全文获取类型
收费全文 | 2032篇 |
免费 | 76篇 |
国内免费 | 7篇 |
专业分类
林业 | 126篇 |
农学 | 37篇 |
基础科学 | 16篇 |
314篇 | |
综合类 | 438篇 |
农作物 | 117篇 |
水产渔业 | 182篇 |
畜牧兽医 | 718篇 |
园艺 | 31篇 |
植物保护 | 136篇 |
出版年
2023年 | 10篇 |
2022年 | 16篇 |
2021年 | 27篇 |
2020年 | 40篇 |
2019年 | 37篇 |
2018年 | 43篇 |
2017年 | 42篇 |
2016年 | 46篇 |
2015年 | 28篇 |
2014年 | 51篇 |
2013年 | 98篇 |
2012年 | 93篇 |
2011年 | 107篇 |
2010年 | 60篇 |
2009年 | 63篇 |
2008年 | 102篇 |
2007年 | 119篇 |
2006年 | 102篇 |
2005年 | 142篇 |
2004年 | 126篇 |
2003年 | 135篇 |
2002年 | 158篇 |
2001年 | 23篇 |
2000年 | 25篇 |
1999年 | 24篇 |
1998年 | 20篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 15篇 |
1990年 | 21篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 18篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1977年 | 7篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1969年 | 7篇 |
1968年 | 6篇 |
1967年 | 6篇 |
1961年 | 8篇 |
排序方式: 共有2115条查询结果,搜索用时 0 毫秒
61.
Field DB Baumgartner TR Charles CD Ferreira-Bartrina V Ohman MD 《Science (New York, N.Y.)》2006,311(5757):63-66
It is currently unclear whether observed pelagic ecosystem responses to ocean warming, such as a mid-1970s change in the eastern North Pacific, depart from typical ocean variability. We report variations in planktonic foraminifera from varved sediments off southern California spanning the past 1400 years. Increasing abundances of tropical/subtropical species throughout the 20th century reflect a warming trend superimposed on decadal-scale fluctuations. Decreasing abundances of temperate/subpolar species in the late 20th century indicate a deep, penetrative warming not observed in previous centuries. These results imply that 20th-century warming, apparently anthropogenic, has already affected lower trophic levels of the California Current. 相似文献
62.
Steve Cherry Gary C. White Kim A. Keating Mark A. Haroldson Charles C. Schwartz 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(2):195-215
Current management of the grizzly bear (Ursus arctos) population in Yellowstone National Park and surrounding areas requires annual estimation of the number of adult female bears
with cubs-of-the-year. We examined the performance of nine estimators of population size via simulation. Data were simulated
using two methods for different combinations of population size, sample size, and coefficient of variation of individual sighting
probabilities. We show that the coefficient of variation does not, by itself, adequately describe the effects of capture heterogeneity,
because two different distributions of capture probabilities can have the same coefficient of variation. All estimators produced
biased estimates of population size with bias decreasing as effort in creased. Based on the simulation results we recommend
the Chao estimator for model M
h
be used to estimate the number of the female bears with cubs of the year; however, the estimator of Chao and Shen may also
be useful depending on the goals of the research. 相似文献
63.
Colin Southwell David Borchers Charles G. M. Paxton Louise Burt William de la Mare 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(1):41-54
We use line transect detection functions together with generalized linear and additive models to estimate detection probability
when detection on the line (“g(0)”) may not be certain. The methods provide a flexible way of modeling detection probability
for independent observer surveys, and for investigating the effects of explanatory variables. Analysis of data from an aerial
survey of pack-ice seals produced g(0) estimates substantially below 1 for some observers (it varied from 0.80 to 0.98), demonstrated a fairly complex dependence
of detection probability on covariates, and showed negative correlation between observers’ search width and their g(0). In addition to illustrating the utility of generalized additive models for capturing the effect of covariates on detection
probability, the analysis suggests that detection functions may be sufficiently variable that use of g(0) correction factors obtained from other surveys would be inadvisable. We recommend that estimation of g(0) be considered for all aerial surveys; if g(0) is found to be very close to 1, estimation from subsequent surveys under the assumption that it is 1 may be reasonable,
but without any estimation of g(0), the assumption that it is 1 is a matter of faith. 相似文献
64.
Charles C. Mischke Matt J. Griffin David J. Wise Terrence E. Greenway 《Journal of the World Aquaculture Society》2016,47(2):212-219
Proliferative gill disease (PGD) in catfish is caused by the myxozoan Henneguya ictaluri. The complex life cycle requires Dero digitata as the oligochaete host. Efforts to control PGD by eradicating D. digitata have been unsuccessful. Smallmouth buffalo, Ictiobus bubalus, (SMB) are opportunistic bottom feeders and a putative option for controlling D. digitata. In 2011, 15 ponds (0.4 ha) were stocked with 5000 channel catfish, Ictalurus punctatus; 7 of these 15 ponds were also stocked with 300 SMB fingerlings. There were no differences in benthic invertebrate numbers or water quality variables between ponds with or without SMB. At harvest, there were no differences in percent survival, total weight, or catfish feed conversion ratio. In the second year, 18 ponds (0.4 ha) were stocked with 6000 channel catfish. Half the ponds were also stocked with 300 SMB. Sentinel fish were used to estimate disease severity, and pond water was collected for molecular estimation of H. ictaluri actinospore concentrations. Similar to the first year, there were no differences between treatments in any variable tested, including PGD severity in sentinel fish and parasite concentrations in pond water. Under these study conditions, presence of SMB did not have a measureable effect on PGD incidence, parasite density, or overall catfish production. 相似文献
65.
绿豆抗豆象基因PCR标记的构建与应用 总被引:5,自引:1,他引:5
采用PCR分子标记技术,对16个绿豆品种(系)进行了遗传分析。在选用的56个随机引物中,发现抗豆象品种与感豆象品种间有一定差异。根据聚类分析结果将它们分成抗豆象野生种(TC1966)、抗豆象栽培种(V2709)、抗豆象杂交后代(VC3890A2/TC1966-23)和混合类型4个大组。以绿豆抗豆象和感豆象品种及抗豆象品种×感豆象品种组合的F2群体为试验材料,利用BSA法,对抗(感)豆象品种池和一个组合F2的抗(感)豆象池进行了鉴定,获得一个共显性标记。经F2分析,在抗豆象个体中扩增出约1.79 kb的特异片段或2个特异片段(1.79 kb/1.03 kb);在感豆象个体中仅扩增出约1.03 kb的特异片段。初步认为此标记与TC1966的抗豆象基因位点紧密联锁,可用于绿豆抗豆象种质鉴定和遗传育种的分子标记辅助选择。 相似文献
66.
Highly extensible Escherichia coli DNA molecules in planar extensional flow were visualized in dilute solution by fluorescence microscopy. For a narrow range of flow strengths, the molecules were found in either a coiled or highly extended conformation, depending on the deformation history of the polymer. This conformation hysteresis persists for many polymer relaxation times and is due to conformation-dependent hydrodynamic forces. Polymer conformational free-energy landscapes were calculated from computer simulations and show two free-energy minima for flow strengths near the coil-stretch transition. Hysteresis cycles may directly influence bulk-solution stresses and the development of stress-strain relations for dilute polymer flows. 相似文献
67.
Seife C 《Science (New York, N.Y.)》2003,301(5638):1300
68.
van Oijen AM Blainey PC Crampton DJ Richardson CC Ellenberger T Xie XS 《Science (New York, N.Y.)》2003,301(5637):1235-1238
We used a multiplexed approach based on flow-stretched DNA to monitor the enzymatic digestion of lambda-phage DNA by individual bacteriophage lambda exonuclease molecules. Statistical analyses of multiple single-molecule trajectories observed simultaneously reveal that the catalytic rate is dependent on the local base content of the substrate DNA. By relating single-molecule kinetics to the free energies of hydrogen bonding and base stacking, we establish that the melting of a base from the DNA is the rate-limiting step in the catalytic cycle. The catalytic rate also exhibits large fluctuations independent of the sequence, which we attribute to conformational changes of the enzyme-DNA complex. 相似文献
69.
70.
Igakura T Stinchcombe JC Goon PK Taylor GP Weber JN Griffiths GM Tanaka Y Osame M Bangham CR 《Science (New York, N.Y.)》2003,299(5613):1713-1716
Cell contact is required for efficient transmission of human T cell leukemia virus- type 1 (HTLV-I) between cells and between individuals, because naturally infected lymphocytes produce virtually no cell-free infectious HTLV-I particles. However, the mechanism of cell-to-cell spread of HTLV-I is not understood. We show here that cell contact rapidly induces polarization of the cytoskeleton of the infected cell to the cell-cell junction. HTLV-I core (Gag protein) complexes and the HTLV-I genome accumulate at the cell-cell junction and are then transferred to the uninfected cell. Other lymphotropic viruses, such as HIV-1, may similarly subvert normal T cell physiology to allow efficient propagation between cells. 相似文献