全文获取类型
收费全文 | 67889篇 |
免费 | 4594篇 |
国内免费 | 17篇 |
专业分类
72500篇 |
出版年
2023年 | 387篇 |
2022年 | 583篇 |
2021年 | 1131篇 |
2020年 | 884篇 |
2019年 | 1099篇 |
2018年 | 1759篇 |
2017年 | 1537篇 |
2016年 | 2178篇 |
2015年 | 2937篇 |
2014年 | 3105篇 |
2013年 | 4038篇 |
2012年 | 4690篇 |
2011年 | 4378篇 |
2010年 | 2867篇 |
2009年 | 2502篇 |
2008年 | 3431篇 |
2007年 | 3346篇 |
2006年 | 3226篇 |
2005年 | 2719篇 |
2004年 | 2716篇 |
2003年 | 2421篇 |
2002年 | 2281篇 |
2001年 | 1736篇 |
2000年 | 1638篇 |
1999年 | 1335篇 |
1998年 | 688篇 |
1997年 | 510篇 |
1996年 | 494篇 |
1995年 | 491篇 |
1994年 | 394篇 |
1993年 | 370篇 |
1992年 | 775篇 |
1991年 | 721篇 |
1990年 | 651篇 |
1989年 | 647篇 |
1988年 | 577篇 |
1987年 | 583篇 |
1986年 | 517篇 |
1985年 | 497篇 |
1984年 | 410篇 |
1983年 | 373篇 |
1982年 | 293篇 |
1979年 | 393篇 |
1978年 | 263篇 |
1975年 | 254篇 |
1974年 | 308篇 |
1973年 | 306篇 |
1972年 | 271篇 |
1970年 | 253篇 |
1969年 | 254篇 |
排序方式: 共有10000条查询结果,搜索用时 73 毫秒
151.
Effect of lead stress on mineral content and growth of wheat (Triticum aestivum) and spinach (Spinacia oleracea) seedlings 总被引:1,自引:0,他引:1
Mostafa Lamhamdi Ouiam El Galiou Ahmed Bakrim Juan Carlos Nóvoa-Muñoz Manuel Arias-Estévez Ahmed Aarab René Lafont 《Saudi Journal of Biological Sciences》2013,20(1):29-36
Lead (Pb) is the most common heavy metal contaminant in the environment. Pb is not an essential element for plants, but they absorb it when it is present in their environment, especially in rural areas when the soil is polluted by automotive exhaust and in fields contaminated with fertilizers containing heavy metal impurities. To investigate lead effects on nutrient uptake and metabolism, two plant species, spinach (Spinacia oleracea) and wheat (Triticum aestivum), were grown under hydroponic conditions and stressed with lead nitrate, Pb(NO3)2, at three concentrations (1.5, 3, and 15 mM).Lead is accumulated in a dose-dependent manner in both plant species, which results in reduced growth and lower uptake of all mineral ions tested. Total amounts and concentrations of most mineral ions (Na, K, Ca, P, Mg, Fe, Cu and Zn) are reduced, although Mn concentrations are increased, as its uptake is reduced less relative to the whole plant’s growth. The deficiency of mineral nutrients correlates in a strong decrease in the contents of chlorophylls a and b and proline in both species, but these effects are less pronounced in spinach than in wheat. By contrast, the effects of lead on soluble proteins differ between species; they are reduced in wheat at all lead concentrations, whereas they are increased in spinach, where their value peaks at 3 mM Pb.The relative lead uptake by spinach and wheat, and the different susceptibility of these two species to lead treatment are discussed. 相似文献
152.
Propriospinal neurones located in the cervical enlargement and projecting bilaterally to sacral segments of the spinal cord were investigated electrophysiologically in eleven deeply anaesthetized cats. Excitatory or inhibitory postsynaptic potentials from forelimb afferents were recorded following stimulation of deep radial (DR), superficial radial (SR), median (Med) and ulnar (Uln) nerves. 26 cells were recorded from C7, 22 from C8 and 3 from Th1 segments. The majority of the cells were located in the Rexed's laminae VIII and the medial part of the lamina VII. In 10 cases no afferent input from the forelimb afferents was found. In the remaining neurones effects were evoked mostly from DR (88%) and Med (63%), less often from SR (46%) and Uln (46%). Inhibitory actions were more frequent than excitatory. The highest number of IPSPs was evoked from high threshold flexor reflex afferents (FRA)--all connections were polysynaptic. However, inhibitory actions were often evoked from group I or II muscle afferents (polysynaptic or disynaptic) and, less frequently, from cutaneous afferents (mostly polysynaptic). Di- or polysynaptic IPSPs often accompanied monosynaptic EPSPs from group I or II muscle afferents. Disynaptic or polysynaptic EPSPs from muscle and cutaneous afferents were also recorded in many neurones, while polysynaptic EPSPs from FRA were observed only exceptionally. Various patterns of convergence in individual neuronal subpopulations indicate that they integrate different types of the afferent input from various muscle and cutaneous receptors of the distal forelimb. They transmit this information to motor centers controlling hind limb muscles, forming a part of the system contributing to the process of coordination of movements of fore--and hind--limbs. 相似文献
153.
154.
155.
M. José Feito Mercedes Jiménez Concha Fernández-Cabrera Germán Rivas Guillermo Giménez-Gallego Rosa M. Lozano 《Analytical biochemistry》2011,411(1):1
Here we describe, for the first time, the design and characterization of a bona fide fluorescently labeled mutant of the human acidic fibroblast growth factor (aFGF). The aFGF–Cys2 mutant was recombinantly synthesized by substituting the second amino acid with a reactive cysteine whose sulfhydryl group’s side chain reactivity facilitated the covalent binding of a fluorescent probe as a thiolyte monobromobimane. Using a combination of biophysical and functional assays, we found that the fluorescently labeled mutant aFGF is characterized by essentially the same global folding, mitogenic activity, and association behavior with heparin, its physiological activator, as the unlabeled wild-type protein. We used this new tracer protein mutant to determine the association behavior of aFGF with heparin in the presence of high concentrations of albumin that mimicked more closely the plasma medium in which aFGF is naturally located and in which it has evolved to function. By exposing the aFGF–Cys2–heparin complex to increasing concentrations of albumin up to physiological plasma levels, we were able to demonstrate that macromolecular crowding does not affect the stoichiometry of the interaction. In summary, the dimeric aFGF–Cys2–heparin complex might represent a biologically relevant complex in physiological media. 相似文献
156.
157.
María Crespo López Carlos Hornillos JerezMónica Bernaldo De Quirós Aragón M. Mar Gómez Gutiérrez 《Revista espa?ola de geriatría y gerontología》2011,46(6):319
Quality of life (QoL) is one of the most important outcome variables in the study of the efficacy of interventions with people with dementia. However, its assessment is difficult 1) because it is a complex construct for which there is no unified theoretical or conceptual approach, and 2) because of the inherent difficulties in the cognitive impairments of the people under study. In this work different methods and instruments to this end are reviewed, and related findings are discussed. It is important to take into account the subjective view of the assessed person, as assessments done by proxies tend to underestimate QoL. In spite of the need for further development in this field, it is concluded that the instrument of choice is the QOL-AD, as it is change-sensitive, it correlates with health measurements, it is translated into several languages and it can be administered to people with low MMSE scores. 相似文献
158.
159.
M. Matabos S. Plouviez S. Hourdez D. Desbruyères P. Legendre A. Warén D. Jollivet E. Thiébaut 《Journal of Biogeography》2011,38(3):575-594
Aim Deep‐sea hydrothermal vents have now been reported along all active mid‐ocean ridges and back‐arc basins, but the boundaries of biogeographic entities remain questionable owing to methodological issues. Here we examine biogeographic patterns of the vent fauna along the East Pacific Rise (EPR) and determine the relative roles of regional and local factors on the distribution of biodiversity associated with mussel beds along a poorly explored zone, the southern EPR (SEPR). Location East Pacific Rise. Methods A species list of macrobenthic invertebrates along the EPR was compiled from the literature and supplemented with data recovered during the French research cruise BIOSPEEDO carried out in 2004 along the SEPR. Biogeographic patterns were assessed by combining the identification of morphological species with a molecular barcoding approach. A multivariate regression tree (MRT) analysis was performed to identify any geographic breaks, and an empirical distribution of species richness was compared with predictions provided by a mid‐domain effect model. Macrofaunal community structure associated with mussel beds along the SEPR was analysed in relation to environmental factors using cluster and canonical redundancy analyses. Results Sequencing of the cytochrome c oxidase subunit I gene revealed the occurrence of several cryptic species complexes along the EPR, with the equator separating the southern and northern clades. Furthermore, during the BIOSPEEDO cruise at least 10 still unnamed species were collected between 7°25′ S and 21°33′ S. The shift in community structure identified by MRT analysis was located south of 17°34′ S or south of 13°59′ S, depending on the data used, suggesting that the southern part of the SEPR (17°25′–21°33′ S) constitutes a biogeographic transition zone in the vent fauna along the EPR. At a regional scale, latitude combined with the type of venting was significantly correlated with the community structure associated with mussel beds. Main conclusions Together, the molecular data, in situ observations, and the distribution of species suggest that the high diversity of vent fauna species presently observed between 17°25′ S and 21°33′ S is probably a result of the overlap of several distinct biogeographic provinces. We argue that this area thus constitutes a biogeographic vent fauna transition zone along the EPR. 相似文献