全文获取类型
收费全文 | 868305篇 |
免费 | 103512篇 |
国内免费 | 891篇 |
出版年
2018年 | 7392篇 |
2016年 | 10031篇 |
2015年 | 14396篇 |
2014年 | 16967篇 |
2013年 | 23649篇 |
2012年 | 26933篇 |
2011年 | 27437篇 |
2010年 | 18292篇 |
2009年 | 16936篇 |
2008年 | 24395篇 |
2007年 | 25436篇 |
2006年 | 23867篇 |
2005年 | 22964篇 |
2004年 | 22886篇 |
2003年 | 21804篇 |
2002年 | 21049篇 |
2001年 | 35968篇 |
2000年 | 36274篇 |
1999年 | 29169篇 |
1998年 | 11018篇 |
1997年 | 11256篇 |
1996年 | 10756篇 |
1995年 | 10334篇 |
1994年 | 10024篇 |
1993年 | 10036篇 |
1992年 | 25079篇 |
1991年 | 24863篇 |
1990年 | 24437篇 |
1989年 | 23712篇 |
1988年 | 22125篇 |
1987年 | 21145篇 |
1986年 | 19928篇 |
1985年 | 20047篇 |
1984年 | 16773篇 |
1983年 | 14699篇 |
1982年 | 11233篇 |
1981年 | 10515篇 |
1980年 | 9791篇 |
1979年 | 16291篇 |
1978年 | 12997篇 |
1977年 | 11760篇 |
1976年 | 11154篇 |
1975年 | 12582篇 |
1974年 | 13433篇 |
1973年 | 13049篇 |
1972年 | 12095篇 |
1971年 | 10872篇 |
1970年 | 9365篇 |
1969年 | 8992篇 |
1968年 | 8217篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Four new species ofCayaponia are described and illustrated: three from Brazil (C. cogniauxiana, C. nitida andC. rugosa) and one from Brazil and Bolivia (C. ferruginea). 相似文献
142.
The Asian Tree Toad genus Pedostibes, as currently understood, exhibits a conspicuously disjunct distribution, posing several immediate questions relating to the biogeography and taxonomy of this poorly known group. The type species, P. tuberculosus and P. kempi, are known only from India, whereas P. hosii, P. rugosus, and P. everetti are restricted to Southeast Asia. Several studies have shown that these allopatric groups are polyphyletic, with the Indian Pedostibes embedded within a primarily South Asian clade of toads, containing the genera Adenomus, Xanthophryne, and Duttaphrynus. Southeast Asian Pedostibes on the other hand, are nested within a Southeast Asian clade, which is the sister lineage to the Southeast Asian river toad genus Phrynoidis. We demonstrate that Indian and Southeast Asian Pedostibes are not only allopatric and polyphyletic, but also exhibit significant differences in morphology and reproductive mode, indicating that the Southeast Asian species’ are not congeneric with the true Pedostibes of India. As a taxonomic solution, we describe a new genus, Rentapia
gen. nov. to accommodate the Southeast Asian species. 相似文献
143.
144.
Genes for the biosynthesis of spinosyns: applications for yield improvement in Saccharopolyspora spinosa 总被引:2,自引:0,他引:2
K Madduri C Waldron P Matsushima M C Broughton K Crawford D J Merlo R H Baltz 《Journal of industrial microbiology & biotechnology》2001,27(6):399-402
Spinosyns A and D are the active ingredients in an insect control agent produced by fermentation of Saccharopolyspora spinosa. Spinosyns are macrolides with a 21-carbon, tetracyclic lactone backbone to which the deoxysugars forosamine and tri-O-methylrhamnose are attached. The spinosyn biosynthesis genes, except for the rhamnose genes, are located in a cluster that
spans 74 kb of the S. spinosa genome. DNA sequence analysis, targeted gene disruptions and bioconversion studies identified five large genes encoding type
I polyketide synthase subunits, and 14 genes involved in sugar biosynthesis, sugar attachment to the polyketide or cross-bridging
of the polyketide. Four rhamnose biosynthetic genes, two of which are also necessary for forosamine biosynthesis, are located
outside the spinosyn gene cluster. Duplication of the spinosyn genes linked to the polyketide synthase genes stimulated the
final step in the biosynthesis — the conversion of the forosamine-less pseudoaglycones to endproducts. Duplication of genes
involved in the early steps of deoxysugar biosynthesis increased spinosyn yield significantly. Journal of Industrial Microbiology & Biotechnology (2001) 27, 399–402.
Received 31 May 2001/ Accepted in revised form 09 July 2001 相似文献
145.
146.
We evaluated the cytotoxic and DNA cross-linking (CL) ability of four second generation platinum coordination complexes (TNO-6, JM-89, JM-8 and JM-9) delivered alone or in combination with 1-beta-D-arabinofuranosyl cytosine (ara-C) to human colon cancer cells (LoVo). Cell survival varied markedly as a function of the particular substitution moiety. JM-8 and JM-9 were virtually ineffective, even at concentrations as high as 50 micrograms/ml. At that concentration cis-diamminedichloroplatinum(II) (cis-DDP) killed greater than 99.99% of the cells. JM-82 was slightly more active while TNO-6 was the only derivative with appreciably higher cytotoxic activity due to an abrogation of the shoulder region of the type C survival curve. The highest CL effect was observed for cis-DDP followed closely by TNO-6. Very little CL effects were demonstrated for the other three analogs JM-82, JM-8 and JM-9 when measured 6 h after treatment. The combination of cis-DDP and ara-C augmented 10-fold the cytotoxic activity of cis-DDP alone, an effect accompanied by an almost 2-fold increase in CL; every other analog failed to interact in a potentiating manner (either cytotoxicity, or CL at 6 h) with the antimetabolite. Thus, it appears clear that the associated moieties of the Pt coordination complex play a fundamental role in reducing the interaction of the analogs with DNA (as reflected by the decreased CL and cytotoxic effects produced by each agent alone) and in totally preventing their interaction with ara-C to yield a potentiating lethal effect. 相似文献
147.
A. V. Nikolayev Yu. V. Kirichek O. I. Ostrovskaya I. I. Maletina K. I. Petko L. M. Yagupol’skii 《Neurophysiology》1999,31(3):191-193
With the use of a patch-clamp technique in the whole-cell configuration, we studied the effects of pinacidil and its fluorine
derivatives on A-type potassium current (I
A) through the membrane of pyramidal neurons of the rat hippocampus. Hydrogen peroxide (10 mM) exerted no influence on the
rate of inactivation ofI
A; therefore, this current is probably mediated by Shal Kv4.2 potassium channels. Pinacidil demonstrated the properties of
a weakI
A blocker: in the 500 μM concentration it blocked about 45% of the current, while 50 μM of pinacidil fluorine derivatives were
capable of blocking up to 30% ofI
A. The effects of pinacidil and its derivatives showed no dependence on the stimulating potential. A similar pattern of the
effects of pinacidil fluorine derivatives, which are an order of magnitude stronger than those of pinacidil itself, allows
us to suppose that the imine nitrogen of the tested compounds is significantly more involved in the molecular interaction
with the site of an A-type potassium channel than the pyridine nitrogen. 相似文献
148.
149.
150.