全文获取类型
收费全文 | 10332篇 |
免费 | 26篇 |
国内免费 | 37篇 |
专业分类
系统科学 | 33篇 |
丛书文集 | 48篇 |
教育与普及 | 30篇 |
理论与方法论 | 25篇 |
现状及发展 | 4029篇 |
研究方法 | 522篇 |
综合类 | 5538篇 |
自然研究 | 170篇 |
出版年
2014年 | 66篇 |
2012年 | 155篇 |
2011年 | 327篇 |
2010年 | 61篇 |
2009年 | 54篇 |
2008年 | 168篇 |
2007年 | 241篇 |
2006年 | 193篇 |
2005年 | 215篇 |
2004年 | 214篇 |
2003年 | 192篇 |
2002年 | 206篇 |
2001年 | 429篇 |
2000年 | 437篇 |
1999年 | 294篇 |
1994年 | 269篇 |
1992年 | 252篇 |
1991年 | 201篇 |
1990年 | 230篇 |
1989年 | 193篇 |
1988年 | 185篇 |
1987年 | 190篇 |
1986年 | 201篇 |
1985年 | 264篇 |
1984年 | 202篇 |
1983年 | 164篇 |
1982年 | 138篇 |
1981年 | 141篇 |
1980年 | 144篇 |
1979年 | 326篇 |
1978年 | 264篇 |
1977年 | 217篇 |
1976年 | 192篇 |
1975年 | 197篇 |
1974年 | 207篇 |
1973年 | 178篇 |
1972年 | 208篇 |
1971年 | 249篇 |
1970年 | 308篇 |
1969年 | 233篇 |
1968年 | 229篇 |
1967年 | 196篇 |
1966年 | 226篇 |
1965年 | 149篇 |
1959年 | 78篇 |
1958年 | 123篇 |
1957年 | 82篇 |
1956年 | 58篇 |
1954年 | 62篇 |
1948年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
961.
962.
Ageing, fitness and neurocognitive function. 总被引:18,自引:0,他引:18
A F Kramer S Hahn N J Cohen M T Banich E McAuley C R Harrison J Chason E Vakil L Bardell R A Boileau A Colcombe 《Nature》1999,400(6743):418-419
963.
964.
Charles T. LaRue 《西北部美国博物学家》2011,54(1)
Two hundred forty-one species of birds have been identified from northern Black Mesa, Arizona. This region's avifauna was poorly known until the late 1970s when large-scale coal mining began. Vegetation of the region is predominantly Great Basin desert scrub, pinyon-juniper woodland, and mixed-conifer woodland. The latter vegetation supports an assemblage of isolated montane bird species unique to the region. Numerous environmental changes have recently affected the bird life of Black Mesa. These include large-scale type conversions (pinyon-juniper clearing and surface mining), pond construction, and establishment of exotic vegetation. 相似文献
965.
Techniques for locating, capturing, and monitoring activities and movements of Flammulated Owls were developed during four years of study. Adults responded to imitated territorial song throughout the nesting season. Nests were found by noting vocalizations and behaviors associated with courtship-feeding and food transfers between the sexes. After fledging, broods were difficult to follow, unless the young or adults were radio-tagged. Adults and young were captured with noose poles, hoop nets, and mist nets. Backpack harnesses and tail-mounted transmitters were used on adults and backpacks on fledglings. Transmitter life was 30–40 nights. 相似文献
966.
D E Humphries G W Wong D S Friend M F Gurish W T Qiu C Huang A H Sharpe R L Stevens 《Nature》1999,400(6746):769-772
967.
The motility of the gastrointestinal tract consists of local, non-propulsive mixing (pendular or segmental) and propulsive (peristaltic) movements. It is generally considered that mixing movements are produced by intrinsic pacemakers which generate rhythmic contractions, and peristalsis by intrinsic excitatory and inhibitory neural reflex pathways, but the relationship between mixing and peristalsis is poorly understood. Peristalsis is compromised in mice lacking interstitial cells of Cajal, suggesting that these pacemaker cells may also be involved in neural reflexes. Here we show that mixing movements within longitudinal muscle result from spontaneously generated waves of elevated internal calcium concentration which originate from discrete locations (pacing sites), spread with anisotropic conduction velocities in al directions, and terminate by colliding with each other or with adjacent neurally suppressed regions. Excitatory neural reflexes control the spread of excitability by inducing new pacing sites and enhancing the overall frequency of pacing, whereas inhibitory reflexes suppress the ability of calcium waves to propagate. We provide evidence that the enteric nervous system organizes mixing movements to generate peristalsis, linking the neural regulation of pacemakers to both types of gut motility. 相似文献
968.
Mechanism of antigen-driven selection in germinal centres 总被引:26,自引:0,他引:26
The high affinity of antibodies produced during responses to T-cell-dependent antigens is associated with somatic mutation in the variable region of the immunoglobulin. Indirect evidence indicates that: (1) this arises by a process of hypermutation, acting selectively on rearranged immunoglobulin variable-region genes, which is activated in centroblasts within germinal centres; and (2) centrocytes, the progeny of centroblasts, undergo selection on the basis of their ability to receive a positive signal from antigen. We have now performed experiments analysing this selection process, and found that, on culture, centrocytes isolated from human tonsil kill themselves within a few hours by apoptosis. This is not a feature of other tonsillar B cells. Centrocytes can be prevented from entering apoptosis if they are activated both through their receptors for antigen and a surface glycoprotein recognized by CD40 antibodies. 相似文献
969.
Transcriptional control. Scissors and helical forks 总被引:2,自引:0,他引:2
970.
Indoxyl derivatives were detected as minor products among the urinary metabolites of two trial drugs, a benzodiazepine (GP 55 129) and a benzophenone (CGP 11 952). Their structures were elucidated by NMR and mass spectroscopy. Presumably, metabolites containing potential aldehyde functions react spontaneously with endogenous indoxyl. Such derivatives have not hitherto been encountered in drug metabolism. 相似文献