全文获取类型
收费全文 | 5972篇 |
免费 | 602篇 |
国内免费 | 295篇 |
专业分类
6869篇 |
出版年
2024年 | 20篇 |
2023年 | 94篇 |
2022年 | 91篇 |
2021年 | 135篇 |
2020年 | 182篇 |
2019年 | 263篇 |
2018年 | 213篇 |
2017年 | 206篇 |
2016年 | 210篇 |
2015年 | 220篇 |
2014年 | 299篇 |
2013年 | 380篇 |
2012年 | 221篇 |
2011年 | 231篇 |
2010年 | 195篇 |
2009年 | 285篇 |
2008年 | 295篇 |
2007年 | 320篇 |
2006年 | 298篇 |
2005年 | 271篇 |
2004年 | 232篇 |
2003年 | 211篇 |
2002年 | 194篇 |
2001年 | 157篇 |
2000年 | 154篇 |
1999年 | 126篇 |
1998年 | 115篇 |
1997年 | 97篇 |
1996年 | 95篇 |
1995年 | 84篇 |
1994年 | 77篇 |
1993年 | 88篇 |
1992年 | 87篇 |
1991年 | 96篇 |
1990年 | 68篇 |
1989年 | 62篇 |
1988年 | 56篇 |
1987年 | 38篇 |
1986年 | 53篇 |
1985年 | 66篇 |
1984年 | 54篇 |
1983年 | 37篇 |
1982年 | 45篇 |
1981年 | 37篇 |
1980年 | 29篇 |
1979年 | 21篇 |
1978年 | 15篇 |
1976年 | 12篇 |
1973年 | 10篇 |
1972年 | 7篇 |
排序方式: 共有6869条查询结果,搜索用时 15 毫秒
991.
The influence of salt and nitrogen on herbivore abundance: direct and indirect effects 总被引:4,自引:0,他引:4
We report on the influence of experimentally increased interstitial salinity and plant nitrogen on the abundance of the delphacid
planthopper, Prokelisia marginata (Van Duzee) (Homoptera: Delphacidae), which feeds on salt marsh cordgrass, Spartina alterniflora. We also report the effects of these treatments on parasitism of P. marginata eggs by the fairyfly parasitoid, Anagrus sophiae (Hymenoptera: Mymaridae). Soil salinity was significantly elevated following the addition of salt pellets broadcast over
the ground and plant foliar nitrogen was significantly increased after the addition of fertilizer. The addition of fertilizer
increased P. marginata densities on Spartina but addition of salt did not. Neither treatment significantly affected levels of egg parasitism by A. sophiae. In this system direct effects of plants on their herbivores via changes in plant chemistry appear more important than indirect
effects of plants on herbivores via their natural enemies.
Received: 1 August 1997 / Accepted: 29 September 1997 相似文献
992.
Animal, fungal, and bacterial consumers can have dramatic effects on the structure of plant communities, often by consuming
dominant competitors and indirectly increasing the abundance of inferior competitors. We investigated the role of a consumer
plant, the parasite Cuscuta salina, on plant zonation in a western salt marsh. Cuscuta had a strong host species preference in experiments, disproportionally infecting Salicornia virginica, the dominant competitor in most of the marsh. In plots with Cuscuta, which infected 18% of our study area over a 3-year period, Salicornia cover decreased and the cover of Arthrocnemum increased substantially in comparison to plots without Cuscuta. Deep in the Salicornia zone, the cover of Arthrocnemum in Cuscuta-infected plots increased by 558% in 1 year relative to uninfected plots. At the ecotone, the cover of Arthrocnemum in Cuscuta-infected plots increased by only 41% during the same time interval. These data suggest that the relative benefit of a consumer
to a less-preferred, subordinate competitor may be strongest where competition is the most asymmetrical as predicted by recent
theoretical models. By weakening the competitive dominant, which in the absence of the parasite can create virtual monocultures,
Cuscuta enhanced community diversity and altered the ecotone between Salicornia and Arthrocnemum. Cuscuta patches were highly dynamic at the ecotone between Salicornia and Arthrocnemum, and thus the changes we measured in our sample plots were likely to be representative of large portions of the marsh. Our
findings emphasize the importance of trophic interactions in salt marsh structure and zonation.
Received: 23 April 1997 / Accepted: 10 October 1997 相似文献
993.
A microsporidan isolate currently considered to represent a mixed infection of Nosema necatrix Kramer, 1965 and Thelohania diazoma Kramer, 1965 was subjected to cultivation in hosts held at various temperatures. The ratio of the Nosema (monospore) to the Thelohania (octospore) forms at these temperatures was found to vary from 1:1 at 16 C to 1:0 at 32 C. Isolation technics using mechanical, temperature and temporal methods separated monosporous from octosporous forms for inoculation purposes. However, microscopic examination of hosts receiving these inocula revealed the presence of both monospores and octospores. Electrophoretic analysis of monospores and monospore-octospore mixtures indicated equivalent hydrophobic protein spectra. These observations suggest that this isolate has the ability to produce either single spores or spores in groups of eight. This microsporidan was not considered a member of the genus Stempellia since spores in groups of 2 or 4 were not observed. Retention of the name Nosema necatrix Kramer is suggested. 相似文献
994.
Additional volatile compounds were isolated from muskmelon fruit by means of a water recycling apparatus, separated by GLC, and identified principally by MS and GLC retention data. Compounds reported for the first time as melon components are: n-hexanol, 1-octen-3-ol, cis-3-nonen-1-ol, n-butyl acetate, isobutyl acetate, 2-methylbutyl acetate, n-hexyl acetate, ethyl n-butyrate, ethyl 2-methylbutyrate, benzyl acetate, β-phenethyl acetate, and γ-phenylpropyl acetate. Muskmelon fruit stored frozen prior to steam distillation-extraction yielded an essence which, when compared with that obtained from freshly harvested fruit, contained considerably larger amounts of trans-2-nonenal, n-nonanol, cis-3-nonen-1-ol, cis-6-nonen-1-ol, and the methyl and ethyl esters of linoleic and linolenic acids. Marked decreases in the relative amounts of benzyl acetate, β-phenethyl acetate, and γ-phenylpropyl acetate resulted from freezing. All 21 compounds examined were present in the essences prepared from fresh, refrigerated, and frozen fruit. 相似文献
995.
996.
Age and growth of the endemic Xingu River stingray Potamotrygon leopoldi validated using fluorescent dyes
下载免费PDF全文
![点击此处可从《Journal of fish biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Between 2003 and 2005, vertebrae of 151 Xingu River Potamotrygon leopoldi (Potamotrygonidae) (75 males and 76 females) were analysed to derive a growth curve for this species. The disc width (W D) was significantly different between sexes, with females measuring 149–700 mm W D and males 109–500 mm W D. The average percentage error for vertebrae readings of the whole sample was 2·7%. The marginal increment ratio (R MI) showed an increasing trend with the highest value in November, decreasing from December on. The majority of vertebrae displaying R MI zero, occurred in September, but the annual periodicity of ring deposition throughout the year was not conclusive. Tetracycline (TCN) injected specimens were held in captivity for 13 months and displayed a fluorescent mark in vertebrae confirming a yearly periodicity of band pair formation with the translucent ring deposited in September–October. The Akaike information criterion (AIC) showed that, among the seven models considered, the best fit was obtained for the von Bertalanffy modified with W 0 (where W 0 = W D at birth) for both sexes. Growth parameters for females were: W 0 = 149 mm; W ∞ = 763·06 mm; k = 0·12 year– 1, whereas for males: W 0 = 109 mm; W ∞ = 536·4 and k = 0·22 year?1. Maximal ages were 7·2 years in males and 14·3 years in females. The species shows sexual dimorphism expressed in the growth pattern, size at maturity, longevity and asymptotic sizes. Concern for sustainability is raised due to the construction of the Belo Monte Hydroelectric Power Plant (2015 and 2016) in the State of Pará causing changes to the habitat of this species, which is endemic to the Xingu River and two of its tributaries. 相似文献
997.
A series of pterostilbene β-amino alcohol derivatives were designed, synthesized and evaluated as multifunctional agents for the treatment of Alzheimer’s disease (AD). In vitro assays demonstrated that most of the derivatives were selective acetylacholinesterase (AChE) inhibitors with moderate multifunctional properties. Among them, compound 5f exhibited the best inhibitory activity for EeAChE (IC50 = 24.04 μM), that was better than pterostilbene under our experimental condition. In addition, compound 5f displayed reasonable antioxidant activity and could confer significant neuroprotective effect against H2O2-induced PC-12 cell injury. Moreover, 5f also showed self-induced Aβ1-42 aggregation inhibitory potency and displayed high BBB permeability in vitro. These multifunctional properties highlight 5f as a promising candidate for further studies directed to the development of novel drugs against AD. 相似文献
998.
Telomere length reveals cumulative individual and transgenerational inbreeding effects in a passerine bird
下载免费PDF全文
![点击此处可从《Molecular ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Kat Bebbington Lewis G. Spurgin Eleanor A. Fairfield Hannah L. Dugdale Jan Komdeur Terry Burke David S. Richardson 《Molecular ecology》2016,25(12):2949-2960
Inbreeding results in more homozygous offspring that should suffer reduced fitness, but it can be difficult to quantify these costs for several reasons. First, inbreeding depression may vary with ecological or physiological stress and only be detectable over long time periods. Second, parental homozygosity may indirectly affect offspring fitness, thus confounding analyses that consider offspring homozygosity alone. Finally, measurement of inbreeding coefficients, survival and reproductive success may often be too crude to detect inbreeding costs in wild populations. Telomere length provides a more precise measure of somatic costs, predicts survival in many species and should reflect differences in somatic condition that result from varying ability to cope with environmental stressors. We studied relative telomere length in a wild population of Seychelles warblers (Acrocephalus sechellensis) to assess the lifelong relationship between individual homozygosity, which reflects genome‐wide inbreeding in this species, and telomere length. In juveniles, individual homozygosity was negatively associated with telomere length in poor seasons. In adults, individual homozygosity was consistently negatively related to telomere length, suggesting the accumulation of inbreeding depression during life. Maternal homozygosity also negatively predicted offspring telomere length. Our results show that somatic inbreeding costs are environmentally dependent at certain life stages but may accumulate throughout life. 相似文献
999.
Isabelle M. C?té Emily S. Darling Christopher J. Brown 《Proceedings. Biological sciences / The Royal Society》2016,283(1824)
Interactions between multiple ecosystem stressors are expected to jeopardize biological processes, functions and biodiversity. The scientific community has declared stressor interactions—notably synergies—a key issue for conservation and management. Here, we review ecological literature over the past four decades to evaluate trends in the reporting of ecological interactions (synergies, antagonisms and additive effects) and highlight the implications and importance to conservation. Despite increasing popularity, and ever-finer terminologies, we find that synergies are (still) not the most prevalent type of interaction, and that conservation practitioners need to appreciate and manage for all interaction outcomes, including antagonistic and additive effects. However, it will not be possible to identify the effect of every interaction on every organism''s physiology and every ecosystem function because the number of stressors, and their potential interactions, are growing rapidly. Predicting the type of interactions may be possible in the near-future, using meta-analyses, conservation-oriented experiments and adaptive monitoring. Pending a general framework for predicting interactions, conservation management should enact interventions that are robust to uncertainty in interaction type and that continue to bolster biological resilience in a stressful world. 相似文献
1000.
Omid Abazari Zahra Shafaei Mahbubeh Eslami-Moghadam Behafarid Ghalandari Ali Akbar Saboury 《Journal of biomolecular structure & dynamics》2016,34(5):1123-1131
In recent years, using heavy metal compounds such as platinum as anticancer agent is one of the common ways in chemical therapy. In this study, a new anticancer compound of glycine derivatives of Pt(II) complex (amyl-glycine1, 10-phenanthroline Platinum nitrate) was designed, and the biological effects of this novel compound on the alterations in the function and structure of human hemoglobin (Hb) at different temperatures of 25 and 37°C were assessed by applying various spectroscopic (fluorescence and circular dichroism (CD)) and theoretical methods. Fluorescence data indicated the strong ability of Pt(II) complex to quench the intrinsic fluorescence of Hb. The binding constant, number of binding sites, and thermodynamic parameters at two temperatures were calculated, and the results indicated the major possibility of occurring van der Waals force or hydrogen bond interactions in the Pt(II) complex–Hb interaction. For evaluating the alteration of secondary structure of Hb upon interaction with various concentrations of complex, far-UV CD spectra were used and it was observed that in high dose of complex, significant changes were occurred which is indicative of some side effects in overdosing of this complex. On the other hand, the molecular docking results illustrate that are well in agreement in obtaining data with spectroscopy. Above results suggested that using Pt(II) complex as an anticancer agent, model drug in high-dose usage might cause some disordering in structure and function of Hb as well as improve understanding of the side effects of newly designed metal anticancer drugs undergoing. 相似文献