全文获取类型
收费全文 | 7886篇 |
免费 | 781篇 |
专业分类
8667篇 |
出版年
2021年 | 82篇 |
2020年 | 71篇 |
2019年 | 68篇 |
2018年 | 80篇 |
2017年 | 73篇 |
2016年 | 143篇 |
2015年 | 272篇 |
2014年 | 262篇 |
2013年 | 363篇 |
2012年 | 407篇 |
2011年 | 373篇 |
2010年 | 273篇 |
2009年 | 196篇 |
2008年 | 296篇 |
2007年 | 289篇 |
2006年 | 271篇 |
2005年 | 285篇 |
2004年 | 267篇 |
2003年 | 212篇 |
2002年 | 224篇 |
2001年 | 211篇 |
2000年 | 215篇 |
1999年 | 228篇 |
1998年 | 115篇 |
1997年 | 90篇 |
1996年 | 121篇 |
1995年 | 88篇 |
1994年 | 99篇 |
1993年 | 103篇 |
1992年 | 226篇 |
1991年 | 158篇 |
1990年 | 145篇 |
1989年 | 190篇 |
1988年 | 169篇 |
1987年 | 150篇 |
1986年 | 130篇 |
1985年 | 150篇 |
1984年 | 122篇 |
1983年 | 109篇 |
1982年 | 70篇 |
1981年 | 78篇 |
1980年 | 73篇 |
1979年 | 92篇 |
1978年 | 75篇 |
1977年 | 62篇 |
1975年 | 56篇 |
1974年 | 99篇 |
1973年 | 65篇 |
1972年 | 68篇 |
1971年 | 66篇 |
排序方式: 共有8667条查询结果,搜索用时 0 毫秒
111.
Detecting marine invasions can be challenging, especially for lesser-known taxa, and requires (a) thorough field surveys of the region of interest for members of the taxon, (b) systematic analyses to identify all species found, (c) literature searches for the worldwide distribution of these species and for previous records of the taxon in this region, and (d) application of rigorous criteria to assess whether each species found is native or introduced. We carried out these steps in order to detect and document kamptozoan (entoproct) invasions on the American mid-Atlantic coast. We report on the occurrence of two colonial kamptozoans (Barentsia benedeni, Loxosomatoides laevis) in Chesapeake Bay (Maryland and Virginia, USA). On the American Atlantic coast, B. benedeni had previously only been reported from Massachusetts, although this species has a worldwide distribution in bays and harbors. The genus Loxosomatoides had not previously been reported from North America and L. laevis was known only from India. Since the genus Loxosomatoides was very poorly characterized, we briefly review all four of its species, which differ only slightly from each other. We have also synonymized L. japonicum with L. laevis. We did not find any of the kamptozoan species previously recorded in surveys of Chesapeake Bay and the American Atlantic coast. This is the first detailed consideration of anthropogenic influences on kamptozoan distributions, and we emphasize that most kamptozoan species are cryptogenic pending further investigation. 相似文献
112.
Mutations in the arylsulfatase A pseudodeficiency allele causing metachromatic leukodystrophy. 下载免费PDF全文
V Gieselmann A L Fluharty T Tnnesen K Von Figura 《American journal of human genetics》1991,49(2):407-413
We identified a patient suffering from late infantile metachromatic leukodystrophy who genetically seemed to be homozygous for the mutations signifying the arylsulfatase A pseudodeficiency allele. Homozygosity for the pseudodeficiency allele is associated with low arylsulfatase A activity but does not cause a disease. Analysis of the arylsulfatase A gene in this patient revealed a C----T transition in exon 2, causing a Ser 96----Phe substitution in addition to the sequence alterations causing arylsulfatase A pseudodeficiency. Although this mutation was found only in 1 of 78 metachromatic leukodystrophy patients tested, five more patients were identified who seemed hetero- or homozygous for the pseudodeficiency allele. The existence of nonfunctional arylsulfatase A alleles derived from the pseudodeficiency allele calls for caution when the diagnosis of arylsulfatase A pseudodeficiency is based solely on the identification of the mutations characterizing the pseudodeficiency allele. 相似文献
113.
Peter J. M. van den Elzen Jeffrey Townsend Kathleen Y. Lee John R. Bedbrook 《Plant molecular biology》1985,5(5):299-302
Summary We show here that plant cells are sensitive to the antibiotic hygromycin-B4. We also show that a chimaeric gene consisting of the nopaline synthase (nos) gene regulatory elements and the E. coli derived hygromycin phosphotransferase (hpt) gene, when transferred to plants' cells, confers resistance to hygromycin B. The chimaeric nos-hpt gene enables efficient selection of DNA transfer to plant cells when used in conjunction with Ti plasmid-derived binary vectors in cocultivation experiments. 相似文献
114.
115.
116.
ATP-dependent translocation of amino phospholipids across the human erythrocyte membrane 总被引:5,自引:0,他引:5
Trace amounts of radiolabeled phospholipids were inserted into the outer membrane leaflet of intact human erythrocytes, using a non-specific lipid transfer protein. Phosphatidylcholine, phosphatidylserine and phosphatidylethanolamine were transferred from the donor lipid vesicles to the membrane of the intact red cell with equal ease, whilst sphingomyelin was transferred 6-times less efficiently. The transbilayer mobility and equilibrium distribution of the labeled phospholipids were assessed by treatment of the intact cells with phospholipases. In fresh erythrocytes, the labeled amino phospholipids appeared to move rapidly towards the inner leaflet. The choline phospholipids, on the other hand, approached an equilibrium distribution which strongly favoured the outer leaflet. In ATP-depleted erythrocytes, the relocation of the amino phospholipids was markedly retarded. 相似文献
117.
Direct assay for phosphotransacetylase and acetyl-coenzyme A carboxylase by high-performance liquid chromatography 总被引:1,自引:0,他引:1
B De Spiegeleer G Mannens G Slegers W Van den Bossche A Claeys 《Analytical biochemistry》1986,158(1):195-200
A simple and specific assay to measure the activity of two coenzyme A derivative-processing enzymes, i.e., phosphotransacetylase (EC 2.3.1.8) and acetyl-coenzyme A carboxylase (EC 6.4.1.2), is described. The assay is based on the HPLC analysis of the short-chain coenzyme A derivatives formed by the enzymatic reaction, viz., acetyl-CoA and malonyl-CoA. For this purpose, ion-pair reversed-phase HPLC conditions are optimized. Furthermore, the influence of several variables on the enzyme reaction is studied in order to get maximum activity. Due to its short analysis time, good selectivity, and chromatogram information, HPLC proves to be an excellent method for the assay of these enzymes. 相似文献
118.
Jan Van den Bulcke Joris Van Acker Marc Stevens 《International biodeterioration & biodegradation》2006,57(4):229-238
A computer technique for assessing blue-stained coated wood has been implemented for evaluating the discoloration of coatings and analysing the interior wood staining of samples subjected to testing according to European Standard EN 152. The comparison of visual assessment and computer-evaluated percentages of blue staining is based on a combination of correlation measures, principal components and cluster analysis. It appears difficult to imitate human evaluation with image processing, since computer ratings represent exact percentages, while subjective evaluations do not. Additionally, more specific techniques for exploring fungal growth in coated wood have been described. As EN 152 was specifically developed for testing efficacy of wood preservatives, a modified test methodology was elaborated for testing the efficacy of wood coatings, called here as the EN 152-reverse method. Furthermore three-dimensional (3D) reconstruction is validated as a tool for in-depth analysis of blue-stain disfigurement. This 3D visualisation indicates important differences in fungal infestation and proves its suitability for blue-stain resistance testing. 相似文献
119.
Although mitral regurgitation (MR) is the most common valvular heart disease, it should be regarded as a complex multifactorial disease that involves 相似文献
120.
An integro-differential equation is proposed to model a general relapse phenomenon in infectious diseases including herpes. The basic reproduction number R(0) for the model is identified and the threshold property of R(0) established. For the case of a constant relapse period (giving a delay differential equation), this is achieved by conducting a linear stability analysis of the model, and employing the Lyapunov-Razumikhin technique and monotone dynamical systems theory for global results. Numerical simulations, with parameters relevant for herpes, are presented to complement the theoretical results, and no evidence of sustained oscillatory solutions is found. 相似文献