全文获取类型
收费全文 | 808084篇 |
免费 | 88946篇 |
国内免费 | 557篇 |
专业分类
897587篇 |
出版年
2018年 | 7644篇 |
2017年 | 7161篇 |
2016年 | 10376篇 |
2015年 | 14299篇 |
2014年 | 16693篇 |
2013年 | 23677篇 |
2012年 | 26517篇 |
2011年 | 26899篇 |
2010年 | 18265篇 |
2009年 | 16621篇 |
2008年 | 23766篇 |
2007年 | 24344篇 |
2006年 | 22960篇 |
2005年 | 22138篇 |
2004年 | 22031篇 |
2003年 | 21148篇 |
2002年 | 20370篇 |
2001年 | 38362篇 |
2000年 | 38572篇 |
1999年 | 30633篇 |
1998年 | 10706篇 |
1997年 | 11202篇 |
1996年 | 10512篇 |
1995年 | 9821篇 |
1994年 | 9552篇 |
1993年 | 9335篇 |
1992年 | 24777篇 |
1991年 | 24033篇 |
1990年 | 23424篇 |
1989年 | 22793篇 |
1988年 | 21153篇 |
1987年 | 19753篇 |
1986年 | 18310篇 |
1985年 | 18134篇 |
1984年 | 15043篇 |
1983年 | 12547篇 |
1982年 | 9515篇 |
1981年 | 8489篇 |
1980年 | 7946篇 |
1979年 | 13485篇 |
1978年 | 10493篇 |
1977年 | 9432篇 |
1976年 | 8537篇 |
1975年 | 9496篇 |
1974年 | 10128篇 |
1973年 | 10026篇 |
1972年 | 8955篇 |
1971年 | 8167篇 |
1970年 | 6971篇 |
1969年 | 6732篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
932.
933.
M G Lee 《Molecular and cellular biology》1996,16(3):1220-1230
934.
K. Sasikala Ch.V. Ramana P. Raghuveer Rao M. Subrahmanyam 《FEMS microbiology letters》1990,72(1-2):23-28
Abstract Photoproduction of hydrogen, nitrogenase activity (acetylene reduction) and hydrogenase activity (methylene blue dye reduction) were studied in free and alginate immobilized whole cells of a purple non-sulfur photosynthetic bacterium Rhodobacter sphaeroides O.U. 001. Four-fold increase in hydrogen production, two-fold increase in nitrogenase activity and 1.2-fold increase in the hydrogenase activity were observed in immobilized cells compared to free cells. Effect of various inhibitors (CO and C2 H2 ) and electron donor (H2 ) on the above three functions by free and immobilized cells has also been studied. 相似文献
935.
An assessment was made of two methods for determining the potency of tissue-type plasminogen activator (TPA). A chromogenic microtitre plate assay was established which contained TPA, plasminogen, a synthetic plasmin substrate (H-D-valyl-L-leucyl-L-lysyl-p-nitroaniline dihydrochloride, S2251) and any one of the following stimulators: native fibrinogen, enzymatic and chemical digests of fibrinogen, poly-D-lysine (PDL) and chemical derivatives of the latter. The chromogen assay was compared with an automated clot-lysis (turbidimetric) assay for sensitivity, reproducibility and validity for potency determination. Reference preparations of TPA were titrated in both assays: in the chromogen assay the dose-response curves were non-parallel, whereas parallelism was observed in the clot-lysis assay. Thus, the chromogen assay was restricted in its applicability and disqualified from any routine regulatory use. The potency of individual lots of recombinant (r)TPA could only be estimated in International Units (IU) of TPA activity with the automated clot-lysis assay and the potency values obtained (IU/vial) were in remarkably close agreement with the manufacturers' values. 相似文献
936.
The objectives of this study were to evaluate the physical structure and the release mechanisms of theophylline microspheres
made of Eudragit S 100 polymer as an enteric polymer, combined with a nonerodible polymer, Eudragit RL 100. In the preparation
process, polymer combinations (1:1) were dissolved in an organic solvent mixture composed of acetone and methanol at a specific
ratio containing a theoretical drug loading of approximately 15%. Two microsphere formulations (LS1 and LS2) were prepared
at two different total polymer concentrations (10% in LS1 and 12.7% in LS2). Dissolution studies were carried out using US
Pharmacopeia Dissolution Apparatus II in an acidic medium for 8 h and in an acidic medium (2 h) followed by a slightly basic-buffered
medium for 10 h. Both LS1 and LS2 microsphere formulations produced particles that were spherical in shape and had very narrow
size distributions with one size fraction comprising 70–80% of the yield. Scanning electron microscopy and quantitative Fourier
transform infrared were used for microsphere physical structure evaluation. Except for the absence of drug crystals, photomicrographs
of both LS microspheres after dissolution in pH 1.2 and 7.2 buffer solutions were similar to those before dissolution. Dissolution
results indicated the ability of LS microspheres to minimize drug release during the acid stage. However, in the slightly
basic medium that followed the acidic stage, the drug release was sustained and controlled in its kinetics and data fitted
to Peppas equation indicated a case II transport suggesting that the drug release is mainly through swelling/erosion mechanism. 相似文献
937.
Relative growth rate correlates negatively with pathogen resistance in radish: the role of plant chemistry 总被引:2,自引:0,他引:2
E. HOFFLAND G. J. NIEMANN J. A. VAN PELT J. B. M. PUREVEEN G. B. EIJKEL J. J. BOON H. LAMBERS 《Plant, cell & environment》1996,19(11):1281-1290
Plant growth rate has frequently been associated with herbivore defence: a large investment in quantitative defence compounds occurs at the expense of growth. We tested whether such a relationship also holds for growth rate and pathogen resistance. For 15 radish (Raphanus sativus L.) cultivars, we determined the potential growth rate and the resistance to fungal wilt disease caused by Fusarium oxysporum. We subsequently aimed to explain a putative negative relationship between growth rate and resistance based on plant chemical composition. Both growth rate and resistance level varied greatly among cultivars. Moreover, there was a strong negative correlation between growth rate and resistance, i.e. there are costs associated with a high resistance level. Roots of slow-growing, resistant cultivars have a higher biomass density. Using pyrolysis mass spectrometry. we part1y explained variation in both growth rate and resistance in terms of the same change in chemical composition. Leaves of slow-growing, resistant cultivars contained more cell wall material. Surprisingly, roots of slow-growing, highly resistant cultivars contained significantly less cell wall material, and more cytoplasmic elements (proteins). We speculate that this higher protein concentration is related to high construction and turn-over costs and high metabolic activity. The latter in turn is thought to be responsible for a rapid and adequate resistance reaction, in which phenols may be involved. 相似文献
938.
A numerical model is described for the oxygenation of blood in lung capillaries by considering the transport mechanisms of molecular diffusion, convection and the facilitated diffusion due to the presence of haemoglobin. In order to represent the oxygen dissociation curve accurately in the model, the nth order one-step kinetics of oxygen uptake by haemoglobin has been used. The resulting system of coupled, non-linear partial differential equations is solved numerically. It is shown that the blood is required to traverse a larger distance in the capillary before becoming fully oxygenated with nth order one-step kinetics in comparison to first-order one-step kinetics. 相似文献
939.
A preliminary analysis of distribution patterns in a large, pantropical genus, Barleria L. (Acanthaceae) 总被引:1,自引:0,他引:1
Barleria L. (Acanthaceae) is a large, polymorphic, widespread genus of herbs and shrubs comprising about 300 species, occurring mainly in Africa and Asia but with one species, Barleria oenotheroides Dum.Cours., extending to the New World tropics. Recent completion of a monographic infra-generic classification of the genus (in which seven sections are recognised, and the names of four of these validated in this paper—see Appendix 1), has facilitated a comprehensive analysis of distribution patterns on a global scale. The richest representation of Barleria is in Africa where there are two centres of diversity, one in tropical East Africa (about eighty species) and the other in southern Africa (about seventy species). The number of species tails off rapidly to both the Far East and the West. Barleria shows a marked trans-Atlantic disjunction between West Africa and the Neotropics, with B. oenotheroides shared by these two regions. This type of disjunction, which is known in other genera of the family, cannot be adequately explained in Barleria on the basis of long-distance dispersal or past continental movements. There is a high degree of regional endemism (e.g. 75% for the Indian subcontinent) at both the species and sectional levels within this genus. The degree of similarity between regions is correspondingly low. The endemics in each region tend to belong to only one or a few of the sections. There are few truly widespread taxa within the genus. East and West Africa are the only regions in which all sections are represented. Sections Barleria and Prionitis C.B. Cl. are the most widespread in the genus; Sections Somalia (Oliv.) Lindau, Fissimura M. Balkwill and Stellatohirtae M. Balkwill are mainly restricted to Africa and Sections Chrysothrix M. Balkwill and Cavirostrata M. Balkwill are the most restricted, occurring mainly in India and Sri Lanka. On a local scale, many of the species show highly restricted, clumped distributions; this is apparently related to particular soil types and possibly to the short-distance, ballistic mode of seed dispersal. This account of the biogeography of Barleria is to be regarded as preliminary, as much taxonomic work at the species level remains to be done before a full-scale cladistic biogeographic account can be undertaken. Particular areas worthy of future investigation include establishing the centre of origin of the genus and investigating the basis for the high degree of endemism shown by many of the species. 相似文献
940.
We have isolated 18 polymorphic microsatellite loci for Cophixalus ornatus from genomic libraries enriched for (AAAG)n, (AACC)n and (AAGG)n repetitive elements. The number of alleles ranges from five to 22 per locus with the observed heterozygosity ranging from 0.10 to 0.92. These markers will be useful for the analysis of population structure in C. ornatus and testing alternative models of speciation. 相似文献