全文获取类型
收费全文 | 3857篇 |
免费 | 439篇 |
国内免费 | 5篇 |
出版年
2021年 | 31篇 |
2020年 | 23篇 |
2019年 | 33篇 |
2018年 | 37篇 |
2017年 | 41篇 |
2016年 | 51篇 |
2015年 | 124篇 |
2014年 | 94篇 |
2013年 | 150篇 |
2012年 | 230篇 |
2011年 | 214篇 |
2010年 | 128篇 |
2009年 | 123篇 |
2008年 | 145篇 |
2007年 | 160篇 |
2006年 | 199篇 |
2005年 | 155篇 |
2004年 | 196篇 |
2003年 | 157篇 |
2002年 | 152篇 |
2001年 | 45篇 |
2000年 | 35篇 |
1999年 | 37篇 |
1998年 | 58篇 |
1997年 | 46篇 |
1996年 | 39篇 |
1995年 | 36篇 |
1994年 | 41篇 |
1993年 | 24篇 |
1992年 | 34篇 |
1991年 | 36篇 |
1990年 | 41篇 |
1989年 | 34篇 |
1988年 | 35篇 |
1987年 | 27篇 |
1986年 | 33篇 |
1985年 | 30篇 |
1984年 | 48篇 |
1983年 | 32篇 |
1982年 | 61篇 |
1981年 | 50篇 |
1980年 | 49篇 |
1979年 | 39篇 |
1978年 | 26篇 |
1977年 | 49篇 |
1976年 | 45篇 |
1975年 | 41篇 |
1974年 | 40篇 |
1973年 | 27篇 |
1970年 | 23篇 |
排序方式: 共有4301条查询结果,搜索用时 250 毫秒
101.
Frederick G. Meyer 《Economic botany》1980,34(4):401-437
The remains of carbonized plants recovered from sites destroyed by Mount Vesuvius in A.D. 79 furnish valuable historical evidence on many staple food plants used by the ancient Campanians of the 1st century A.D. Carbonized seeds,grains, nuts, and fruits of 24 species identified from documented materials at Pompeii, Herculaneum, and the Roman villa at Torre Annunziata include: filbert, Corylus avellana;broadbean, Vicia fabavar. minor;bittervetch. Vicia ervilia;chickpea, Cicer arietinum;carob, Ceratonia siliqua;lentil, Lens culinaris;European chestnut, Castanea sativa;English or Persian walnut, Juglans regia;common onion, Allium cepa;garlic, Allium sativum;fig, Ficus carica;olive, Olea europaea;date, Phoenix dactylifera;stone pine, Pinus pinea;six-rowed barley, Hordeum vulgare;emmer wheat, Triticum dicoccon;common millet, Panicum miliaceum;Italian millet, Setaria italica;oat, Avena sativa;almond, Prunus dulcis;sour cherry, Prunus cerasus;pear, Pyrus communis;crabapple, Malussp.;and grape, Vitis vinifera. 相似文献
102.
Thiobacillus ferrooxidans became significantly more tolerant to mercury stress after culturing in media of increasing mercury(II) concentrations. When mercuric chloride was added to the growth medium, the resistant organisms were found to volatilize elemental mercury (Hg0).T. ferrooxidans may be an important factor in the natural mercury cycle, since the environments whereT. ferrooxidans is found typically contain elevated levels of heavy metals, including mercury. 相似文献
103.
High biological activity of the synthetic replicates of somatostatin-28 and somatostatin-25 总被引:4,自引:0,他引:4
Paul Brazeau Nicholas Ling Frederick Esch Peter Böhlen Robert Benoit Roger Guillemin 《Regulatory peptides》1981,1(4):255-264
We have isolated form extracts of ovine hypothalami two molecules characterized as somatostatin-28 and somatostatin-4-28 (referred to as somatostatin-25). They were reproduced by solid hase synthesis. In equimolar ratio and depending upon the experimental conditions, synthetic somatostatin-28 ans somatostatin-25 are 3-14 times more potent than somatostatin-14 to inhibit the basal in vitro secretion of growth hormone or as stimulated by prostaglandin (PGE2). In early studies in vivo, somatostatin-28 and somatostatin-25 are also more potent than somatostatin-14 in inhibiting the secretion of growth hormone acutely stimulated in the rat by injection of morphine; somatostatin-28 is also longer-acting than somatostatin-14. These results suggest that somatostatin-14, as originally isolated, is a biologically active fragment of a larger molecule of greater specific activity; it should be considered as another form of somatostatin with high biological activity present in some tissues and likely secreted y the tissues along with somatostatin-14 and possibly other somatostatin-peptides of diverse sizes. 相似文献
104.
William A. Bonner Neal E. Blair Frederick M. Dirbas 《Origins of life and evolution of the biosphere》1981,11(1-2):119-134
Our earlier experiments are briefly reviewed, involving the abiotic generation of optical activity by exposure of DL-amino acids to various chiral physical forces. The enantiomeric enrichments so obtained were low, however, and additional experiments were undertaken with the objective of abiotically enhancing such small enantiomeric excesses. DL Mixtures of leucine N-carboxy anhydride gave enantiomerically enriched polymers on partial polymerization, while valine NCA mixtures behaved oppositely. Leucine polymers were also found to hydrolyze stereoselectively, providing for additional enantiomenic enhancement. A repetitive sequence of partial polymerization-hydrolysis steps is suggested as a possible mechanism for the abiotic genesis of optically enriched polypeptides on the primitive Earth. 相似文献
105.
106.
107.
108.
109.
From February 1972 to August 1974 ten immatureCebus albifrons monkeys were weighed and vaginal swabbing performed at monthly or shorter intervals to determine age and weight at the onset
of puberty. The average weight (±S.E.M.) at birth and at puberty was 226±5.8 g and 1,617±32.45 g, respectively. The average
age at puberty was 3.59±0.17 years. The average weight velocity for all ten monkeys shows the maximum rate of weight gain
to occur shortly after birth and decrease rapidly to its smallest prepubertal increment at nine months of age (weaning). From
nine months there is a post-weaning weight spurt which reaches its greatest velocity at an average age of 15 months. Thereafter,
the weight velocity decreases to its lowest level. Individual weight velocity curves of each of the ten animals show a slight
prepubertal weight spurt which is not obvious in the average growth curve. 相似文献
110.
Chin Y. Yeh Peter P. Fu Frederick A. Beland Ronald G. Harvey 《Bioorganic chemistry》1978,7(4):497-506
molecular orbital theoretical calculations performed on the anti and syn diolepoxides (1 and 2) of the potent carcinogen benzo[a]pyrene provide insight into the molecular structure and reactivity of these mutagenic and carcinogenic hydrocarbon metabolites. Hydrogen-bonded interaction between the 7-HO proton and the epoxide oxygen atom of 2 is shown to be absent in the normal semichair conformation of the tetrahydro ring, (H…O bond distance = 2.7 Å), but is energetically favored in a somewhat distorted puckered structure (H…O bond distance = 1.7 Å). Unexpectedly, internal H-bonding alters the relative electron density at C9 and C10, leading to prediction of the former as the more electrophilic center. Since all reactions of 2 take place exclusively at C10, transannular H-bonding is concluded not to contribute significantly to the structure of 2. Diolepoxide reactions with both weak and strong nucleophiles and with DNA are discussed and the mechanisms interpreted in terms of molecular structure as determined by the theoretical calculations. 相似文献