全文获取类型
收费全文 | 23558篇 |
免费 | 2261篇 |
国内免费 | 7篇 |
出版年
2023年 | 118篇 |
2022年 | 288篇 |
2021年 | 575篇 |
2020年 | 342篇 |
2019年 | 398篇 |
2018年 | 468篇 |
2017年 | 442篇 |
2016年 | 680篇 |
2015年 | 1172篇 |
2014年 | 1267篇 |
2013年 | 1467篇 |
2012年 | 1911篇 |
2011年 | 2032篇 |
2010年 | 1224篇 |
2009年 | 1074篇 |
2008年 | 1469篇 |
2007年 | 1395篇 |
2006年 | 1317篇 |
2005年 | 1157篇 |
2004年 | 1206篇 |
2003年 | 1069篇 |
2002年 | 1036篇 |
2001年 | 253篇 |
2000年 | 179篇 |
1999年 | 251篇 |
1998年 | 265篇 |
1997年 | 174篇 |
1996年 | 143篇 |
1995年 | 125篇 |
1994年 | 131篇 |
1993年 | 152篇 |
1992年 | 149篇 |
1991年 | 115篇 |
1990年 | 122篇 |
1989年 | 115篇 |
1988年 | 98篇 |
1987年 | 104篇 |
1986年 | 72篇 |
1985年 | 97篇 |
1984年 | 78篇 |
1983年 | 65篇 |
1982年 | 85篇 |
1981年 | 92篇 |
1980年 | 82篇 |
1979年 | 60篇 |
1978年 | 69篇 |
1977年 | 51篇 |
1976年 | 57篇 |
1974年 | 47篇 |
1972年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
David Philip Arthur Craig James W. Grice Chris A. Varnon B. Gibson Michel B. C. Sokolowski Charles I. Abramson 《PloS one》2012,7(10)
Free-flying honey bees (Apis mellifera L.) reactions were observed when presented with varying schedules of post-reinforcement delays of 0 s, 300 s, or 600 s. We measured inter-visit-interval, response length, inter-response-time, and response rate. Honey bees exposed to these post-reinforcement delay intervals exhibit one of several patterns compared to groups not encountering delays, and had longer inter-visit-intervals. We observed no group differences in inter-response time. Honey bees with higher response rates tended to not finish the experiment. The removal of the delay intervals increased response rates for those subjects that completed the trials. 相似文献
52.
F N Craig 《Journal of applied physiology》1972,33(5):611-615
53.
J.F. Kuo Eugene J. Malveaux Janice G. Patrick Craig W. Davis Albert W. Pruitt 《Biochimica et Biophysica Acta (BBA)/General Subjects》1977,497(3):785-796
Possible involvement of cyclic GMP-dependent and cyclic AMP-dependent protein kinases, protein kinase modulators and cyclic nucleotide phosphodiesterases in functions of vascular tissues were investigated in the dog. All of the above activities, localized in the smooth muscle-rich inner layer of the blood vessels, were found to be higher in the arteries than in the veins. The peripheral arteries were disproportionately richer in cyclic GMP-dependent protein kinase (as indicated by high ratios of cyclic GMP-dependent to cyclic AMP-dependent protein kinase) than were the veins, with the exception of the pulmonary artery, an atypical arterial tissue exposed to low blood pressure. Interestingly, the protein kinase ratio for the aorta, an artery with no significant role in blood pressure regulation, was not higher than that for the vena cava. Creation of femoral arteriovenous fistulae in the dogs led to preferential reductions in the cyclic GMP-dependent enzyme activity both in the proximal and distal arteries, whereas it was elevated in the stressed vein distal to the anastomotic site. The cyclic GMP-dependent enzyme was preferentially reduced in the saphenous artery distal to occlusion. Changes in the cyclic GMP-dependent enzyme activity appeared to precede gross atrophy or hypertrophy of the vessels. It is suggested that the vascular cyclic GMP-dependent protein kinase may be closely related to peripheral resistance and its regulation. 相似文献
54.
Evaluating Hypotheses on the Evolution of Helping Behaviour in the Bell Miner (Manorina melanophrys)
In an analysis of the pattern of helping at the nest in bell miners (Manorina melanophrys), Clarke (1989) examined five hypotheses that could explain the evolution of helping behaviour in this species. We applaud the attempt by Clarke to consider many different hypotheses, but believe that more careful formulation of the hypotheses, more appropriate statistical analyses, and eventual experimental testing are required before his conclusions can be accepted. 相似文献
55.
Jonathan E. Reyman 《American anthropologist》2000,102(3):635-636
The Land of Prehistory:. Critical History of American Archaeology. Alice Beck Kehoe. New York: Routledge, 1998. 288 pp. 相似文献
56.
Moran Jonathan A.; Merbach Marlis A.; Livingston Nigel J.; Clarke Charles M.; Booth Webber E. 《Annals of botany》2001,88(2):307-311
Old World pitcher plants (Nepenthes spp., Nepenthaceae) trapand digest invertebrate prey to derive nutrients, primarilynitrogen (N). In the majority of lowland Nepenthes species studiedto date, ants (Hymenoptera, Formicidae) are numerically thedominant prey taxon. Nepenthes albomarginata is unusual in showingan apparent bias towards the capture of termites (Isoptera).We tested the hypothesis that N. albomarginata derives N fromtermite capture, by comparison of foliar stable N isotope abundance( 相似文献
57.
Jonathan A. Newman 《Global Change Biology》2004,10(1):5-15
Experimental evidence regarding the responses of cereal aphids to rising atmospheric CO2 has been ambiguous. Some studies suggest increased population sizes under future CO2 levels, others suggest decreased population sizes, and still others suggest little or no difference. Recently, Newman et al. (2003) constructed a general mathematical model of the aphid–grass interaction to investigate whether or not we should, in fact, expect a general aphid response to rising CO2. They concluded that aphid populations are likely to be larger under future CO2 concentrations if soil N levels are high, the aphid species' nitrogen requirement is low and the aphid species' density‐dependent response in winged morph production is weak. In that model, and in field experiments, CO2 concentration influences aphid population dynamics through the effect it has on plant quality. However, future CO2 concentrations are also likely to be accompanied by higher ambient temperatures, a combination that has received little focus to date. In the present paper, the Newman et al. model is used to consider the combined effects of increased CO2 concentrations and temperature on aphid population sizes. It is concluded that, when both factors are elevated, aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone. This result has important implications for future experimentation. 相似文献
58.
59.
Craig Hadley 《American journal of physical anthropology》2007,132(1):160-161
60.
Thomas M. Becker Jack D. Poland Thomas J. Quan Mark E. White Jonathan M. Mann Allan M. Barnes 《The Western journal of medicine》1987,147(5):554-557
Meningitis caused by Yersinia pestis developed in 6 (6%) of a total of 105 patients with plague reported to the Centers for Disease Control from 1970 to 1979. Five of the six cases occurred in children aged 10 to 15 years. All six patients received antibiotic therapy before meningitis developed, which appeared between the 9th and 14th days after the onset of acute illness in five of the six patients. There were no neurologic sequelae. The antigenic and biochemical profiles of the Y pestis strains isolated from cerebrospinal fluid in the meningitis cases did not differ from those of the Y pestis strains obtained from blood and bubo aspirates in the other 99 patients, and neither did in vitro studies suggest antibiotic resistance. While plague meningitis is an uncommon complication of acute plague infection, physicians in the western United States should be aware that it may develop as much as 14 days after antibiotic therapy for the acute plague infection has been initiated. 相似文献