首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   105768篇
  免费   1291篇
  国内免费   1684篇
  108743篇
  2024年   29篇
  2023年   137篇
  2022年   332篇
  2021年   600篇
  2020年   368篇
  2019年   483篇
  2018年   12167篇
  2017年   10913篇
  2016年   7871篇
  2015年   1399篇
  2014年   1147篇
  2013年   1326篇
  2012年   5369篇
  2011年   13719篇
  2010年   12542篇
  2009年   8732篇
  2008年   10491篇
  2007年   11906篇
  2006年   856篇
  2005年   984篇
  2004年   1363篇
  2003年   1354篇
  2002年   1059篇
  2001年   522篇
  2000年   380篇
  1999年   238篇
  1998年   162篇
  1997年   147篇
  1996年   144篇
  1995年   129篇
  1994年   121篇
  1993年   123篇
  1992年   154篇
  1991年   154篇
  1990年   88篇
  1989年   109篇
  1988年   77篇
  1987年   61篇
  1986年   58篇
  1985年   64篇
  1984年   25篇
  1983年   41篇
  1982年   30篇
  1980年   26篇
  1979年   20篇
  1977年   19篇
  1972年   259篇
  1971年   278篇
  1962年   24篇
  1944年   16篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Parent–offspring attachment is important for animals which have offspring that require parental care for their development. Infant attachment to the mother has been examined in macaques, but it remains poorly understood in common marmosets. Here, we examined the abilities of 14 common marmoset infants to show preference for their parents over adults from another group at the ages of 4, 10, and 15 weeks. Each infant was exposed to its parent and an adult from another group in an I-shaped maze. Although 4-week-old infants did not show a significant difference between approach behaviors toward their parents and other adults, 10- and 15-week-old infants approached and stayed longer near their parents than adults from another group. These results suggest selective approach behavior develops in marmosets by the age of 10 weeks.  相似文献   
192.
We investigated replicated differentiation in four lineages of livebearing fishes (two with the genus Poecilia and two within Gambusia), which inhabit freshwater habitats and have also colonized sulfide springs in Oklahoma, Mexico, and Venezuela. Sulfide springs are characterized by extreme hypoxia and high concentrations of toxic hydrogen sulfide, which provide a strong source of divergent selection compared to adjacent non-sulfidic habitats. Using geometric morphometric analysis of body shape, we found that sulfide spring populations significantly differ from relatives from regular freshwater habitats in all four lineages investigated. Differentiation is characterized by strong patterns of convergent evolution, with sulfide spring populations consistently exhibiting an increase in head size, even though the magnitude and nature differences varied across lineages. Head size is strongly correlated with an increase in gill size in sulfide spring populations of the genus Poecilia, which facilitates efficient oxygen acquisition in the hypoxic H2S-environment and directly affects survival. The convergent patterns of differentiation support previous findings about the effects of sulfide on trait evolution.  相似文献   
193.
Examination of herbarium specimens and field studies showed that an unnamed taxon collected from Malmesbury, Western Cape for phylogenetic studies in the Campanulaceae, is a distinct species within the monotypic genus Treichelia. This species previously described as Wahlenbergia depressa Wolley-Dod is resurrected as Treichelia dodii to replace a later homonym. The differences between Treichelia longibracteata (H. Buek) Vatke and W. depressa are explained. A key to the species and a distribution map are provided.  相似文献   
194.
Silvio Fici 《Kew Bulletin》2011,66(2):299-301
A new species of Capparis, C. tchaourembensis Fici, is described and illustrated. Its conservation status is assessed and its affinities with related taxa are discussed.  相似文献   
195.
This work aims to examine the global behavior of a Gause type predator–prey model considering two aspects: (i) the functional response is Holling type III and, (ii) the prey growth is affected by the Allee effect. We prove the origin of the system is an attractor equilibrium point for all parameter values. It has also been shown that it is the ω-limit of a wide set of trajectories of the system, due to the existence of a separatrix curve determined by the stable manifold of the equilibrium point (m,0), which is associated to the Allee effect on prey. When a weak Allee effect on the prey is assumed, an important result is obtained, involving the existence of two limit cycles surrounding a unique positive equilibrium point: the innermost cycle is unstable and the outermost stable. This property, not yet reported in models considering a sigmoid functional response, is an important aspect for ecologists to acknowledge as regards the kind of tristability shown here: (1) the origin; (2) an interior equilibrium; and (3) a limit cycle of large amplitude. These models have undoubtedly been rather sensitive to disturbances and require careful management in applied conservation and renewable resource contexts.  相似文献   
196.
β-Pinene, an oxygenated monoterpene, is one of the major monoterpenes emitted into the atmosphere from forest areas and trees. Besides, it is a principal component of essential oils of a number of aromatic plants, which are involved in a variety of ecological interactions, including allelopathy, in the natural environment. However, studies pertaining to phytotoxicity and biochemical effect(s) of β-pinene are largely lacking. We investigated the effect of β-pinene (0.02, 0.04, 0.08, 0.20, 0.40 and 0.80 mg/ml) in a dose- and time-dependent manner on early seedling growth, dry weight accumulation, photosynthetic pigments and changes in macromolecule (protein and carbohydrate) content and activities of enzymes—proteases, α- and β-amylases, polyphenol oxidases and peroxidases- in rice (Oryza sativa) after 3rd, 5th and 7th day of exposure. β-pinene (≥0.04 mg/ml) significantly reduced the root (by 13–87%) and coleoptile (by 5–80%) length of rice. Exposure to β-pinene reduced total chlorophyll content in rice coleoptiles suggesting a negative impact on photosynthesis. The content of macromolecules (proteins and carbohydrates) enhanced significantly in response to β-pinene, whereas the activities of hydrolyzing enzymes—proteases, α-amylases, and β-amylases—declined (by 30–85, 26–84, 27–74%, respectively) in β-pinene-exposed seedlings. In contrast, the activities of peroxidases (POX) and polyphenol oxidases (PPO) enhanced significantly (by 16–152 and 53–290%, respectively) in rice roots in response to β-pinene in a dose- and time-dependent manner. Increased activities of POX and PPO indicate their involvement in providing protection and/or conferring resistance against β-pinene-induced stress. The study concludes that β-pinene inhibits the early growth of rice by altering the plant biochemical status and enhancing activities of POXs and PPOs involved in general plant defense.  相似文献   
197.
Selenium-containing amino acids, selenocystine (CysSeSeCys), methylselenocysteine (MeSeCys), and selenomethionine (SeMet) have been examined for anti-hemolytic and peroxyl radical scavenging ability. Effect of these compounds on membrane lipid peroxidation, release of hemoglobin, and loss of intracellular K+ ion as a consequence of peroxyl radicals-induced oxidation of human red blood cells were used to evaluate their anti-hemolytic ability. The peroxyl radicals were generated from thermal degradation of 2,2′-azobis(2-methylpropionamidine) dihydrochloride. Significant delay (t eff) was observed in oxidative damage in the presence of the selenium compounds. From the IC50 values for the inhibition of hemolysis, lipid peroxidation, and K+ ion leakage, the relative anti-hemolytic ability of the compounds were found to be in the order of CysSeSeCys > MeSeCys > SeMet. The anti-hemolytic abilities of the compounds, when compared with sodium selenite (Na2SeO3) under identical experimental conditions, were found to be better than Na2SeO3. Relative rate constants estimated for the reaction of MeSeCys and SeMet with peroxyl radicals by competition kinetics using ABTS2− as a reference confirmed that all the compounds are efficient peroxyl radical scavengers. Comparison of the GPx-like activity of these compounds, by NADPH–GSH reductase coupled assay, indicated that CysSeSeCys exhibits the highest activity. Based on these results, it is concluded that among the compounds examined, CysSeSeCys, possessing the ability to reduce peroxyl radicals and hydroperoxides showed efficient anti-hemolytic activity.  相似文献   
198.
199.
A high concentration of vanadium (V) has toxic effects on human and animals and is one of environmental pollutants. In the present study, we have conducted a fundamental study using cultured Vero cells from monkey kidney for the future environmental monitoring. Orthovanadate (VAN), one of V compounds, of 10−10 and 10−8 M did not affect the cell growth although the higher concentration of above 10−6 M VAN inhibited the cell growth accompanied with the decrease in cell numbers and morphological changes. Given that the washing method with ice-cold Li is also effective for determination of the cellular Na content, we used this method for the determination of the V content of the Vero cells. The V distributions in Vero cell; in the 10−3 M VAN solution, extracellular and intracellular were obtained as 1:0.564:0.036 and 1:0.662:0.098 at 60 and 120 min after the treatment of VAN. The intracellular V content was 10% of the applied concentration of VAN. Consequently, it was suggested that V concentration of 10−7 and 10−6 M in the tissue and environment, respectively, might become the threshold concentration; a criterion of the environmental contamination when we carry out environmental monitoring.  相似文献   
200.
Numerous studies have concluded that primates move about their environments in a nonrandom manner, frequently traveling between consecutive foraging sites along relatively straight-line paths. However, primates do not always take the most direct path between resources, and a number of species have been observed to travel repeatedly along a network of the same arboreal pathways. In this study, I used spatially explicit techniques to examine quantitatively what mantled howler monkey groups on Barro Colorado Island, Panama, accomplish by selecting nonlinear paths between resources and by repeatedly using the same paths within an arboreal network. Results show that chosen arboreal paths between sites where foraging occurred have higher levels of resource availability and canopy connectivity than comparable straight-line paths between the same sites. When comparing the relative importance of these factors, autologistic models of pathway choice indicate that though canopy connectivity is related to the location of repeatedly used arboreal pathway networks, the most statistically significant predictor is resource availability (both on a path and within a visual detection distance of a path). These results provide support for the hypothesis that repeated use of arboreal pathway networks aids in resource monitoring and acquisition. In addition, statistical models developed from 1 primary focal group’s travel patterns had high predictive value when employed to generate likely locations for arboreal pathways in the home ranges of 3 neighboring groups. This finding has important implications for studies of primate habitat use and seed dispersal, as it suggests that different groups consistently use similar characteristics when deciding on travel paths.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号