全文获取类型
收费全文 | 10206篇 |
免费 | 987篇 |
国内免费 | 11篇 |
专业分类
11204篇 |
出版年
2021年 | 122篇 |
2020年 | 51篇 |
2019年 | 90篇 |
2018年 | 84篇 |
2017年 | 86篇 |
2016年 | 143篇 |
2015年 | 299篇 |
2014年 | 308篇 |
2013年 | 431篇 |
2012年 | 614篇 |
2011年 | 577篇 |
2010年 | 422篇 |
2009年 | 362篇 |
2008年 | 546篇 |
2007年 | 588篇 |
2006年 | 529篇 |
2005年 | 602篇 |
2004年 | 622篇 |
2003年 | 589篇 |
2002年 | 550篇 |
2001年 | 104篇 |
2000年 | 70篇 |
1999年 | 127篇 |
1998年 | 180篇 |
1997年 | 106篇 |
1996年 | 116篇 |
1995年 | 114篇 |
1994年 | 114篇 |
1993年 | 95篇 |
1992年 | 112篇 |
1991年 | 90篇 |
1990年 | 95篇 |
1989年 | 80篇 |
1988年 | 89篇 |
1987年 | 82篇 |
1986年 | 78篇 |
1985年 | 94篇 |
1984年 | 130篇 |
1983年 | 107篇 |
1982年 | 134篇 |
1981年 | 140篇 |
1980年 | 115篇 |
1979年 | 77篇 |
1978年 | 93篇 |
1977年 | 87篇 |
1976年 | 84篇 |
1975年 | 87篇 |
1974年 | 83篇 |
1973年 | 94篇 |
1972年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
We conducted an experiment in a northern mixed-grass prairie at Wind Cave National Park, South Dakota, USA to evaluate the effect of defoliation frequency on aboveground net primary production (ANPP), shoot nitrogen concentration, and aboveground N yield of graminoids. ANPP was significantly reduced at weekly and biweekly defoliation frequencies, but unaffected relative to unclipped controls at monthly and bimonthly frequencies. By contrast, clipping at all frequencies increased shoot N concentration above that of controls, and this increase was greatest at monthly or more frequent defoliations. Total aboveground N yield and potential N yield to grazers were greatest at intermediate (bimonthly to biweekly) frequencies. We suggest that grazers may maximize their nutritional status in this system by periodically regrazing areas at frequencies near the approximately monthly optimum that we observed. 相似文献
12.
13.
A set of procedures is offered for assessing interraler reliability and certain aspects of validity of codes in cross-cultural studies. The method assumes that at least two independent raters have coded more than one trait. Each trait coded by one rater is correlated with each trait coded by a second, and all the codings by a single rater are intercorrelated with each other. The results are presented in a multitrait-multi-rater matrix. From this matrix it is possible to determine the interrater reliability and discriminant validity of trails in addition to a higher order concept based on pairs of traits. 相似文献
14.
Ronald L. Rutowski 《Animal behaviour》1982,30(1):108-112
Free-flying males of the checkered white butterfly, Pieris protodice, were presented with tethered females that varied with respect to both size (as measured by forewing length) and age (as measured by wing wear). Because males make a substantial investment in their offspring through nutrients passed to the female during copulation, they were expected to court young and large females for longer times than older and smaller females, and they did so. Additional experiments further suggest that size discriminations are made on the basis of apparent wing area and age discriminations are made on the basis of an age-related increase in ultraviolet reflectance that occurs in females. The discussion examines the adaptive value of these discriminations. 相似文献
15.
An Ecteola-cellulose chromatography assay for 3′-phosphoadenosine 5′-phosphosulfate: Phenol sulfotransferase 总被引:1,自引:0,他引:1
Ronald T. Borchardt Anna Baranczyk-Kuzma Carol L. Pinnick 《Analytical biochemistry》1983,130(2):334-338
A new assay procedure for phenol sulfotransferase which employs [35S]-3'-phosphoadenosine-5'-phosphosulfate as a sulfate donor and a variety of phenols as sulfate acceptors was developed. The appearance of the 35S-sulfated products or the disappearance of the [35S]-3'-phosphoadenosine-5'-phosphosulfate are determined simultaneously by chromatography of the assay incubation mixtures on Ecteola-cellulose columns, eluting with an NH4HCO3 step gradient. Various acidic, neutral, and basic phenols can be employed as substrates for phenol sulfotransferase using this procedure. 相似文献
16.
Abstract Klebsiella pneumoniae M5a1 grows readily on two compounds, 4-hydroxyphenylacetate and 4-aminobutyrate, whose catabolism produces succinic semialdehyde. A single succinic semialdehyde dehydrogenase was detected, native molecular weight 52000, that has NAD as the preferred cofactor and is induced by succinic semialdehyde functions in the oxidation of succinic semialdehyde during growth on both 4-hydroxyphenyl-acetate and 4-aminobutyrate. This contrasts with the situation for Escherichia coli and Pseudomonas putida where two distinct forms of succinic semialdehyde dehydrogenase have been observed. 相似文献
17.
18.
19.
20.
Ronald J.A. Wanders Carlo Van Roermund Cor Lof Alfred J. Meijer 《Analytical biochemistry》1983,129(1):80-87
A simple fluorimetric assay for the determination of carbamoyl phosphate in tissue extracts is described. In the assay, production of ATP from carbamoyl phosphate and ADP by carbamate kinase is coupled to the formation of NADPH, using glucose, hexokinase, NADP+, and glucose-6-phosphate dehydrogenase. Production of NADPH in this system proved to be equal to the amount of carbamoyl phosphate present. 相似文献