全文获取类型
收费全文 | 3268篇 |
免费 | 321篇 |
国内免费 | 3篇 |
专业分类
3592篇 |
出版年
2023年 | 11篇 |
2022年 | 24篇 |
2021年 | 62篇 |
2020年 | 34篇 |
2019年 | 39篇 |
2018年 | 43篇 |
2017年 | 40篇 |
2016年 | 80篇 |
2015年 | 123篇 |
2014年 | 137篇 |
2013年 | 189篇 |
2012年 | 278篇 |
2011年 | 232篇 |
2010年 | 162篇 |
2009年 | 144篇 |
2008年 | 211篇 |
2007年 | 195篇 |
2006年 | 213篇 |
2005年 | 170篇 |
2004年 | 214篇 |
2003年 | 155篇 |
2002年 | 154篇 |
2001年 | 51篇 |
2000年 | 25篇 |
1999年 | 51篇 |
1998年 | 37篇 |
1997年 | 34篇 |
1996年 | 23篇 |
1995年 | 25篇 |
1994年 | 21篇 |
1993年 | 33篇 |
1992年 | 28篇 |
1991年 | 21篇 |
1990年 | 23篇 |
1989年 | 28篇 |
1988年 | 25篇 |
1987年 | 19篇 |
1986年 | 19篇 |
1985年 | 28篇 |
1984年 | 13篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 9篇 |
1979年 | 12篇 |
1976年 | 10篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1969年 | 15篇 |
1968年 | 8篇 |
排序方式: 共有3592条查询结果,搜索用时 15 毫秒
1.
2.
Investing in ecosystem health: Using rivers as a case study 总被引:2,自引:0,他引:2
Jeff Bennett 《Ecological Management & Restoration》2002,3(2):104-107
3.
4.
Marianne E. Walsh Charles M. Collins Thomas F. Jenkins Alan D. Hewitt Jeff Stark Karen Myers 《Soil & Sediment Contamination》2003,12(5):631-645
Fort Greely, Alaska has an extensive complex of weapon training and testing areas located on lands withdrawn from the public domain under the Military Lands Withdrawal Act (PL106-65). The Army has pledged to implement a program to identify possible munitions contamination. Because of the large size (344,165,000 m2) of the high hazard impact areas, characterization of these constituents will be difficult. We used an authoritative sampling design to find locations most likely to contain explosives-residues on three impact areas. We focused our sampling on surface soils and collected multi-increment and discrete samples at locations of known firing events and from areas on the range that had craters, pieces of munitions, targets, or a designation as a firing point. In the two impact areas used primarily by the Army, RDX was the most frequently detected explosive. In the impact area that was also used by the Air Force, TNT was the most frequently detected explosive. Where detected, the explosives concentrations generally were low (<0.05 mg/kg) except in soils near low-order detonations, where the explosive-filler was in contact with the soil surface. These low-order detonations potentially can serve as localized sources for groundwater contamination if positioned in recharge areas. 相似文献
5.
Jeff A Johnson Heather RL Lerner Pamela C Rasmussen David P Mindell 《BMC evolutionary biology》2006,6(1):65-12
Background
Populations of the Oriental White-backed Vulture (Gyps bengalensis) have declined by over 95% within the past decade. This decline is largely due to incidental consumption of the non-steroidal anti-inflammatory veterinary pharmaceutical diclofenac, commonly used to treat domestic livestock. The conservation status of other Gyps vultures in southern Asia is also of immediate concern, given the lack of knowledge regarding status of their populations and the continuing existence of taxonomic uncertainties. In this study, we assess phylogenetic relationships for all recognized species and the majority of subspecies within the genus Gyps. The continuing veterinary use of diclofenac is an unknown but potential risk to related species with similar feeding habits to Gyps bengalensis. Therefore, an accurate assessment of the phylogenetic relationships among Gyps vultures should aid in their conservation by clarifying taxonomic uncertainties, and enabling inference of their respective relatedness to susceptible G. bengalensis. 相似文献6.
Exercise thermoregulation after 14 days of bed rest 总被引:1,自引:0,他引:1
7.
The preparation, by the phosphotriester approach, of d[C-T-A-T-T-C-C-A-G-A-A-G-T] from one tetranucleoside triphosphate and three trinucleoside diphosphate blocks is described. The use of the o-dibromomethylbenzoyl (DBMB) protecting group in oligodeoxyribonucleotide synthesis is described for the first time. Internucleotide linkages are protected by o-chlorophenyl groups which are finally removed by treatment with the N1, N1, N3, N3-tetramethylguanidinium salt of syn-4-nitrobenzaldoxime. The first phosphorylation step (leading to phosphodiester intermediates) is carried out by treatment with o-chlorophenyl phosphorodi-(1,2,4-triazolide) followed by treatment with water and triethylamine. 1-Mesitylenesulphonyl-3-nitro-1,2,4-triazole (MSNT) is used as the activating agent in the second phosphorylation step in which 5'-protected mono- and di-nucleotides are condensed with nucleoside building blocks containing unprotected 3'-hydroxy functions. 相似文献
8.
William D. Reese 《Brittonia》1987,39(2):225-237
Refinements in the taxonomy of the genus Calymperes (Musci: Calymperaceae) allow increasingly accurate estimates of the total number of species worldwide and new assessments of world ranges of the species. Instead of the 272 correct names of Calymperes listed in Volume 1 of Index Muscorum, the world total of “good” species now appears to be in the range of 30 to 40, and contrary to the high degree of narrow endemism implied from the compilation of Index Muscorum, many species of Calymperes have broad ranges in the paleo- or neotropics. A few species, including C. afzelii, C. erosum, C. palisotii, C. lonchophyllum, and C. tenerum, are pantropical. World ranges of species of Calymperes are mapped and implications for historical patterns of dispersal and speciation are discussed. Suites of endemic species and species groups in the neotropics differ from those in the paleotropics, demonstrating different ancestries, a long period of geographic separation, and independent evolution in two fundamentally different environments. It is suggested that Calymperes shares close phylogenetic relationship with limbate Syrrhopodon. 相似文献
9.
Effects of mot gene expression on the structure of the flagellar motor 总被引:33,自引:0,他引:33
Direct freezing procedures have enabled us to visualize distinctive intramembrane particle ring structures in the cytoplasmic membranes of peritrichously flagellated bacteria by means of freeze-fracture electron microscopy. These structures were identified as flagellar motor components because their distribution matched that of flagella, and because they were absent in non-flagellated mutants of Escherichia coli. Particle rings were present in both the Gram-positive Streptococcus and the Gram-negative E. coli. In E. coli, a non-functional mocha operon produced flagellated but immotile cells lacking the particle rings. Simultaneous introduction of the motA and motB genes, led to recovery of both motility and the ring structures but neither gene alone was sufficient. The concomitant loss of the rings and motility is consistent with the ring particles having a central role in the flagellar motor. 相似文献
10.
F J Hornicek D H Reese H K Lo T I Malinin G I Malinin 《Biochemical and biophysical research communications》1988,154(2):606-612
The blastogenic transformation of lymphocytes by periodic acid was investigated to determine if blastogenesis induced by this mitogen was preceded by phosphoinositide turnover as previously shown for the lectins. Although periodate oxidation stimulated nucleic acid synthesis and interleukin-2 production, no changes in phosphoinositide turnover could be detected when compared to control lymphocyte cultures. These data indicate that increased phosphoinositide turnover is not an absolute prerequisite for lymphocyte proliferation. 相似文献