年 | 628篇 |
2007年 | 657篇 |
2006年 | 591篇 |
2005年 | 678篇 |
2004年 | 690篇 |
2003年 | 668篇 |
2002年 | 617篇 |
2001年 | 187篇 |
2000年 | 144篇 |
1999年 | 173篇 |
1998年 | 201篇 |
1997年 | 128篇 |
1996年 | 142篇 |
1995年 | 148篇 |
1994年 | 127篇 |
1993年 | 116篇 |
1992年 | 159篇 |
1991年 | 134篇 |
1990年 | 142篇 |
1989年 | 132篇 |
1988年 | 135篇 |
1987年 | 123篇 |
1986年 | 106篇 |
1985年 | 132篇 |
1984年 | 164篇 |
1983年 | 136篇 |
1982年 | 154篇 |
1981年 | 166篇 |
1980年 | 138篇 |
1979年 | 98篇 |
1978年 | 115篇 |
1977年 | 116篇 |
1976年 | 103篇 |
1975年 | 114篇 |
1974年 | 117篇 |
1973年 | 125篇 |
1971年 | 79篇 |
1970年 | 74篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Anaerobic Oxidation of Acetylene by Estuarine Sediments and Enrichment Cultures 总被引:22,自引:18,他引:4 下载免费PDF全文
Charles W. Culbertson Alexander J. B. Zehnder Ronald S. Oremland 《Applied microbiology》1981,41(2):396-403
Acetylene disappeared from the gas phase of anaerobically incubated estuarine sediment slurries, and loss was accompanied by increased levels of carbon dioxide. Acetylene loss was inhibited by chloramphenicol, air, and autoclaving. Addition of 14C2H2 to slurries resulted in the formation of 14CO2 and the transient appearance of 14C-soluble intermediates, of which acetate was a major component. Acetylene oxidation stimulated sulfate reduction; however, sulfate reduction was not required for the loss of C2H2 to occur. Enrichment cultures were obtained which grew anaerobically at the expense of C2H2. 相似文献
102.
103.
104.
105.
106.
Phytoplankton assemblage and seston composition of the San José's gulf (Argentine, 42° S) were studied during a period of two years. Cell counts, chlorophyll, particulate nitrogen and particulate carbon determination were made. Phytoplankton synecology was studied using principal component analysis. 相似文献
107.
The tautomeric composition of a solution of ammonium 3-deoxy-d-manno-octulosonate (KDO, 1a) in D2O at 28° was assessed by means of 13 C-F.t.-n.m.r. spectroscopy. The results revealed the presence of 6?0 and 11 % of the α and β anomers of the pyranose, and 20 and 9 % of the two furanoses, and suggested, but did not unequivocally prove, that the major furanose form is the α anomer. To facilitate interpretation of the spectral results for 1, ammonium 3,5-dideoxy-d-arabino(or ribo)-octulosonate (3a) was prepared by the reaction of 5-deoxy-d-erythro-pentose with sodium oxalacetate at pH 11. A chromatographically homogeneous, noncrystalline sample of 3 was obtained by lyophilization, and characterized as its (4-nitrophenyl)hydrazone (m.p. 162-163°). The 13C-n.m.r. spectrum of a solution of 3a in D2O revealed it to be substantially all in the α-pyranose form. No signals were obtained for the possible 1,4-lactone of 3. As the 1,5-lactone and furanose forms are impossible for 3, it exhibited no signals analogous to those attributed to furanoid 1. On the basis of these results for 3, the two lactone forms of 1 were excluded from consideration, and the three pairs of 13C-n.m.r. signals observed at ≈45, 86, and 104 p.p.m. were assigned to the furanose forms of 1. 相似文献
108.
Parameters involved in reconstitution of the outer membrane permeability described by Brunner, Caputo, and Treick [3] were
examined. The most efficient reconstitution was obtained when divalent cations accompanied the addition of exogenous outer
membrane material. Studies indicated that the effectiveness of Ca2+ or Mg2+ to promote reassociation of outer membrane material, and subsequent protection against actinomycin D, was dependent upon
the strain ofEscherichia coli. More specifically, the data suggest that the effectiveness of the different divalent cations in promoting reassociation
was determined by the relative amounts of F1 and F2 fractions released by ethylenediaminetetraacetate (EDTA). Reconstitution
was shown by cell survival to be as high as 25% and dependent upon the total amount of material released from the outer membrane
by EDTA. Between 50 and 80% of the bound material could be removed from the cells by subsequent EDTA treatment. 相似文献
109.
The postoperative course of six patients with primary hyperparathyroidism and obvious radiological evidence of bone disease pretreated with 1-alpha-hydroxy vitamin D3 (1 alpha HCC) was indistinguishable from that of six patients with a similar clinical and radiological picture who were not pretreated. 1alphaHCC may increase the hypercalcaemia in some cases and cannot be recommended for the routine preparation of such patients for surgery. 相似文献
110.
This study describes and quantifies the social organization and agonistic behaviour of adult male collared lemmings (Dicrostonyx groenlandicus) in an indoor enclosure. Five groups of four lemmings were observed during separate 8-day tests. Experimental design permitted an increase in group size while density remained constant. When in groups of two, males established stable dominant-subordinate relationships and, when in groups of four, they formed stable non-linear dominance hierarchies. For either group size, frequency of agonistic behaviour declined over time. Habituation, formation of dominance relationships and spatio-temporal partitioning of space are suggested as possible explanations of this decline. Percentage of initial body weight lost during the experiments varied inversely with social rank. Possible causes of this weight loss are discussed. Social relationships in this study are discussed in light of field observations of male lemming home range distribution. 相似文献