全文获取类型
收费全文 | 38680篇 |
免费 | 3176篇 |
国内免费 | 4035篇 |
出版年
2024年 | 42篇 |
2023年 | 715篇 |
2022年 | 736篇 |
2021年 | 1057篇 |
2020年 | 1178篇 |
2019年 | 1542篇 |
2018年 | 1230篇 |
2017年 | 1262篇 |
2016年 | 1354篇 |
2015年 | 1462篇 |
2014年 | 2047篇 |
2013年 | 2727篇 |
2012年 | 1597篇 |
2011年 | 1838篇 |
2010年 | 1512篇 |
2009年 | 1927篇 |
2008年 | 2060篇 |
2007年 | 2033篇 |
2006年 | 1908篇 |
2005年 | 1816篇 |
2004年 | 1602篇 |
2003年 | 1471篇 |
2002年 | 1350篇 |
2001年 | 1053篇 |
2000年 | 920篇 |
1999年 | 933篇 |
1998年 | 752篇 |
1997年 | 705篇 |
1996年 | 635篇 |
1995年 | 683篇 |
1994年 | 668篇 |
1993年 | 549篇 |
1992年 | 547篇 |
1991年 | 495篇 |
1990年 | 403篇 |
1989年 | 367篇 |
1988年 | 368篇 |
1987年 | 316篇 |
1986年 | 264篇 |
1985年 | 286篇 |
1984年 | 314篇 |
1983年 | 196篇 |
1982年 | 240篇 |
1981年 | 155篇 |
1980年 | 135篇 |
1979年 | 128篇 |
1978年 | 93篇 |
1977年 | 54篇 |
1976年 | 52篇 |
1974年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
D. Pangallo K. Chovanov H. Drahovska F. De Leo C. Urzì 《International biodeterioration & biodegradation》2009,63(7):868-872
The aim of this work was the evaluation of fluorescence ITS-PCR (f-ITS) as a molecular tool to analyze the microbial community involved in the biodeterioration of cultural heritage surfaces. As a case study we analyzed by f-ITS ninety-two bacterial strains isolated from a medieval fresco and the surrounding air environment. The internal transcribed spacer between the 16S and 23S rRNA genes was amplified, and then the fluorescently labeled PCR products were separated by capillary electrophoresis. Bacterial strains were identified by 16S rDNA sequencing. The f-ITS electropherograms showed different profiles coherent with the affiliation of the strains at the genus and species levels. Among the isolates obtained from the fresco surface, those belonging to the genus Bacillus were the most prevailing exhibiting 8 different f-ITS profiles. The airborne bacilli exhibited only 2 of these 8 profiles. Staphylococcus were mostly isolated from air and produced 4 different profiles. Pseudomonas isolates presented 3 different profiles, and one of them was typical of Pseudomonas putida. Members of the other genera produced their distinctive profiles. Our results show that f-ITS is a promising molecular tool for the rapid selection and clustering of strains isolated from different sources. 相似文献
72.
F. MONTEALEGRE‐Z 《Journal of evolutionary biology》2009,22(2):355-366
Male katydids (Orthoptera: Tettigoniidae) produce mating calls by rubbing the wings together, using specialized structures in their forewings (stridulatory file, scraper and mirror). A large proportion of species (ca. 66%) reported in the literature produces ultrasonic signals as principal output. Relationships among body size, generator structures and the acoustic parameters carrier frequency (fc) and pulse duration (pd), were studied in 58 tropical species that use pure‐tone signals. A comparative analysis, based on the only available katydid phylogeny, shows how changes in sound generator form are related to changes in fc and pd. Anatomical changes of the sound generator that might have been selected via fc and pd are mirror size, file length and number of file teeth. Selection for structures of the stridulatory apparatus that enhance wing mechanics via file‐teeth and scraper morphology was crucial in the evolution of ultrasonic signals in the family Tettigoniidae. 相似文献
73.
Contrary to theoretical predictions, female 2‐spot ladybirds (Adalia bipunctata) mate many more times than necessary to maintain high fertilisation success and may gain through the acquisition of material or genetic benefits. In order to investigate this mating system in detail, microsatellite markers have been isolated using a modified enrichment technique. Thirty‐nine loci were successfully amplified by polymerase chain reaction (PCR), of which only two were monomorphic. Detailed characterization of ten loci revealed very high levels of polymorphism. These markers are likely to be invaluable tools with which to study population genetics and patterns of paternity in this species. 相似文献
74.
75.
This paper examines family resemblance for five anthropometric measurements (height, weight, triceps skinfold, upper arm circumference relaxed [UACR] and flexed [UACF] and for systolic and diastolic blood pressure in a group of adult Caribbean islanders of primarily African ancestry. Six hypotheses about family resemblance are tested by using path analysis and likelihood ratios. Significant intergenerational transmission is found only for height and UACR. For weight, UACF, and diastolic blood pressure, non-transmissible sibling resemblance is the primary component of family resemblance, although significant marital resemblance exists for diastolic blood pressure. Triceps skinfold and systolic blood pressure show no evidence of any family resemblance. Although results for highly heritable traits such as height are comparable to reports from other populations, measurements with a large contribution from common family environment or residual environmental effects, such as triceps skinfold or blood pressure, have much lower family resemblance in this population than in other populations. We hypothesize that this difference is due to the fact that adult children and their parents do not share a common household in this culture and to the presence of major nonfamilial environmental factors contributing to obesity and hypertension in this population. 相似文献
76.
L B Flander 《American journal of physical anthropology》1978,49(1):103-110
Metric data on 200 sacra of known sex, age and race are analyzed to determine the usefulness of conventional observations for determining sex in this bone. Results of the univariate analysis show that significant sex differences in the sacrum involve primarily the top portion of the bone for both whites and blacks. However, measurements of curvature are important sex differences in the sample of blacks. A new index relating the S1 body to sacral breadth is proposed as more useful in classifying the bones by sex than indices involving other measurements. Discriminant analysis shows that the sample of whites can be analyzed significantly better by this method than by using an index. The choice of univariate or multivariate method must depend on the condition of the bone, and will be influenced to some extent by the race from which the sample is drawn. 相似文献
77.
Laryngeal air sacs are circular out-pocketings, located in the hyoid bone with their ostium in the midline of the anterior part of the larynx. From previous cadaver studies of the rhesus monkey it was deduced that the function of the air sac is to act as a resonating chamber. The present study was designed to test this hypothesis. Recordings were made of three rhesus monkeys before and after surgical removal of the air sac. Spectrographic analysis of the monkeys' vocalizations indicated that differences in formant frequency characteristics between pre-and post-surgical recordings were negligible. This finding suggests that the laryngeal air sac does not play an important role in the resonant properties of the monkeys' vocal tracts. 相似文献
78.
Akinbo A. Adesomoju Joseph I. Okogun Michael P. Cava Patrick J. Carroll 《Phytochemistry》1983,22(11):2535-2536
A tricyclic diterpene ketone, roseadione, has been isolated from Hypoestes rosea and its structure established as a dicyclopenta[a, d]cyclo 相似文献
79.
Claude Courties Roland Perasso Marie-Josèphe Chrétiennot-Dinet Manolo Gouy Laure Guillou Marc Troussellier 《Journal of phycology》1998,34(5):844-849
Ostreococcus tauri Courties et Chrétiennot-Dinet is the smallest described autotrophic eukaryote dominating the phytoplanktonic assemblage of the marine Mediterranean Thau lagoon (France). Its taxonomic position was partly elucidated from ultrastructure and high-pressure liquid chromatography (HLPC) pigment analysis. The sequence analysis of the 18S rDNA gene of O. tauri measured here is available in EMBL Nucleotide Sequence Database (accession number: Y15814) and allowed to clarify its phylogenetic position. O. tauri belongs to the Prasinophyceae and appears very close to Mantoniella, a typical scaly Prasinophyceae, morphologically very different from the naked and coccoid Ostreococcus. An electrophoretic analysis of O. tauri shows that the nucleus contains 10.20 mbp. This small genome, fragmented into 14 chromosomes ranging in size from 300 to 1500 kbp, confirms the minimalist characteristics of Ostreococcus tauri. 相似文献
80.
Selective inhibition of carbonic anhydrase (CA) enzyme is an active area of research for medicinal chemists. In the current account, a hybrid pharmacophore approach was employed to design sulfonamide, amide and amine containing new series of potent carbonic anhydrase II inhibitors. The aromatic fragment associated with pharmacophore was altered suitably in order to find effective inhibitors of CA-II. All the derivatives 4a-4m showed better inhibition compared to the standard acetazolamide. In particular, compound 4l exhibited significant inhibition with IC50 value of 0.01796 ± 0.00036 µM. The chemo-informatics analysis justified that all the designed compounds possess <10 HBA and <5 HBD. The ligands-protein binding analyses showed that 4l confined in the active binding pocket with three hydrogen bonds observed with His63, Asn66 and Thr197 residues. 相似文献