全文获取类型
收费全文 | 83671篇 |
免费 | 3606篇 |
国内免费 | 4569篇 |
出版年
2023年 | 727篇 |
2022年 | 904篇 |
2021年 | 2115篇 |
2020年 | 1463篇 |
2019年 | 1850篇 |
2018年 | 1568篇 |
2017年 | 1250篇 |
2016年 | 1858篇 |
2015年 | 3711篇 |
2014年 | 7123篇 |
2013年 | 6757篇 |
2012年 | 5581篇 |
2011年 | 6190篇 |
2010年 | 4466篇 |
2009年 | 4168篇 |
2008年 | 4374篇 |
2007年 | 4701篇 |
2006年 | 3350篇 |
2005年 | 2991篇 |
2004年 | 2136篇 |
2003年 | 1906篇 |
2002年 | 1854篇 |
2001年 | 1373篇 |
2000年 | 1181篇 |
1999年 | 1113篇 |
1998年 | 972篇 |
1997年 | 787篇 |
1996年 | 863篇 |
1995年 | 1002篇 |
1994年 | 896篇 |
1993年 | 838篇 |
1992年 | 779篇 |
1991年 | 791篇 |
1990年 | 670篇 |
1989年 | 600篇 |
1988年 | 610篇 |
1987年 | 477篇 |
1986年 | 445篇 |
1985年 | 675篇 |
1984年 | 970篇 |
1983年 | 621篇 |
1982年 | 831篇 |
1981年 | 813篇 |
1980年 | 598篇 |
1979年 | 581篇 |
1978年 | 388篇 |
1977年 | 391篇 |
1976年 | 346篇 |
1974年 | 258篇 |
1973年 | 268篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
1.
2.
3.
4.
5.
Previous researches mainly focused on the runoff responses to landuse change based on annual, seasonal or monthly time scales, there are few studies based on daily scale. We conducted a comprehensive investigation into runoff responses on the daily scale as well as annual and monthly time scales using SWAT, and compared the impacts of time scales with different time indicators quantitatively. Jinjiang, a coastal catchment of southeast China with a humid sub-tropical climate, was used for simulations. A calibrated SWAT model produced satisfactory reproduction of annual, monthly and daily runoff processes over a nine-year (2002–2010) period at three gauging stations. Runoff was then simulated and compared using the same meteorological input but two different landuse scenarios (1985 and 2006, with reduced forest and increased cropland and urbanized area). The results showed varying change in runoff among three time scales and three catchments. The annual runoff had the smallest increase between two scenarios, monthly runoffs had medium rates (increasing in all months except October–November), and daily runoff had the largest rates with the increase in flood peaks but decrease in drought flows, because of the variable influence on interception/evapotranspiration loss, percolation and antecedent soil water storage. Indicators of different time scales (annual runoff, monthly runoff, maximum 1-day and 5-day flood runoff, minimum 1-day and 7-day runoff) proved appropriate for analysing landuse change impacts. 相似文献
6.
7.
Understanding the state of forest resource utilization in China and correctly evaluating the role and function of forest resources in national economic development are essential for realizing balanced development of forests, the environment, and the economy. This is especially true given the present situation of increasingly scarce forest resources. Using data from Chinese forest industry statistical yearbooks, forestry development reports, and other documents, this paper examines the current state of forest resource utilization in China from the angle of combining quantities and values based on input–output tables. We show that demand for and input use of forest resources varies greatly across industrial sectors; the paper products and furniture manufacturing industries have the greatest direct consumption coefficient for timber use. When considering direct and indirect demand, it is clear that forest resources restrict different industrial sectors in diverse ways. These results provide an important set of reference values regarding the utilization of forest resources and coordinated industrial development in China. 相似文献
8.
Maria Weinert Tharakeswari Selvakumar Travis S. Tierney Kambiz N. Alavian 《Journal of visualized experiments : JoVE》2015,(96)
Degeneration of mesencephalic dopaminergic (mesDA) neurons is the pathological hallmark of Parkinson’s diseae. Study of the biological processes involved in physiological functions and vulnerability and death of these neurons is imparative to understanding the underlying causes and unraveling the cure for this common neurodegenerative disorder. Primary cultures of mesDA neurons provide a tool for investigation of the molecular, biochemical and electrophysiological properties, in order to understand the development, long-term survival and degeneration of these neurons during the course of disease. Here we present a detailed method for the isolation, culturing and maintenance of midbrain dopaminergic neurons from E12.5 mouse (or E14.5 rat) embryos. Optimized cell culture conditions in this protocol result in presence of axonal and dendritic projections, synaptic connections and other neuronal morphological properties, which make the cultures suitable for study of the physiological, cell biological and molecular characteristics of this neuronal population. 相似文献
9.
An essential part in the development of informative linkage maps is to include genetic markers that have been anchored by physical mapping. Here a set of 18 porcine cosmid-derived genetic markers are reported that have been mapped by linkge analysis, and that also have been physically localized by fluorescence in situ hybridization (FISH). Three different strategies were used to establish polymorphic markers from the cosmid clones. Firstly, dinucleotide microsatellite loci were derived by sequencing cosmid subclones containing (CA), repeats. Secondly, variable SINE 3′ poly(A) tracts (SINEVA) were identified by direct SINE-PCR amplification of cosmid clones. Thirdly, the cosmids were used in Southern blot hybridization to detect restriction fragment length polymorphisms (RFLPs). Compared with the most recent consensus compilation of the porcine gene map, the present assignment of markers to chromosomes Zp, 3, 4, 10, 12q, and 16 represents the first loci mapped to these chromosomes, for which linkage as well as in situ data are now available. 相似文献
10.
Abstract. Anopheles stephensi mosquitoes which had fed upon mice infected with Plasmodium yoelii nigeriensis malaria parasites produced significantly fewer eggs than mosquitoes fed on an uninfected mouse. Fecundity reduction was more pronounced when the bloodmeal contained malaria gametocytes and the mosquitoes developed oocysts. Egg production and haematin excretion were correlated for uninfected bloodfed mosquitoes; the presence of P.y. nigeriensis in the blood affected this relationship. Reduced fecundity was associated with a significant reduction of bloodmeal size (measured by haematin excretion) in mosquitoes which ingested gametocytaemic blood. The bloodmeal size in mosquitoes fed on parasitaemic blood without gametocytes was not significantly reduced. The use of haematin assays for determination of bloodmeal size in mosquitoes is discussed. 相似文献