全文获取类型
收费全文 | 9509篇 |
免费 | 722篇 |
出版年
2023年 | 94篇 |
2022年 | 133篇 |
2021年 | 265篇 |
2020年 | 143篇 |
2019年 | 188篇 |
2018年 | 231篇 |
2017年 | 222篇 |
2016年 | 333篇 |
2015年 | 511篇 |
2014年 | 565篇 |
2013年 | 732篇 |
2012年 | 819篇 |
2011年 | 789篇 |
2010年 | 456篇 |
2009年 | 470篇 |
2008年 | 562篇 |
2007年 | 497篇 |
2006年 | 471篇 |
2005年 | 437篇 |
2004年 | 403篇 |
2003年 | 362篇 |
2002年 | 339篇 |
2001年 | 108篇 |
2000年 | 72篇 |
1999年 | 91篇 |
1998年 | 100篇 |
1997年 | 83篇 |
1996年 | 67篇 |
1995年 | 52篇 |
1994年 | 51篇 |
1993年 | 53篇 |
1992年 | 43篇 |
1991年 | 30篇 |
1990年 | 33篇 |
1989年 | 30篇 |
1988年 | 24篇 |
1987年 | 33篇 |
1986年 | 15篇 |
1985年 | 32篇 |
1984年 | 35篇 |
1983年 | 25篇 |
1982年 | 24篇 |
1981年 | 24篇 |
1980年 | 16篇 |
1978年 | 15篇 |
1977年 | 23篇 |
1975年 | 13篇 |
1974年 | 17篇 |
1973年 | 12篇 |
1972年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
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
12.
13.
14.
Esteve Padrs Mireia Duach Antoni Morros Manuel Sabs Joan Maosa 《Trends in biochemical sciences》1984,9(12):508-510
Fourth-derivative spectrophotometry offers several advantages over classical absorption or difference spectrophotometry in examining the characteristics of aromatic amino acids in proteins. The basic principles of the technique and its applications are outlined. 相似文献
15.
16.
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
17.
18.
Javier J Gonzalez-Rosa Manuel Vazquez-Marrufo Encarnacion Vaquero Pablo Duque Monica Borges Carlos M Gomez-Gonzalez Guillermo Izquierdo 《BMC neurology》2011,11(1):64-19
Background
Event-related potentials (ERPs) may be used as a highly sensitive way of detecting subtle degrees of cognitive dysfunction. On the other hand, impairment of cognitive skills is increasingly recognised as a hallmark of patients suffering from multiple sclerosis (MS). We sought to determine the psychophysiological pattern of information processing among MS patients with the relapsing-remitting form of the disease and low physical disability considered as two subtypes: 'typical relapsing-remitting' (RRMS) and 'benign MS' (BMS). Furthermore, we subjected our data to a cluster analysis to determine whether MS patients and healthy controls could be differentiated in terms of their psychophysiological profile. 相似文献19.
20.
Miguel Gandra Jorge Assis Manuel Ramos Martins David Abecasis 《Molecular biology and evolution》2021,38(4):1402
Knowledge on genetic structure is key to understand species connectivity patterns and to define the spatiotemporal scales over which conservation management plans should be designed and implemented. The distribution of genetic diversity (within and among populations) greatly influences species ability to cope and adapt to environmental changes, ultimately determining their long-term resilience to ecological disturbances. Yet, the drivers shaping connectivity and structure in marine fish populations remain elusive, as are the effects of fishing activities on genetic subdivision. To investigate these questions, we conducted a meta-analysis and compiled genetic differentiation data (FST/ΦST estimates) for more than 170 fish species from over 200 published studies globally distributed. We modeled the effects of multiple life-history traits, distance metrics, and methodological factors on observed population differentiation indices and specifically tested whether any signal arising from different exposure to fishing exploitation could be detected. Although the myriad of variables shaping genetic structure makes it challenging to isolate the influence of single drivers, results showed a significant correlation between commercial importance and genetic structure, with widespread lower population differentiation in commercially exploited species. Moreover, models indicate that variables commonly used as proxy for connectivity, such as larval pelagic duration, might be insufficient, and suggest that deep-sea species may disperse further. Overall, these results contribute to the growing body of knowledge on marine genetic connectivity and suggest a potential effect of commercial fisheries on the homogenization of genetic diversity, highlighting the need for additional research focused on dispersal ecology to ensure long-term sustainability of exploited marine species. 相似文献