全文获取类型
收费全文 | 49240篇 |
免费 | 3859篇 |
国内免费 | 7671篇 |
专业分类
60770篇 |
出版年
2024年 | 241篇 |
2023年 | 973篇 |
2022年 | 1298篇 |
2021年 | 1603篇 |
2020年 | 1712篇 |
2019年 | 2281篇 |
2018年 | 1948篇 |
2017年 | 1747篇 |
2016年 | 1715篇 |
2015年 | 1628篇 |
2014年 | 2396篇 |
2013年 | 3406篇 |
2012年 | 1953篇 |
2011年 | 2246篇 |
2010年 | 1742篇 |
2009年 | 2353篇 |
2008年 | 2286篇 |
2007年 | 2522篇 |
2006年 | 2239篇 |
2005年 | 2100篇 |
2004年 | 1806篇 |
2003年 | 1728篇 |
2002年 | 1610篇 |
2001年 | 1291篇 |
2000年 | 1126篇 |
1999年 | 1074篇 |
1998年 | 998篇 |
1997年 | 934篇 |
1996年 | 836篇 |
1995年 | 784篇 |
1994年 | 760篇 |
1993年 | 699篇 |
1992年 | 686篇 |
1991年 | 623篇 |
1990年 | 547篇 |
1989年 | 496篇 |
1988年 | 493篇 |
1987年 | 410篇 |
1986年 | 421篇 |
1985年 | 642篇 |
1984年 | 689篇 |
1983年 | 388篇 |
1982年 | 590篇 |
1981年 | 547篇 |
1980年 | 455篇 |
1979年 | 340篇 |
1978年 | 246篇 |
1977年 | 256篇 |
1976年 | 219篇 |
1973年 | 187篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Tapas K. Nandi Hridoy R. Bairagya Bishnu P. Mukhopadhyay K. Sekar Dipankar Sukul Asim K. Bera 《Journal of biosciences》2009,34(1):27-34
The role of invariant water molecules in the activity of plant cysteine protease is ubiquitous in nature. On analysing the
11 different Protein DataBank (PDB) structures of plant thiol proteases, the two invariant water molecules W1 and W2 (W220
and W222 in the template 1PPN structure) were observed to form H-bonds with the Ob atom of Asn 175. Extensive energy minimization and molecular dynamics simulation studies up to 2 ns on all the PDB and solvated
structures clearly revealed the involvement of the H-bonding association of the two water molecules in fixing the orientation
of the asparagine residue of the catalytic triad. From this study, it is suggested that H-bonding of the water molecule at
the W1 invariant site better stabilizes the Asn residue at the active site of the catalytic triad. 相似文献
42.
43.
For the first time the expression of C3 and CAM in the leaves of different age of Marrubium frivaldszkyanum Boiss, is reported. With increasing leaf age a typical C3 photosynthesis pattern and high transpiration rate were found. In older leaves a shift to CAM occurred and the 24-h transpiration water loss decreased. A correlation was established between leaf area and accumulation of malate. Water loss at early stages of leaf expansion may be connected with the shift to CAM and the water economy of the whole plant. 相似文献
44.
Peter J. Carrick 《植被学杂志》2003,14(5):761-772
Abstract. The nearest‐neighbour technique is used to infer competition and facilitation between the three most abundant species in a semi‐arid region of western South Africa. Relationships among the shrubs Leipoldtia schultzei and Ruschia robusta, which are leaf‐succulent members of the Mesembryanthemaceae (‘mesembs’) and Hirpicium alienatum a non‐succulent Asteraceae, were compared on two adjacent sites with different histories of browsing intensity. Competition was more prevalent and more important than facilitation. The only evidence for facilitation was found at the heavily‐browsed site where the palatable Hirpicium was larger under the unpalatable Leipoldtia. Generally the prevalence and importance of competition was reduced at the heavily‐browsed site. Strong evidence was obtained for intraspecific competition in each of the three species; also, competition was evident between the two mesembs, where Leipoldtia was competitively dominant over Ruschia, although neither species inhibited Hirpicium. Minimal competition between the mesembs and the asteraceous shrub was interpreted in terms of differentiation in rooting depth, and competition within the mesembs, in terms of overlap in rooting depth. The mesembs had the bulk of their roots in the top 5 cm of soil, while the asteraceous shrub had the bulk of its roots, and all its fine roots, at greater depths. The shallow‐rooted morphology of the mesembs is well adapted to utilize small rainfall events, which occur frequently in the Succulent Karoo, and do not penetrate the soil deeply. Modifications of existing methods are applied for analysing nearest‐neighbour interactions. 相似文献
45.
The crenistria Limestone is a set of three autochthonous massive limestone beds occurring with great lithological persistence in the Kulm Facies (cd III , upper Viséan) of the eastern Rheinisches Schiefergebirge. Microfacies analysis reveals mainly minipeloidal fabrics and homogeneous micrite. Uncrushed, sediment-filled conchs of goniatites represent loci of sheltered preservation of primary carbonate textures. Calcified radiolarians are abundant, forming between 20 and 80% of total rock volume. The alleged algal genus Rectangulina is common in the crenistria Limestone. It is reinterpreted to represent the faeces of goniatites. For the first time the presence of in situ preserved sponges is reported. They can be recognized as delicate networks of microsparitic needles embedded in peloidal fabrics. Hexactinellids with primary spicule arrangements can be found embedded in homogeneous micrite. The carbonate forming the limestone beds was produced microbially during decomposition of soft tissue of the radiolarians and sponges. During the cd III , anoxia in the bottom waters of the Kulm Basin persisted for long periods due to stable density stratification of the water column under humid climatic conditions. Oxic conditions in the bottom waters during formation of the limestone are indicated by bioturbation, the presence of sponges and the high Mn-contents of the carbonate. The latter derived from reduction of Mn-oxides during microbial carbonate formation. 相似文献
46.
Quince (Cydonia oblonga Mill.), pear (Pyrus communis L.) and olive (Olea europaea L.) genotypes were evaluated for their tolerance to iron deficiency stress by growing young plants in three types of aerated nutrient solutions: (1) with iron, (2) without iron or (3) low in iron and with 10 mM bicarbonate. Plants were obtained either from rooted softwood cuttings or from germination of seeds. The degree of tolerance was evaluated with several indices: (1) the chlorophyll content, (2) the root Fe3+ reducing capacity and (3) the whole plant relative growth. Fifteen hours before Fe3+ reducing capacity determination, iron was applied to the roots of plants with iron-stress, since this method resulted in increasing the reductase activity. All quince and pear genotypes increased the root Fe3+ reducing capacity when grown in the treatments for iron-stress, in relation to control plants of the same genotypes. In olive cultivars, the Fe3+ reducing capacity was lower in the iron-stress treatments than in the control one. Studying the relationship between relative growth and chlorophyll content for each genotype under iron-stress, in relation to both indices in control plants, a classification of species and genotypes was established. According to that, most olive cultivars and some pear rootstocks and cultivars appear more iron-efficient than quince rootstocks. Our study shows that in some woody species, determining root Fe3+ reducing capacity is not the best method to establish tolerance to iron deficiency stress. 相似文献
47.
48.
49.
A bacterial clone carrying sequences coding for elongation factor EF-1 alpha from Artemia 总被引:2,自引:0,他引:2
A bacterial cDNA clone was identified carrying one third of the nucleotides coding for elongation factor EF-1 alpha from the brine shrimp Artemia. The sequence of codons corresponds with the known sequence of amino acids of EF-1 alpha in the region involved. 相似文献
50.