全文获取类型
收费全文 | 26173篇 |
免费 | 1927篇 |
国内免费 | 21篇 |
出版年
2023年 | 140篇 |
2022年 | 342篇 |
2021年 | 649篇 |
2020年 | 398篇 |
2019年 | 521篇 |
2018年 | 689篇 |
2017年 | 558篇 |
2016年 | 912篇 |
2015年 | 1387篇 |
2014年 | 1514篇 |
2013年 | 1891篇 |
2012年 | 2201篇 |
2011年 | 2126篇 |
2010年 | 1342篇 |
2009年 | 1147篇 |
2008年 | 1558篇 |
2007年 | 1463篇 |
2006年 | 1351篇 |
2005年 | 1232篇 |
2004年 | 1157篇 |
2003年 | 1057篇 |
2002年 | 896篇 |
2001年 | 254篇 |
2000年 | 258篇 |
1999年 | 242篇 |
1998年 | 170篇 |
1997年 | 155篇 |
1996年 | 136篇 |
1995年 | 120篇 |
1994年 | 133篇 |
1993年 | 130篇 |
1992年 | 123篇 |
1991年 | 124篇 |
1990年 | 128篇 |
1989年 | 103篇 |
1988年 | 108篇 |
1987年 | 80篇 |
1986年 | 98篇 |
1985年 | 96篇 |
1984年 | 89篇 |
1983年 | 79篇 |
1982年 | 47篇 |
1981年 | 61篇 |
1980年 | 43篇 |
1979年 | 73篇 |
1978年 | 56篇 |
1977年 | 58篇 |
1976年 | 52篇 |
1972年 | 42篇 |
1970年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
Anna Spalletti 《Photochemical & photobiological sciences》2004,3(7):695-699
Irradiation of the ZZ stereoisomer of 1,4-di-(2'-quinolylethenyl)-benzene was found to cause direct adiabatic (one photon-two bond) isomerization to a product having the same lifetime as the EE isomer but a rather different spectrum with respect to that obtained by direct excitation of the EE one. To clarify this unexpected behaviour, the conformational equilibria of the EE stereoisomer have been studied in non-polar solvent by fluorimetry. The most abundant conformers, formed by the hindered rotation of the condensed-ring groups around the quasi-single bond with the ethenic carbons, have been characterized by the selective effect of the excitation energy on the fluorescence spectrum. The combined application of the principal component analysis allowed the separation of the spectral properties of three conformers to be achieved. Information on their structures was obtained by theoretical calculations. The results of the present conformational study clearly indicated that the fluorescence spectrum of the photoproduct of ZZ belongs to a specific component of the conformer mixture of the EE isomer. 相似文献
53.
Anna Nowakowska-Waszczuk Andrzej Sokoŀowski 《Applied microbiology and biotechnology》1987,26(4):363-364
Summary In an air-lift fermenter, the following production was obtained from 125 g sucrose in mineral medium at pH 2.5 : 15.76 g mycelium dry wt, 107.2 g citric acid anhydrous and 0.594 mol CO2 within 138 h (run I) and 13.72 g mycelium dry wt, 114.28 g citric acid and 0.516 mol CO2 within 144 h (run II). Initially, the carbon content of consumed sugar and products did not balance. At the end of fermentation, the carbon content of the products was 0.9%–5.5% higher than that of the consumed sugar. For the purpose of the calculations the carbon content in mycelium was accepted as 0.462.The work was a part of Project No 04.11 CPBP, topic No 2.24 相似文献
54.
A mouse model for poliovirus neurovirulence identifies mutations that attenuate the virus for humans. 总被引:20,自引:16,他引:4 下载免费PDF全文
A mutation in the genome of poliovirus type 3 that is known to reduce neurovirulence in humans similarly reduces neurovirulence in mice when incorporated into a mouse-adapted-human poliovirus recombinant. Viral recombinants with a uracil at nucleotide position 472 in the 5'-noncoding regions of their genomes are unable to replicate in the mouse brain. Viral recombinants with a cytosine at this position are neurovirulent in mice. Neurovirulence of poliovirus in mice may therefore prove to be a useful indicator of the genetic stability of new attenuating mutations created by site-directed mutagenesis. 相似文献
55.
Summary The relationship between ichthyotoxicity and predation-related defensive functional morphology was examined in alcyonacean soft corals of the central and northern regions of the Great Barrier Reef (GBR), Australia. Approximately 170 specimens were assessed encompassing a number of genera within three families: 1) the Alcyoniidae (Lobophytum, Sarcophytum, Sinularia, Cladiella, Parerythropodium, and Alcyonium); 2) Neptheidae (Lemnalia, Paralemnalia, Capnella, Lithophyton, Nephthea, Dendronephthya, Scleronephthya, and Stereonephthya), and 3) Xeniidae (Anthelia, Efflatounaria, Cespitularia, Heteroxenia, and Xenia). Ichthyotoxicity data were derived from earlier studies which used Gambusia affinis Baird and Girard (Vertebrata, Pisces) as a test organism. These data were compared to morphological data collected from specimens in the field and laboratory. Three sets of statistical analyses were performed, each considering a progressively narrower group of taxa. The first included 68 specimens and considered 16 morphological characters in each, falling into the general categories of gross colony form, colony texture, presence of mucus, colony color, polyp retractility, and sclerite morphology and distribution. These were tested for independence against ichthyotoxicity data. The second set of analyses involved a more restricted morphological data set derived from 28 species of Sinularia in combination with 28 species within the Nephtheidae, comparing them to their respective toxicity ranks. The third analysis considered the previous two taxonomic groups separately in relation to their toxicity levels.The attempt to consider many morphological characters in a taxonomically diverse collection did not reveal any general association in the Alcyonacea between defensive morphology and toxicity, and those associations which did emerge were clearly erroneous. The second analysis, considering only Sinularia spp. and nephtheids, demonstrated a negative association between ichthyotoxicity and the morphological characters of a) polypary armament, b) microarmament of the individual polyp, and c) strong mineralization of the coenenchyme. The third analysis revealed that the negative association found between toxicity and the first two characters was derived entirely from the nephtheids while the association detected between toxicity and the third character was restricted to Sinularia. It is concluded that a relationship between toxicity and morphology can be demonstrated, but it is heavily dependent upon which specific morphological characters are being considered and at what level of taxonomic resolution the analysis is being performed. An approach utilizing many characters over many taxa is unlikely to yield significant, reliable, or meaningful results.Australian Institute of Marine Science Contribution Number 383 相似文献
56.
Halińska, A. and Lewak, St. 1987. Free and conjugated gibberellins in dormancy and germination of apple seeds.
The presence of gibberellin A4 (GA4 ) was confirmed in partly stratified seeds of apple ( Malus domestica Borb., cv. Antonówka) by mass spectrometry of the methyl ester. Levels of free and conjugated gibberellins A4+7 and A9 changed during drying of mature seeds, during cold and warm stratification, as well as during germination of dormant and non-dormant embryos. The temporary rise in GA4+7 during cold stratification and during the culture of dormant embryos as well as the lack of it under conditions of warm stratification, allowed us to postulate a role for GA4+7 in the removal of dormancy. In addition, GA9 was absent in dormant embryos and increased during cold stratification and during the culture of non-dormant embryos. This suggests the involvement of GA9 , in induction of normal development of the seedling. The equivalence between changes in free and conjugated GAs suggests that formation and hydrolysis of conjugates are involved in the control of the physiologically active levels of free GA4+7 and GA9 . 相似文献
The presence of gibberellin A
57.
F. Rollo A. La Marca A. Amici 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(4):501-505
Summary Twelve seed specimens of varying ages and from different archaeological sites were analyzed for the presence of polymerized DNA and RNA. Amongst the samples tested, one of Vitis vinifera from an archaeological site in Iran (2,000–3,000 B.C.) was found to be completely devoid of nucleic acids. Zea mais seeds of Precolumbial age from Peru (about 800 A.D.) contained depolymerized DNA and RNA. Samples of Vitis vinifera and Rubus sp. from a Lombard archaeological site (800 A.D.) as well as radiocarbon dated seeds from the site of the Spring Sanctuary near Metaponto (I–IV century B.C.) were found to contain polymerized DNA and rRNA bands. However the electrophoretic properties of the rRNAs in one case and hybridization experiments performed with cloned seed DNA in the other, clearly demonstrated that the polymerized nucleic acids were not of plant origin. 相似文献
58.
59.
Anna Chrtková 《Plant Systematics and Evolution》1987,155(1-4):307-309
A new species,Lotus digii, has been found in Morocco growing on the coastal sandy soils. Further localities are from Algeria and Egypt. It should be expected also in Libya. 相似文献
60.