全文获取类型
收费全文 | 9384篇 |
免费 | 999篇 |
国内免费 | 3篇 |
出版年
2021年 | 141篇 |
2020年 | 95篇 |
2019年 | 122篇 |
2018年 | 152篇 |
2017年 | 100篇 |
2016年 | 183篇 |
2015年 | 329篇 |
2014年 | 369篇 |
2013年 | 438篇 |
2012年 | 583篇 |
2011年 | 557篇 |
2010年 | 348篇 |
2009年 | 311篇 |
2008年 | 467篇 |
2007年 | 465篇 |
2006年 | 408篇 |
2005年 | 437篇 |
2004年 | 409篇 |
2003年 | 357篇 |
2002年 | 374篇 |
2001年 | 243篇 |
2000年 | 208篇 |
1999年 | 197篇 |
1998年 | 126篇 |
1997年 | 83篇 |
1996年 | 81篇 |
1995年 | 64篇 |
1994年 | 79篇 |
1993年 | 67篇 |
1992年 | 144篇 |
1991年 | 130篇 |
1990年 | 119篇 |
1989年 | 141篇 |
1988年 | 117篇 |
1987年 | 126篇 |
1986年 | 93篇 |
1985年 | 110篇 |
1984年 | 81篇 |
1983年 | 94篇 |
1982年 | 78篇 |
1981年 | 85篇 |
1980年 | 73篇 |
1979年 | 84篇 |
1978年 | 72篇 |
1977年 | 93篇 |
1976年 | 71篇 |
1975年 | 67篇 |
1974年 | 68篇 |
1973年 | 60篇 |
1972年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Modern nutritional studies have found that diverse diets are linked to lower infant mortality rates and longer life expectancies in humans. This is primarily because humans require more than fifty essential nutrients for growth and cell maintenance and repair; most of these essential nutrients must come from outside food sources rather than being manufactured by the body itself; and a diversity of food types is required to consume the full suite of essential nutrients necessary for optimal human health. These principles and their related affects on human adaptations and demography are the hallmarks of a theoretical paradigm defined as nutritional ecology. This essay applies concepts derived from nutritional ecology to the study of human evolution. Principles of nutritional ecology are applied to the study of the Middle‐to‐Upper Paleolithic transition in order to broadly illustrate the interpretive ramifications of this approach. At any stage in human evolution, those hominid populations that chose to diversify their subsistence base may have had a selective advantage over competitors who restricted their diet principally to one food type, such as terrestrial mammals. 相似文献
2.
3.
In C4 plants phosphoenolpyruvate (PEP) of the C4 cycle may betransported on a chloroplast transporter which also transports3-phosphoglycerate (3-PGA) and triosephosphates. In C3 plantsPEP is not considered to be effectively transported on the chloroplastphosphate translocator. The influences of certain organic phosphates,having a similar structure to either PEP or triose-phosphates,on 3-PGA dependent O2 evolution by C4 (Digitaria sanquinalisL. Scop.) and C3 (Hordeum vulgare L.) mesophyll chloroplastswere investigated. In the C4 mesophyll chloroplasts phosphoglycolatewas a competitive inhibitor (Ki = 2.1 mM) of 3-PGA dependentO2 evolution, and was as effective as previously reported forPEP. 2-Phosphoglycerate was also a competitive inhibitor (Kt= 8.6 mM) of O2 evolution in the C4 mesophyll chloroplasts with3-PGA as substrate, while phospholactate was a weak inhibitorand glyphosate had no effect. Neither PEP, phosphoglycolatenor 2-phosphoglycerate were effective inhibitors of 3- PGA dependentO2 evolution in the C3 chloroplasts. Phosphohydroxypyruvatewas a competitive inhibitor of 3-PGA dependent O22 evolutionin both chloroplast types. The selectivity in inhibition ofO2 evolution with 3-PGA as substrate suggests that the C4 mesophyllchloroplasts can recognize certain organic phosphates with thephosphate in the C-2 or C-3 position but that the C4 mesophyllchloroplasts can only effectively recognize certain organicphosphates with the phosphate in the C-3 position. The resultsalso support the view that 3-PGA and PEP are transported onthe same phosphate translocator in C4 mesophyll chloroplasts.
1 Current address: Department of Horticulture, 2001 Fyffe Court,The Ohio State University, Columbus, Ohio 43210-1096. (Received March 24, 1987; Accepted April 16, 1987) 相似文献
4.
5.
A. J. Edwards J. D. Hamilton W. D. Nichol G. W. Taylor A. M. Dawson 《BMJ (Clinical research ed.)》1970,1(5692):342-345
Seven patients with “coeliac axis compression syndrome” are reported. Five were treated surgically, but only two did well. A survey of 200 healthy adults showed epigastric bruits in 6·5%; only one of these had dyspepsia, though dyspepsia was present in 12·5% overall.Caution is urged in attributing a causal relationship between coeliac axis compression and pain and in proceeding to arteriography when compression is suspected on clinical grounds. 相似文献
6.
Plant and Soil - Agricultural practices often have persistent effects on soil physicochemical properties and soil biota, which can feedback to influence plant performance. We investigated... 相似文献
7.
Jonathan Porter Joan Robinson Roger Pickup Clive Edwards 《FEMS microbiology letters》1995,133(1-2):195-199
Abstract The effectiveness of immunofluorescence flow cytometry and cell sorting to detect, quantify and separate indigenous bacterial populations present in low concentrations in sewage outflow was investigated. Preparatory experiments for targeted recovery revealed indigenous, immunoglobulin-G-binding particles present at low levels in sewage outflow samples taken from Coniston Water. Fluorescence-activated cell sorting of this population was employed to enrich for these particles, which were confirmed as bacterial cells. This cell population comprised approximately 23% of the total plate count on MacConkey agar before cell sorting, rising to approximately 95% after sorting. These results corresponded to cell densities of less than 5% of the total plate count on R2A agar. Taxonomic tests suggested the bacterium to be Ochrobactrum anthropi . 相似文献
8.
9.
We used Chelex 100 chelating resin to prepare DNA for the polymerase chain reaction (PCR) from two species of Hymenopteran parasitoids, Trioxys pallidus and Diglyphus begini. Chelex 100 produces consistent DNA yields for both species, as measured with Hoescht dye fluorometry. Approximately twice as much DNA was obtained from individual D. begini wasps than from T. pallidus wasps, but there were no differences in yield between sexes. We used this DNA to perform random amplified polymorphic DNA (RAPD) analysis, a PCR technique that amplifies various regions of the genome using arbitrarily chosen 10-base primers. Of the 120 primers tested using T. pallidus, 92 produced a total of 342 scorable bands, 118 of which exhibited presence/absence polymorphism. Of the 25 primers tested using D. begini, 18 produced a total of 53 scorable bands, 30 of which exhibited presence/absence polymorphism. The level of genetic variation detected using this technique was greater than any found in Hymenoptera using allozymes. Scorable bands segregated as dominant Mendelian traits. Potential uses of RAPD-PCR in genetic analyses on parasitic Hymenoptera are discussed. 相似文献
10.
Joshua R Edwards Evangelos A Diamantakos Jacob D Peuler Peter C Lamar Walter C Prozialeck 《BMC physiology》2007,7(1):1