全文获取类型
收费全文 | 1911篇 |
免费 | 182篇 |
国内免费 | 225篇 |
专业分类
2318篇 |
出版年
2024年 | 6篇 |
2023年 | 40篇 |
2022年 | 37篇 |
2021年 | 57篇 |
2020年 | 85篇 |
2019年 | 79篇 |
2018年 | 68篇 |
2017年 | 79篇 |
2016年 | 63篇 |
2015年 | 74篇 |
2014年 | 78篇 |
2013年 | 141篇 |
2012年 | 90篇 |
2011年 | 89篇 |
2010年 | 72篇 |
2009年 | 84篇 |
2008年 | 81篇 |
2007年 | 102篇 |
2006年 | 105篇 |
2005年 | 100篇 |
2004年 | 56篇 |
2003年 | 58篇 |
2002年 | 74篇 |
2001年 | 59篇 |
2000年 | 50篇 |
1999年 | 54篇 |
1998年 | 38篇 |
1997年 | 23篇 |
1996年 | 35篇 |
1995年 | 29篇 |
1994年 | 26篇 |
1993年 | 34篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 13篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 20篇 |
1986年 | 10篇 |
1985年 | 25篇 |
1984年 | 19篇 |
1983年 | 5篇 |
1982年 | 13篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1978年 | 14篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1974年 | 4篇 |
排序方式: 共有2318条查询结果,搜索用时 15 毫秒
101.
Laszlo Nagy Jennifer Nagy Colin J. Legg David I. Sales David Horsfield 《Plant Ecology & Diversity》2013,6(2):191-207
Summary A study was made in the Cairngorms, Scotland to make recommendations for a monitoring scheme capable of detecting changes in the vegetation caused by recreational pressure following the development of a funicular railway. Four methods were used in field trials to assess percentage cover of plant species and gravel, rock and bare ground, where appropriate, in two vegetation types (open and closed). The methods used were visual estimates in 50 × 40 cm quadrats (Q), the mean of visual estimates in twenty 10 × 10 cm sub-quadrats of the 50 × 40 cm quadrats (Q20), a modified point intercept method (RL) and photography. Variances between observers and between-quadrats were estimated for the different methods. The sampling design for detecting change was based on a model of variance, constructed from field trial data. Between-observer and between-quadrat variances were related to mean percentage cover and approximated to a binomial distribution. The between-quadrat variance was larger than observer variance. The Q20 method achieved appreciably better precision than the other methods. Analysis of half of the 10 × 10 cmsub-quadrats (1/2Q20) selected in a checker board design achieved a relative efficiency of 78% compared with the Q20. This result suggests that comparable precision to the Q20 method could be achieved by choosing about 14 sub-quadrats in a larger quadrat, thus saving some time. Variation between quadrats also suggested that the Q20 method was the one of choice for maximising precision. The precision of the photographic method was based on fewer data points, so is less accurate than other estimates. Minimum sample sizes were estimated for detecting a 10% relative change of a species in open vegetation with 30% cover (i.e. a change from 30% to <27 or to >33% cover). With a 10 % Type II error rate and 5 % Type I error rate the minimum sample sizes were 47 quadrats for Q, 18 for Q 20, 43 for RL, and 23 for the means of ten 10 × 10 cm sub-quadrats in open vegetation. The most time-efficient field recording appeared to be the use of Q despite the required sample size being 2.6 times higher than that of Q20. The far lower time requirement per quadrat, however, compensated for the higher numbers. The number of quadrats would depend on the specified change in percentage cover and on the statistical significance level used. For example, to detect a 10% absolute change in cover (i.e. from 30% to either <20 % or >40 % cover) at 95 % probability the net effective recording time is estimated at 5 h per vegetation type while to detect a 5 % change at 99 % probability would require c. 25 h. Larger samples may be required for other species or for species with a low initial cover. 相似文献
102.
Vera Y. Chung Rebecca Konietzny Philip Charles Benedikt Kessler Roman Fischer Benjamin W. Turney 《Fly》2016,10(2):91-100
Kidney stone disease is a major health burden with a complex and poorly understood pathophysiology. Drosophila Malpighian tubules have been shown to resemble human renal tubules in their physiological function. Herein, we have used Drosophila as a model to study the proteomic response to crystal formation induced by dietary manipulation in Malpighian tubules. Wild-type male flies were reared in parallel groups on standard medium supplemented with lithogenic agents: control, Sodium Oxalate (NaOx) and Ethylene Glycol (EG). Malpighian tubules were dissected after 2 weeks to visualize crystals with polarized light microscopy. The parallel group was dissected for protein extraction. A new method of Gel Assisted Sample Preparation (GASP) was used for protein extraction. Differentially abundant proteins (p<0.05) were identified by label-free quantitative proteomic analysis in flies fed with NaOx and EG diet compared with control. Their molecular functions were further screened for transmembrane ion transporter, calcium or zinc ion binder. Among these, 11 candidate proteins were shortlisted in NaOx diet and 16 proteins in EG diet. We concluded that GASP is a proteomic sample preparation method that can be applied to individual Drosophila Malpighian tubules. Our results may further increase the understanding of the pathophysiology of human kidney stone disease. 相似文献
103.
Cassandra S. James Stephen J. Mackay Angela H. Arthington Samantha J. Capon Anna Barnes Ben Pearson 《Ecology and evolution》2016,6(16):5950-5963
The primary objective of this study was to test the relevance of hydrological classification and class differences to the characteristics of woody riparian vegetation in a subtropical landscape in Queensland, Australia. We followed classification procedures of the environmental flow framework ELOHA – Ecological Limits of Hydrologic Alteration. Riparian surveys at 44 sites distributed across five flow classes recorded 191 woody riparian species and 15, 500 individuals. There were differences among flow classes for riparian species richness, total abundance, and abundance of regenerating native trees and shrubs. There were also significant class differences in the occurrence of three common tree species, and 21 indicator species (mostly native taxa) further distinguished the vegetation characteristics of each flow class. We investigated the influence of key drivers of riparian vegetation structure (climate, depth to water table, stream‐specific power, substrate type, degree of hydrologic alteration, and land use) on riparian vegetation. Patterns were explained largely by climate, particularly annual rainfall and temperature. Strong covarying drivers (hydrology and climate) prevented us from isolating the independent influences of these drivers on riparian assemblage structure. The prevalence of species considered typically rheophytic in some flow classes implies a more substantial role for flow in these classes but needs further testing. No relationships were found between land use and riparian vegetation composition and structure. This study demonstrates the relevance of flow classification to the structure of riparian vegetation in a subtropical landscape, and the influence of covarying drivers on riparian patterns. Management of environmental flows to influence riparian vegetation assemblages would likely have most potential in sites dominated by rheophytic species where hydrological influences override other controls. In contrast, where vegetation assemblages are dominated by a diverse array of typical rainforest species, and other factors including broad‐scale climatic gradients and topographic variables have greater influence than hydrology, riparian vegetation is likely to be less responsive to environmental flow management. 相似文献
104.
105.
106.
107.
108.
109.
110.
Md. Dilshad Manzar Wassilatul Zannat Jamal Ali Moiz David Warren Spence Seithikurippu R. Pandi-Perumal Ahmed S. Bahammam 《Biological Rhythm Research》2016,47(6):851-864
The Pittsburgh Sleep Quality Index (PSQI) is a rigorously validated questionnaire with extensive use in sleep assessment. Findings from numerous factor analytic studies of the PSQI have been interpreted to support a heterogeneous factor structure model for the test. Nevertheless, the literature continues to lack a focused evaluation of whether this heterogeneous factor structure is justified. A consideration of this issue led to a conclusion that a closer analysis of the PSQI’s factor structure was merited. To address this need a comparative confirmatory factor analysis for assessing the performance of the accepted factors models of the PSQI was conducted. A sample of university students (n = 418), age = 20.92 ± 1.81 years, BMI = 23.30 ± 2.57 kg/m2 completed the multi-structured sleep survey at Jamia Millia Islamia, New Delhi, India. Seventeen putative factor structures (three 1-Factor, eight 2-Factor, and six 3-Factor) of the PSQI from the existing literature were selected for analysis. Fourteen models (82.35%) had almost similar values for model fit indices. Two models were misfits, and one model was a poor fit. The two misfit models incorporated gender and age as covariates. The third poor fit model was used to produce a unique path diagram, which made it distinct from the remaining 16 models. The overlapping values in the fit range of the model fit indices did not support the often projected heterogeneous factor structures of the PSQI for the vast majority of the models. 相似文献