全文获取类型
收费全文 | 5791篇 |
免费 | 373篇 |
国内免费 | 1篇 |
出版年
2023年 | 48篇 |
2022年 | 111篇 |
2021年 | 228篇 |
2020年 | 150篇 |
2019年 | 180篇 |
2018年 | 214篇 |
2017年 | 212篇 |
2016年 | 287篇 |
2015年 | 366篇 |
2014年 | 418篇 |
2013年 | 484篇 |
2012年 | 513篇 |
2011年 | 448篇 |
2010年 | 275篇 |
2009年 | 220篇 |
2008年 | 307篇 |
2007年 | 258篇 |
2006年 | 234篇 |
2005年 | 197篇 |
2004年 | 198篇 |
2003年 | 159篇 |
2002年 | 155篇 |
2001年 | 78篇 |
2000年 | 54篇 |
1999年 | 55篇 |
1998年 | 31篇 |
1997年 | 21篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 14篇 |
1986年 | 10篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1975年 | 5篇 |
1973年 | 4篇 |
1970年 | 4篇 |
1969年 | 9篇 |
1968年 | 5篇 |
1965年 | 5篇 |
排序方式: 共有6165条查询结果,搜索用时 46 毫秒
991.
Bonomi S Settembrini F Salval A Rapisarda V 《Plastic and reconstructive surgery》2011,128(4):373e-374e; author reply 374e-375e
992.
Population density in cetaceans can be estimated through photo-identification, mark-recapture, land-based observations and visual estimative. We the aim to contribute with conservation strategies, we used line transects (distance method) to estimate the population density of the river dolphin, S. guianensis, in the estuarine region of Cananéia, Southeastern Brazil. The study, developed from May 2003 until April 2004, during dry and rainy seasons and different times of the day, included a sampling area divided into three sectors according to their proximity to the open sea: Sector I (the closest to the open sea); Sector II (with a large flow of fresh water and a salient declivity); and Sector III (with a large flow of fresh water and non salient declivity). Onboard random sampling was carried out in all three sectors, and dolphins seen from the bow to 90 degrees on both port and starboard sides, were registered along with their position and distance from the boat. The total density found was 12.41 ind/km2 (CV = 25.53%) with an average of 2.2 individuals per group for both periods of the day, morning and afternoon. Densities also varied between dry and rainy seasons, being lower in the first with 5.77 ind/km2 (CV = 27.87%) than in the second 20.28 ind/km2 (CV = 31.95%), respectively. Regarding the three sectors, a non-causal heterogeneous distribution was found: Sector I was the most populated (D = 33.10 ind/km2, CV = 13.34%), followed by Sector II (D = 7.8 ind/km2, CV = 21.07%) and Sector III (D = 3.04 ind/km2, CV = 34.04%). The aforementioned area, due to its proximity to the open sea, has the highest salinity level and therefore has the greatest chance of holding most of the marine fish schools which can be cornered by dolphins on high declivity areas during fishing activities. This suggests that food availability may be the most important factor on the river dolphin's distribution in the estuary. Similar studies will contribute to a better understanding of these populations and are essential for future conservation strategies. 相似文献
993.
Studies investigating the impacts of biodiversity loss on ecosystem processes have often reached different conclusions, probably
because insufficient attention has been paid to some aspects including (1) which biodiversity measure (e.g., species number,
species identity or trait) better explains ecosystem functioning, (2) the mechanisms underpinning biodiversity effects, and
(3) how can environmental context modulates biodiversity effects. Here, we investigated how species number (one to three species)
and traits of aquatic fungal decomposers (by replacement of a functional type from an unpolluted site by another from a metal-polluted
site) affect fungal production (biomass acumulation) and plant litter decomposition in the presence and absence of metal stress.
To examine the putative mechanisms that explain biodiversity effects, we determined the contribution of each fungal species
to the total biomass produced in multicultures by real-time PCR. In the absence of metal, positive diversity effects were
observed for fungal production and leaf decomposition as a result of species complementarity. Metal stress decreased diversity
effects on leaf decomposition in assemblages containing the functional type from the unpolluted site, probably due to competitive
interactions between fungi. However, dominance effect maintained positive diversity effects under metal stress in assemblages
containing the functional type from the metal-polluted site. These findings emphasize the importance of intraspecific diversity
in modulating diversity effects under metal stress, providing evidence that trait-based diversity measures should be incorporated
when examining biodiversity effects. 相似文献
994.
Carignatto CR Oliveira KS de Lima VM de Oliva Neto P 《Indian journal of microbiology》2011,51(3):283-288
Xanthan is a important biopolymer for commercial purpose and it is produced in two stages by Xanthomonas campestris. In the first one, the bacterium is cultivated in the complex medium enriched in nitrogen and the biomass produced is used
as inoculum for the next stage in which the gum is produced in another medium. In this work a new medium for the first stage
is proposed in place of currently used YM medium. Different formulated growth media were studied and the correspondent biomass
produced was analysed as inoculum for the second stage. The inoculum and gum were produced by batch process in shaker at 27°C
in pH 6.0 and at 30°C in pH 7.0, respectively. The gum was precipitated with ethanol (3:1 v/v). The dryed biomass and xathan
gum produced were determined by drying in oven at 105 and 40°C, respectively. The viscosity of the fermentation broth and
1% gum solution in water were determined in Brookfield viscometer. The formulated medium presented the increase in gum production
(30%), broth (136%) and 1% gum solution viscosity (60%) compared to YM, besides the inferior cost. The results showed the
importance of the quality of the inoculum from the first stage of the culture which influenced on the gum viscosity in the
second stage. 相似文献
995.
Pires RH Santos JM Zaia JE Martins CH Mendes-Giannini MJ 《Memórias do Instituto Oswaldo Cruz》2011,106(6):646-654
Candida parapsilosis, currently divided into three distinct species, proliferates in glucose-rich solutions and has been associated with infections resulting from the use of medical devices made of plastic, an environment common in dialysis centres. The aims of this study were (i) to screen for Candida orthopsilosis and Candida metapsilosis (100 environmental isolates previously identified as C. parapsilosis), (ii) to test the ability of these isolates to form biofilm and (iii) to investigate the in vitro susceptibility of Candida spp biofilms to the antifungal agents, fluconazole (FLC) and amphotericin B (AMB). Isolates were obtained from a hydraulic circuit collected from a haemodialysis unit. Based on molecular criteria, 47 strains were re-identified as C. orthopsilosis and 53 as C. parapsilosis. Analyses using a formazan salt reduction assay and total viable count, together with microscopy studies, revealed that 72 strains were able to form biofilm that was structurally similar, but with minor differences in morphology. A microtitre-based colorimetric assay used to test the susceptibility of fungal biofilms to AMB and FLC demonstrated that the C. parapsilosis complex displayed an increased resistance to these antifungal agents. The results from these analyses may provide a basis for implementing quality controls and monitoring to ensure the microbiological purity of dialysis water, including the presence of yeast. 相似文献
996.
Vieira CS Almeida DB de Thomaz AA Menna-Barreto RF dos Santos-Mallet JR Cesar CL Gomes SA Feder D 《Memórias do Instituto Oswaldo Cruz》2011,106(2):158-165
Semiconductor nanoparticles, such as quantum dots (QDs), were used to carry out experiments in vivo and ex vivo with Trypanosoma cruzi. However, questions have been raised regarding the nanotoxicity of QDs in living cells, microorganisms, tissues and whole animals. The objective of this paper was to conduct a QD nanotoxicity study on living T. cruzi protozoa using analytical methods. This was accomplished using in vitro experiments to test the interference of the QDs on parasite development, morphology and viability. Our results show that after 72 h, a 200 μM cadmium telluride (CdTe) QD solution induced important morphological alterations in T. cruzi, such as DNA damage, plasma membrane blebbing and mitochondrial swelling. Flow cytometry assays showed no damage to the plasma membrane when incubated with 200 μM CdTe QDs for up to 72 h (propidium iodide cells), giving no evidence of classical necrosis. Parasites incubated with 2 μM CdTe QDs still proliferated after seven days. In summary, a low concentration of CdTe QDs (2 μM) is optimal for bioimaging, whereas a high concentration (200 μM CdTe) could be toxic to cells. Taken together, our data indicate that 2 μM QD can be used for the successful long-term study of the parasite-vector interaction in real time. 相似文献
997.
Abreu PA da Silva VA Santos FC Castro HC Riscado CS de Souza MT Ribeiro CP Barbosa JE dos Santos CC Rodrigues CR Lione V Correa BA Cunha AC Ferreira VF de Souza MC Paixão IC 《Current microbiology》2011,62(5):1349-1354
Herpes simplex virus is an important human pathogen responsible for a range of diseases from mild uncomplicated mucocutaneous infections to life-threatening ones. Currently, the emergence of Herpes simplex virus resistant strains increased the need for more effective and less cytotoxic drugs for Herpes treatment. In this work, we synthesized a series of oxoquinoline derivatives and experimentally evaluated the antiviral activity against acyclovir resistant HSV-1 strain as well as their cytotoxity profile. The most active compound (3b), named here as Fluoroxaq-3b, showed a promising profile with a better cytotoxicity profile than acyclovir. The theoretical analysis of the structure-activity relationship of these compounds revealed some stereoelectronic properties such as lower LUMO energy and lipophilicity, besides a higher polar surface area and number of hydrogen bond acceptor groups as important parameters for the antiviral activity. Fluoroxaq-3b showed a good oral theoretical bioavailability, according to Lipinski rule of five, with a promising profile for further in vivo analysis. 相似文献
998.
Russo-Abrahão T Cosentino-Gomes D Gomes MT Alviano DS Alviano CS Lopes AH Meyer-Fernandes JR 《FEMS microbiology letters》2011,317(1):34-42
Candida parapsilosis is considered to be an emerging fungal pathogen because it is associated with an increasing range of infections. In this work, we biochemically characterized ecto-5'-nucleotidase activity on the surface of living, intact C. parapsilosis cells. At a pH of 4.5, intact cells were able to hydrolyze 5'-AMP at a rate of 52.44 ± 7.01 nmol Pi h(-1) 10(-7) cells. 5'-AMP, 5'-IMP and 5'-UMP were hydrolyzed at similar rates, whereas 5'-GMP and 5'-CMP hydrolyzed at lower rates. Enzyme activity was increased by about 42% with addition of Mg(2+) or Ca(2+), and the optimum pH was in the acidic range. An inhibitor of phosphatase activities, sodium orthovanadate, showed no effect on AMP hydrolysis; however, as expected, ammonium molybdate, a classical nucleotidase inhibitor, inhibited the activity in a dose-dependent manner. The results indicated that the existence of an ecto-5'-nucleotidase could play a role in the control of extracellular nucleotide concentrations. 相似文献
999.
Marcos Cabral de Melo Pedro Paulo Goulart Taucce Eduardo Leite Borba 《Plant Systematics and Evolution》2011,293(1-4):161-176
We studied the floral biology and performed experimental intra- and interspecific pollinations in populations of a complex of four Acianthera (Orchidaceae) species occurring in Brazilian campo rupestre vegetation (A.?hamosa, A.?limae, A.?modestissima, and A.?prolifera). All four species flower synchronously, are partially intercompatible, and exhibited some degree of self-sterility. Floral morphology is similar in all the species, with their principal differences associated with size of the floral structures. The four species were visited only by Diptera species of the families Phoridae (Megaselia spp.) and Chloropidae, but visits are rare and fruit set is very low. Sympatric species were not pollinated by the same Diptera species. Acianthera hamosa and A.?modestissima have the smallest flowers, and no marked morphological differences between them were observed; they were both pollinated by very similar Megaselia species. Both prepollination barriers and postpollination events are important to maintaining the isolation of the species, functioning as overlapping filters that diminish the possibility of gene flow between them. However, putative hybrids between A.?prolifera and A.?limae have been found. Conversely, A.?hamosa and A.?modestissima, which are recognized only by vegetative characters that show high phenotypic plasticity, seem only to be isolated by geographical barriers, and they may actually constitute a single species or be sister species. 相似文献
1000.
Azucena González-Coloma Fernanda Delgado Jesus M. Rodilla Lucia Silva Jesus Sanz Jesus Burillo 《Biochemical Systematics and Ecology》2011
The chemical composition and biological activity of wild Lavandula luisieri (Rozeira) Rivas-Martínez populations from the south Beira interior region in central eastern Portugal are described. The essential oils of these four populations have been analyzed by GC–MS, and their antifeedant effects evaluated against Spodoptera littoralis, Myzus persicae and Rhopalosiphum padi. Relative concentration data (% values) was subjected to principal component and cluster analysis. Relative concentration data and antifeedant effects were subjected to a stepwise multiple regression analysis. Additionally, one of these populations has been adapted to experimental field cultivation. 相似文献