全文获取类型
收费全文 | 4672篇 |
免费 | 277篇 |
国内免费 | 3篇 |
出版年
2023年 | 23篇 |
2022年 | 62篇 |
2021年 | 109篇 |
2020年 | 68篇 |
2019年 | 81篇 |
2018年 | 132篇 |
2017年 | 96篇 |
2016年 | 195篇 |
2015年 | 240篇 |
2014年 | 244篇 |
2013年 | 360篇 |
2012年 | 402篇 |
2011年 | 386篇 |
2010年 | 256篇 |
2009年 | 164篇 |
2008年 | 271篇 |
2007年 | 269篇 |
2006年 | 243篇 |
2005年 | 211篇 |
2004年 | 201篇 |
2003年 | 178篇 |
2002年 | 174篇 |
2001年 | 51篇 |
2000年 | 42篇 |
1999年 | 41篇 |
1998年 | 38篇 |
1997年 | 30篇 |
1996年 | 25篇 |
1995年 | 28篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 42篇 |
1991年 | 26篇 |
1990年 | 16篇 |
1989年 | 20篇 |
1988年 | 16篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 18篇 |
1984年 | 11篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 8篇 |
排序方式: 共有4952条查询结果,搜索用时 15 毫秒
941.
942.
The removal of conifers through commercial timber harvesting has been successful in restoring aspen, however many aspen stands are located near streams, and there are concerns about potential aquatic ecosystem impairment. We examined the effects of management-scale conifer removal from aspen stands located adjacent to streams on water quality, solar radiation, canopy cover, temperature, aquatic macroinvertebrates, and soil moisture. This 8-year study (2003–2010) involved two projects located in Lassen National Forest. The Pine-Bogard Project consisted of three treatments adjacent to Pine and Bogard Creeks: (i) Phase 1 in January 2004, (ii) Phase 2 in August 2005, and (iii) Phase 3 in January 2008. The Bailey Project consisted of one treatment adjacent to Bailey Creek in September 2006. Treatments involved whole tree removal using track-laying harvesters and rubber tire skidders. More than 80% of all samples analyzed for NO3-N, NH4-N, and PO4-P at Pine, Bogard, and Bailey Creeks were below the detection limit, with the exception of naturally elevated PO4-P in Bogard Creek. All nutrient concentrations (NO3-N, NH4-N, PO4-P, K, and SO4-S) showed little variation within streams and across years. Turbidity and TSS exhibited annual variation, but there was no significant increase in the difference between upstream and downstream turbidity and TSS levels. There was a significant decrease in stream canopy cover and increase in the potential fraction of solar radiation reaching the streams in response to the Pine-Bogard Phase 3 and Bailey treatments; however, there was no corresponding increase in stream temperatures. Macroinvertebrate metrics indicated healthy aquatic ecosystem conditions throughout the course of the study. Lastly, the removal of vegetation significantly increased soil moisture in treated stands relative to untreated stands. These results indicate that, with careful planning and implementation of site-specific best management practices, conifer removal to restore aspen stands can be conducted without degrading aquatic ecosystems. 相似文献
943.
Kristijan Skok Lidija Gradišnik Uroš Maver Nejc Kozar Monika Sobočan Iztok Takač Darja Arko Rajko Kavalar 《Journal of cellular and molecular medicine》2021,25(8):3680-3698
Cell lines are widely used for various research purposes including cancer and drug research. Recently, there have been studies that pointed to discrepancies in the literature and usage of cell lines. That is why we have prepared a comprehensive overview of the most common gynaecological cancer cell lines, their literature, a list of currently available cell lines, and new findings compared with the original studies. A literature review was conducted via MEDLINE, PubMed and ScienceDirect for reviews in the last 5 years to identify research and other studies related to gynaecological cancer cell lines. We present an overview of the current literature with reference to the original studies and pointed to certain inconsistencies in the literature. The adherence to culturing rulesets and the international guidelines helps in minimizing replication failure between institutions. Evidence from the latest research suggests that despite certain drawbacks, variations of cancer cell lines can also be useful in regard to a more diverse genomic landscape. 相似文献
944.
Klichowska Ewelina Ślipiko Monika Nobis Marcin Szczecińska Monika 《Molecular biology reports》2018,45(4):639-643
Molecular Biology Reports - Stipa pennata (Poaceae), has become a rare and endangered species in Central Europe due habitat loss and fragmentation. This species is characterized by high... 相似文献
945.
Inderpaul Singh Sehgal Sahajal Dhooria Nalini Gupta Amanjit Bal Babu Ram Ashutosh Nath Aggarwal Digambar Behera Ritesh Agarwal 《PloS one》2016,11(4)
BackgroundThere is sparse literature on whether training in endobronchial ultrasound (EBUS)-guided transbronchial needle aspiration (TBNA) improves the diagnostic yield of conventional TBNA (cTBNA).ObjectivesThe aim of this study was to evaluate the diagnostic yield of cTBNA before and after the introduction of EBUS.MethodsThis was a retrospective analysis of patients who underwent cTBNA at our center. The study was divided into two periods, before and after the introduction of EBUS at our facility. The diagnostic yield of cTBNA was compared between the study periods. Rapid on-site cytological examination was not available.ResultsA total of 1,050 patients (61.6% men; mean age 45.6 years) underwent cTBNA during the study period (849 before EBUS; 201 after EBUS). Sarcoidosis (n = 527) followed by bronchogenic carcinoma (n = 222) formed the most common indications for performing cTBNA. There was a significant increase in both the success of obtaining a representative sample (from 71% to 85%), and the diagnostic yield (from 33% to 49.5%) of cTBNA, after the introduction of EBUS. The increase in the diagnostic yield of cTBNA after introduction of EBUS remained significant even after adjusting for years of performing cTBNA and the type of anesthesia (topical vs. sedation and topical) on a multivariate analysis.ConclusionThe diagnostic yield of cTBNA at our facility increased after the introduction of EBUS-TBNA. However, given the retrospective nature of the study, prospective studies are required to confirm our findings. 相似文献
946.
947.
Tomasz Molcan Sylwia Swigonska Anna Nynca Agnieszka Sadowska Monika Ruszkowska Karina Orlowska Renata E. Ciereszko 《Biochimica et Biophysica Acta (BBA)/General Subjects》2019,1863(2):291-303
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most difficult to biodegradate and the most toxic dioxin congener. Previously, we demonstrated in silico the ability of pig CYP1A1 to hydroxylate 2,7-dichlorodibenzo-p-dioxin (DiCDD), but not TCDD. To increase our knowledge concerning the low effectiveness of TCDD biodegradability, we analyzed in silico the binding selectivity and affinity between pig CYP1B1 and the two dioxins by means of molecular modeling. We also compared the effects of TCDD and DiCDD on CYP1B1 gene expression (qRT-PCR) and catalytic (EROD) activity in porcine granulosa cells. It was found that DiCDD and TCDD were stabilized within the pig CYP1B1 active site by hydrophobic interactions. The analysis of substrate channel availability revealed that both dioxins opened the exit channel S, allowing metabolites to leave the enzyme active site. Moreover, DiCDD and TCDD increased the CYP1B1 gene expression and catalytic activity in porcine granulosa cells. On the other hand, TCDD demonstrated higher than DiCDD calculated affinity to pig CYP1B1, hindering TCDD exit from the active site. The great distance between CYP1B1's heme and TCDD also might contribute to the lower hydroxylation effectiveness of TCDD compared to that of DiCDD. Moreover, the narrow active site of pig CYP1B1 may immobilize TCDD molecule, inhibiting its hydroxylation. The results of the access channel analysis and the distance from pig CYP1B1's heme to TCDD suggest that the metabolizing potential of pig CYP1B1 is higher than that of pig CYP1A1. However, this potential is probably not sufficiently high to considerably improve the slow TCDD biodegradation. 相似文献
948.
949.
950.
Kozioł Agata Macegoniuk Katarzyna Grela Ewa Grabowiecka Agnieszka Biernat Monika Lochyński Stanisław 《Molecular biology reports》2019,46(1):51-58
Molecular Biology Reports - Urease is an important virulence factor for a variety of pathogenic bacteria strains such as Helicobacter pylori, which colonizes human gastric mucosa, and Proteus sp.,... 相似文献