全文获取类型
收费全文 | 3351篇 |
免费 | 276篇 |
出版年
2023年 | 11篇 |
2022年 | 28篇 |
2021年 | 48篇 |
2020年 | 23篇 |
2019年 | 31篇 |
2018年 | 33篇 |
2017年 | 45篇 |
2016年 | 71篇 |
2015年 | 123篇 |
2014年 | 129篇 |
2013年 | 171篇 |
2012年 | 273篇 |
2011年 | 198篇 |
2010年 | 136篇 |
2009年 | 128篇 |
2008年 | 245篇 |
2007年 | 177篇 |
2006年 | 188篇 |
2005年 | 165篇 |
2004年 | 170篇 |
2003年 | 152篇 |
2002年 | 158篇 |
2001年 | 37篇 |
2000年 | 36篇 |
1999年 | 47篇 |
1998年 | 50篇 |
1997年 | 38篇 |
1996年 | 31篇 |
1995年 | 35篇 |
1994年 | 32篇 |
1993年 | 29篇 |
1992年 | 39篇 |
1991年 | 28篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 28篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 29篇 |
1984年 | 23篇 |
1983年 | 18篇 |
1982年 | 23篇 |
1981年 | 31篇 |
1980年 | 25篇 |
1978年 | 30篇 |
1977年 | 18篇 |
1976年 | 15篇 |
1975年 | 10篇 |
1974年 | 25篇 |
1973年 | 14篇 |
排序方式: 共有3627条查询结果,搜索用时 15 毫秒
1.
Henry C. Stevens Elaine M. Metz Percy Saboya Del Castillo Juan Díaz Alvn Mark T. Bowler 《Journal of Field Ornithology》2019,90(1):70-79
White‐sand forests are patchily distributed ecosystems covering just 5% of Amazonia that host many specialist species of birds not found elsewhere, and these forests are threatened due to their small size and human exploitation of sand for construction projects. As a result, many species of birds that are white‐sand specialists are at risk of extinction, and immediate conservation action is paramount for their survival. Our objective was to evaluate current survey methods and determine the relative effect of the size of patches of these forests on the presence or absence of white‐sand specialists. Using point counts and autonomous recorders, we surveyed avian assemblages occupying patches of white‐sand forest in the Peruvian Amazon in April 2018. Overall, we detected 126 species, including 21 white‐sand forest specialists. We detected significantly more species of birds per survey point with autonomous recorders than point counts. We also found a negative relationship between avian species richness and distance from the edge of patches of white‐sand forest, but a significant, positive relationship when only counting white‐sand specialists. Although we detected more species with autonomous recorders, point counts were more effective for detecting canopy‐dwelling passerines. Therefore, we recommend that investigators conducting surveys for rare and patchily distributed species in the tropics use a mixed‐method approach that incorporates both autonomous recorders and visual observation. Finally, our results suggest that conserving large, continuous patches of white‐sand forest may increase the likelihood of survival of species of birds that are white‐sand specialists. 相似文献
2.
3.
4.
5.
Rutherford T. Johnstone 《The Western journal of medicine》1956,84(4):265-268
Within the past decade the chemist has evolved a new group of insecticides having a greater range of toxicity to insects—and to man. Since their use is ubiquitous, it is imperative that information be widely disseminated regarding proper cautionary measures as well as the early signs and symptoms of toxicity. 相似文献
6.
Rutherford T. Johnstone 《The Western journal of medicine》1958,89(2):117-120
The traditional diagnostic procedure adhered to in general medicine is too frequently neglected when a physician is confronted with a possible occupational disease. Also, the proper diagnosis in occupational medicine requires additional effort along lines of investigation not necessary in general medicine. Failure to conform to the basic steps as outlined in this paper will distort known entities or “create” false ones. 相似文献
7.
8.
Molecular Forms of Acetylcholinesterase and Butyrylcholinesterase in Human Plasma and Cerebrospinal Fluid 总被引:1,自引:0,他引:1
John R. Atack Elaine K. Perry James R. Bonham Robert H. Perry 《Journal of neurochemistry》1987,48(6):1845-1850
The measurement of cholinesterase activities in either plasma or cerebrospinal fluid (CSF) may ultimately prove to be relevant in the diagnosis of neurological and neuropsychiatric disorders. However, studies to date have examined only total enzyme activities. Therefore in the present study we have examined the distribution of the individual molecular forms of both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in plasma and CSF using sucrose density gradient centrifugation. Although the total activities of AChE were of the same order of magnitude in plasma and CSF, there was a considerable difference (120-500-fold) between total BChE activity in the CSF and the BChE-rich plasma. The analysis of the individual molecular forms revealed that the predominant molecular species of AChE and BChE in the CSF--both lumbar and ventricular--was the G4 form. The G4 form also constituted the majority of the plasma BChE activity and, on average, over half (56%) of the plasma AChE activity. The significance of the AChE and BChE molecular form compositions of both plasma and CSF and their possible relationship to pathological states are discussed. 相似文献
9.
Purification and properties of a germination-specific cortex-lytic enzyme from spores of Bacillus megaterium KM. 总被引:12,自引:0,他引:12 下载免费PDF全文
Two peptidoglycan-lytic enzyme activities were isolated from spores of Bacillus megaterium KM. Surface-bound lytic enzyme was extracted from dormant spores and hydrolysed a variety of peptidoglycan substrates including isolated spore cortex, but did not cause refractility changes in permeabilized spores. Germination-specific lytic enzyme activity appeared early in germination and had minimal activity on isolated peptidoglycan substrates, but caused refractility changes in permeabilized spores of several Bacillus isolated peptidoglycan substrates, but caused refractility changes in permeabilized spores of several Bacillus species. The germination-specific lytic enzyme was shown to be a heat-sensitive 29 kDa protein with maximal activity at pH 6.5. It catalysed post-commitment muramic acid delta-lactam synthesis and displayed an inhibitor profile similar to that for post-commitment A600 loss. The relationship of the germination-specific enzyme to a recently proposed model of spore germination is discussed. 相似文献
10.
Differential control of proto-oncogene c-myc and c-fos expression in lymphocytes and fibroblasts. 总被引:1,自引:0,他引:1 下载免费PDF全文
In lymphocytes stimulated with the mitogen phytohaemagglutinin, an inhibitor of the enzyme ADP-ribosyltransferase (ADPRT) completely blocks the proliferative response and the increase in expression of the proto-oncogene c-myc without affecting c-fos significantly. Conversely, in fibroblasts the serum-induced growth is not affected by the ADPRT inhibitor, and both oncogenes are dramatically super-induced. Hence there are differences between lymphocyte and fibroblast early responses to mitogenic stimulation and also between regulation of c-fos and c-myc gene expression. 相似文献