全文获取类型
收费全文 | 41914篇 |
免费 | 68篇 |
国内免费 | 120篇 |
专业分类
系统科学 | 254篇 |
丛书文集 | 919篇 |
教育与普及 | 112篇 |
理论与方法论 | 245篇 |
现状及发展 | 18382篇 |
研究方法 | 1698篇 |
综合类 | 19896篇 |
自然研究 | 596篇 |
出版年
2013年 | 264篇 |
2012年 | 589篇 |
2011年 | 1212篇 |
2010年 | 245篇 |
2008年 | 735篇 |
2007年 | 762篇 |
2006年 | 801篇 |
2005年 | 804篇 |
2004年 | 777篇 |
2003年 | 763篇 |
2002年 | 757篇 |
2001年 | 1256篇 |
2000年 | 1153篇 |
1999年 | 795篇 |
1992年 | 740篇 |
1991年 | 600篇 |
1990年 | 644篇 |
1989年 | 616篇 |
1988年 | 619篇 |
1987年 | 656篇 |
1986年 | 643篇 |
1985年 | 785篇 |
1984年 | 654篇 |
1983年 | 538篇 |
1982年 | 469篇 |
1981年 | 451篇 |
1980年 | 589篇 |
1979年 | 1324篇 |
1978年 | 1107篇 |
1977年 | 1118篇 |
1976年 | 795篇 |
1975年 | 857篇 |
1974年 | 1251篇 |
1973年 | 1066篇 |
1972年 | 1091篇 |
1971年 | 1329篇 |
1970年 | 1744篇 |
1969年 | 1278篇 |
1968年 | 1219篇 |
1967年 | 1315篇 |
1966年 | 1101篇 |
1965年 | 797篇 |
1964年 | 200篇 |
1959年 | 471篇 |
1958年 | 707篇 |
1957年 | 592篇 |
1956年 | 501篇 |
1955年 | 419篇 |
1954年 | 484篇 |
1948年 | 301篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
The INDSCAL individual differences scaling model is extended by assuming dimensions specific to each stimulus or other object, as well as dimensions common to all stimuli or objects. An alternating maximum likelihood procedure is used to seek maximum likelihood estimates of all parameters of this EXSCAL (Extended INDSCAL) model, including parameters of monotone splines assumed in a quasi-nonmetric approach. The rationale for and numerical details of this approach are described and discussed, and the resulting EXSCAL method is illustrated on some data on perception of musical timbres. 相似文献
222.
223.
224.
A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean 总被引:70,自引:0,他引:70
Coale KH Johnson KS Fitzwater SE Gordon RM Tanner S Chavez FP Ferioli L Sakamoto C Rogers P Millero F Steinberg P Nightingale P Cooper D Cochlan WP Landry MR Constantinou J Rollwagen G Trasvina A Kudela R 《Nature》1996,383(6600):495-501
The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability. 相似文献
225.
Insect motion detectors matched to visual ecology 总被引:1,自引:0,他引:1
To detect motion, primates, birds and insects all use local detectors to correlate signals sampled at one location in the image with those sampled after a delay at adjacent locations. These detectors can adapt to high image velocities by shortening the delay. To investigate whether they use long delays for detecting low velocities, we compared motion-sensitive neurons in ten species of fast-flying insects, some of which encounter low velocities while hovering. Neurons of bee-flies and hawkmoths, which hover, are tuned to lower temporal frequencies than those of butterflies and bumblebees, which do not. Tuning to low frequencies indicates longer delays and extends sensitivity to lower velocities. Hoverflies retain fast temporal tuning but use their high spatial acuity for sensing low-velocity motion. Thus an unexpectedly wide range of spatio-temporal tuning matches motion detection to visual ecology. 相似文献
226.
S. F. Wang J. C. Braekman D. Daloze J. Pasteels P. Soetens N. V. Handjieva P. Kalushkov 《Cellular and molecular life sciences : CMLS》1996,52(6):628-630
The isolation of N-quinaldyl-L-arginine·HCl (1) from the CoccinellidaeSubcoccinella-24-punctata is reported. The structure, first established on the basis of the analysis of the spectral properties of1, has been confirmed by synthesis. The alkaloid is of endogenous origin and markedly deterrent to ants. 相似文献
227.
Molecular evidence for increased hematopoietic proliferation in the spleen of the b/b laboratory rat
S. Savković S. Pavlović T. Mitrović M. Joksimović J. Marjanović V. Glišin Z. Popović 《Cellular and molecular life sciences : CMLS》1996,52(8):807-811
The splenomegaly and the appearance of a significant number of CFU-E (erythroid colony-forming units) and BFU-E1 (erythroid burst-forming units) in the Belgrade laboratory rat (b/b) spleen prompted us to analyse further the molecular evidence for increased hematopoietic proliferation in the b/b spleen. Messenger RNAs (mRNAs) specific for globins, proteins for iron transport and deposition and the band 3 protein were used in rat erythropoietic tissues as markers for proliferation and erythroid differentiation. In the b/b spleen, all mRNAs analysed display an erythroid-specific pattern of expression. This analysis also revealed an enhanced level of mRNA for ferritin in the +/b spleen, whereas erythrocyte-specific mRNA production was normal. 相似文献
228.
Antiviral activities of anthraquinones,bianthrones and hypericin derivatives from lichens 总被引:2,自引:0,他引:2
P. A. Cohen J. B. Hudson G. H. N. Towers 《Cellular and molecular life sciences : CMLS》1996,52(2):180-183
The antiviral activities of some naturally occurring anthraquinones bianthrones, and hypericin derivatives were compared by the end-point CPE (viral cytopathic effects) method and plaque assays. Under optimal conditions of exposure to light, hypericin, 7,7-dichlorohypericin and 5,7-dichloroemodin exhibited strong inhibitory activity against HSV-1 (herpes simplex virus type 1) in both assays. Partial inactivation of the virus was shown by emodin, 7-chloroemodin and 7-chloro-1-O-methylemodin; the bianthrones and other anthraquinones were found to be inactive. Antiviral activity appeared to be, positively correlated with increasing substitution of chlorine in the anthraquinone structure. In the absence of light, only hypericin and 7,7-dichlorohypericin displayed detectable activity. 相似文献
229.
230.