全文获取类型
收费全文 | 21047篇 |
免费 | 2103篇 |
国内免费 | 18篇 |
专业分类
23168篇 |
出版年
2022年 | 204篇 |
2021年 | 429篇 |
2020年 | 233篇 |
2019年 | 299篇 |
2018年 | 384篇 |
2017年 | 310篇 |
2016年 | 452篇 |
2015年 | 880篇 |
2014年 | 856篇 |
2013年 | 1183篇 |
2012年 | 1496篇 |
2011年 | 1447篇 |
2010年 | 910篇 |
2009年 | 826篇 |
2008年 | 1166篇 |
2007年 | 1186篇 |
2006年 | 1098篇 |
2005年 | 1150篇 |
2004年 | 1084篇 |
2003年 | 1008篇 |
2002年 | 1014篇 |
2001年 | 215篇 |
2000年 | 165篇 |
1999年 | 260篇 |
1998年 | 275篇 |
1997年 | 182篇 |
1996年 | 169篇 |
1995年 | 158篇 |
1994年 | 149篇 |
1993年 | 170篇 |
1992年 | 155篇 |
1991年 | 144篇 |
1990年 | 136篇 |
1989年 | 162篇 |
1988年 | 153篇 |
1987年 | 131篇 |
1986年 | 123篇 |
1985年 | 146篇 |
1984年 | 142篇 |
1983年 | 139篇 |
1982年 | 186篇 |
1981年 | 192篇 |
1980年 | 143篇 |
1979年 | 115篇 |
1978年 | 136篇 |
1977年 | 104篇 |
1976年 | 107篇 |
1975年 | 91篇 |
1974年 | 99篇 |
1973年 | 84篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
William R. Tolbert Joseph Peder Richard C. Kimes 《In vitro cellular & developmental biology. Plant》1981,17(10):885-890
Summary A system has been developed for growth and maintenance of mammalian cells in suspension culture at high density. In principle,
the maintenance of constant levels of required nutrients coupled with the removal of toxic cell byproducts can support much
higher suspension cell densities than may be obtained in conventional spinners. The system consisted of 4- or 40-liter reaction
vessels equipped with a vertically supported rotating cylindrical filter. Agitation was provided by the magnetically driven,
rotating filter. Fresh medium was supplied at a rate of 10 to 20 ml/h per 109 cells and the expended medium free of cells was withdrawn through the rotating filter. Both pH and dissolved O2 and CO2 were monitored and regulated. Walker 256 carcinosarcoma cells have been grown in these reactors to densities 10-to 30-fold
greater than that obtained in Bellco spinners. In addition to high cell densities, the yield of cells per liter of medium
used was 2- to 3-fold that obtained in the conventional systems. Both 4-and 40-liter versions of this reactor have been operated
without the use of antibiotics. The 40-liter reactor also has been modified for chemostat operation. In a single run, for
example, the Walker cell density was maintained between 6 and 10×106 cells/ml with a total yield of 8.7×1011 cells from 360 liters of medium. 相似文献
83.
The effect of human peripheral lymph on cell growth in vitro 总被引:1,自引:0,他引:1
84.
Elaine J. Davis Joel M. Blatt Eva K. Henderson Joseph J. Whittaker Julius H. Jackson 《Molecular & general genetics : MGG》1977,156(3):239-249
Summary Spontaneous mutants (146) of Escherichia coli K-12 were selected that were resistant to inhibition of growth by 1.2 mM L-valine (Valr). The Valr isolates, containing acetohydroxy acid synthase resistant to feedback inhibition by L-valine (AHASr), were classed according to cotransduction of the mutation with leu. Several mutations resulting in an AHASr phenotype were found to be cotransducible with glyA. However, no mutations causing a Valr phenotype were linked to ilv. AHAS activity was more closely examined in representatives of three classes of mutants with Valr linked to leu, labeled ilv-660, ilv-661, and ilv-662. The ilvE503 allele in E. coli K-12, known to cause a two- to three-fold derepression of AHAS, was found to affect regulation of synthesis of both valine-sensitive AHAS (AHASs) and AHASr in the mutants containing ilv-660 and ilv-661, whereas it affected repression of AHASs, only, in the mutant containing ilv-662. Further, both AHASs and AHASr in the ilv-661 mutant were repressed by valine, whereas valine did not repress AHASr synthesis in the strain carrying ilv-660 and only partially repressed AHASr in the strain carrying ilv-662. Unexpectedly, AHASr synthesis in strains carrying ilv-660 or ilv-662 was repressible by leucine. The ilv-660 locus appears to be similar in position to ilvH and encodes a product that confers valine-sensitivity upon AHAS activity in the wild-type E. coli K-12. The ilv-660 and ilv-662 loci may normally encode products that influence both the feedback sensitivity of AHAS and control of AHAS biosynthesis. 相似文献
85.
The effects of salinity, temperature, and N-fertilization on growth and yield of Rhodes grass were investigated. High levels of all three factors had a negative effect on tillering and consequently on the total yield. Though high salt-stress lowered the yields, it also lowered the content of free-NO3
– in the shoots, thus, raising the quality of the crops obtained.In partial fulfillment of the requirements for the degree of M.Sc. at The Tel Aviv University.In partial fulfillment of the requirements for the degree of M.Sc. at The Tel Aviv University. 相似文献
86.
Leaf pretreatment with senescence retardants as a basis for oat protoplast improvement 总被引:1,自引:0,他引:1
Kaur-Sawhney Ravindar; Adams Whitney R. Jr.; Tsang Joseph; Galston Arthur W. 《Plant & cell physiology》1977,18(6):1309-1317
Protoplasts obtained from oat leaves floated on buffer for 18hr show high nuclease activity, low rates of incorporation ofamino acids and nucleosides into macromolecules, and high ratesof spontaneous lysis. Addition to the leaf flotation mediumof the senescence retardants cycloheximide or kinetin, of thedibasic amino acids L-lysine or L-arginine, or of the diaminesputrescine or cadaverine reduces the rise in nuclease activityand spontaneous lysis of protoplasts, and increases the rateor extent of presumptive protein and nucleic acid synthesis.The diamines, which also retard chlorophyll degradation in theexcised leaves, appear to act both on the membrane and on systemscontrolling macromolecular synthesis and breakdown. By contrast,the senescence promoter L-serine hastens chlorophyll degradationfrom excised leaves and does not improve protoplasts derivedfrom those leaves. (Received July 4, 1977; ) 相似文献
87.
Sequential events characterize the interaction of viruses with parenchymal cells, and acute lytic infections of tissues and organs have broad biological attributes. A knowledge of these permits a keener understanding of persistent, intermittent herpesvirus infections and persistent, continuous respiratory virus infections. In addition to unique biochemical mechanisms which may permit the latter chronic infections to evolve, the roles of defective and mutant strains of virus, viral interference, and the genetic, developmental and immunological expressions of the host are of considerable and provocative importance.The traditional view of viral infections embraces a broad spectrum of acute pathological and inflammatory events. The relationship of measles virus to subacute sclerosing panencephalitis, the elucidation of the latency of herpes simplex virus, and the slow unmasking of the pathogenesis of multiple sclerosis have illustrated the subtle elements of persistent viral infections of the human being. These chronic neurological diseases have provided the opportunity and stimulus for sharp dissection of the biological and biochemical processes which embellish the logical link of viral infections to other forms of chronic human illness. 相似文献
88.
89.
32P labelled 5S RNA isolated fromMycobacterium smegmatis was digested withT 1 and pancreatic ribonucleases separately and fingerprinted by two dimensional high voltage electrophoresis on thin-layer DEAE-cellulose plates. The radioactive spots were sequenced and their molar yields were determined. The chain length of the 5S RNA was found to be 120. It showed resemblances to both prokaryotic and eukaryotic 5S RNAs. 相似文献
90.
Joseph Westermeyer 《Culture, medicine and psychiatry》1978,2(2):139-150
Opium addiction has been reported among virtually all large ethnic groups in Asia. Conspicuous by its absence has been any mention of addiction among the Lao, a people surrounded by poppy-growing tribal groups. A sample of Lao patient-addicts are here compared to expatriate Asian addicts in Laos.Lao and expatriate addicts show marked similarity in their sociodemographic profiles and patterns of narcotic use. Some differences in their recent use of narcotic drugs appear related to the greater cash income of the expatriate Asians and their greater access to heroin. No specifically cultural factors for explaining ethnic differences in addiction have yet been identified.Acknowledgement is expressed to Dr. Charles Weldon, Dr. Chomchan Soudaly, Mr. Larry Berger, and Mr. Boun Ke for their support and assistance in this study. Ms. Grace Peng and Ms. Beth Stone assisted in the tabulation and analysis of the data. The project was supported by the Minnesota Medical Foundation, the International Programs Office at the University of Minnesota, and the National Institute of Drug Abuse (grant no. 5 T01 DA 00023-02 and grant no. 1 R01 DA 01599-01). 相似文献