全文获取类型
收费全文 | 9597篇 |
免费 | 627篇 |
国内免费 | 1篇 |
出版年
2024年 | 12篇 |
2023年 | 100篇 |
2022年 | 179篇 |
2021年 | 280篇 |
2020年 | 234篇 |
2019年 | 258篇 |
2018年 | 339篇 |
2017年 | 275篇 |
2016年 | 416篇 |
2015年 | 544篇 |
2014年 | 610篇 |
2013年 | 690篇 |
2012年 | 824篇 |
2011年 | 789篇 |
2010年 | 470篇 |
2009年 | 394篇 |
2008年 | 496篇 |
2007年 | 517篇 |
2006年 | 483篇 |
2005年 | 374篇 |
2004年 | 341篇 |
2003年 | 297篇 |
2002年 | 291篇 |
2001年 | 107篇 |
2000年 | 97篇 |
1999年 | 91篇 |
1998年 | 59篇 |
1997年 | 50篇 |
1996年 | 55篇 |
1995年 | 47篇 |
1994年 | 33篇 |
1993年 | 31篇 |
1992年 | 44篇 |
1991年 | 34篇 |
1990年 | 31篇 |
1989年 | 36篇 |
1988年 | 27篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 22篇 |
1984年 | 19篇 |
1983年 | 18篇 |
1982年 | 20篇 |
1981年 | 12篇 |
1980年 | 13篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1975年 | 12篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Gerardo H. Vázquez Nin Ernestina Flores Olga M. Echeverría Hilde Merkert Rodolfo Wettstein Ricardo Benavente 《Chromosoma》1993,102(7):457-463
The distribution of DNA in synaptonemal complexes of rat and mouse spermatocytes, and of chick oocytes was investigated by immunogold electron microscopy. Except for a few specific sites, DNA was not immunolocalized in the space between lateral elements of the complex. Some labeled fibrils connecting the lateral elements with the central element were observed associated with recombination nodules or near them. However, other labeled fibrils in the space between lateral elements did not appear to present any relationship to recombination nodules. The immunocytochemical approaches used here confirmed the presence of significant amounts of DNA in the lateral elements as previously indicated by preferential DNA staining methods. Furthermore, our findings support the view that recombination nodules are the site of chiasma formation. 相似文献
72.
Anastasia Klimova Ricardo Rodríguez-Estrella Guanliang Meng Jesús Neftalí Gutiérrez-Rivera María Luisa Jimenez-Jimenez Shanlin Liu 《Diversity & distributions》2023,29(3):438-461
Aim
Desert springs or oases are the only permanent mesic environments in highly water-limited arid regions. Oases have immense cultural, evolutionary and ecological importance for people and a high number of endemic and relic species. Nevertheless, they are also highly vulnerable ecosystems, with invasive species, overexploitation and climate change being the primary threats. We used the arthropod communities' spatiotemporal diversity and distribution patterns as a proxy to understand biodiversity dynamics in two geographically close but ecologically contrasting and highly threatened ecosystems: deserts and oases.Location
Baja California Peninsula, Mexico.Methods
Arthropod communities at five oases and surrounding desert scrub areas were sampled in two seasons. Using DNA metabarcoding and traditional taxonomic surveys, we tried to identify what biotic and abiotic characteristics of the habitat are important drivers of arthropod diversity and how these characteristics can change across spatial and temporal scales.Results
Over 6200 individuals representing 23 orders were collected. In oasis samples, the community composition fluctuated more in space (i.e. among sites) than in time (i.e. seasons). Thus, seasonal changes did not affect oasis community diversity and composition, but the dissimilarity among sites increased with geographic distance. Moreover, anthropic activities negatively correlated with arthropod diversity in oases. On the other hand, the season, geography (e.g. latitude) and biotic characteristics of the habitat (e.g. sampled scrub species) significantly affected the diversity and composition of the desert arthropod communities.Main Conclusions
Neutral dynamics (e.g. historical climatic events, dispersal limitation and spatial component) and human impact significantly influenced the biodiversity patterns of each oasis. In contrast, the habitat's seasonal variation and biotic characteristics were the most important variables influencing the diversity of the desert communities. Baja California oases harbour distinct invertebrate communities; therefore, each oasis should be conserved individually to preserve these unique assemblages. 相似文献73.
Camila Tamburrini Silvia Lucrecia Dahinten Rubén Ricardo Romero Saihueque María C. Ávila-Arcos María Laura Parolin 《American journal of physical anthropology》2023,182(2):161-176
Ethical discussions around ancient DNA (aDNA) research predate the technological breakthroughs that led to the accelerated generation of ancient genomic data, revealing a long-due need to address these aspects in the field. Given the diverse conflicts that genomics has raised towards the communities associated with the Non-living Human Ancestors under study, it has been suggested that the ethical and legal implications of genetically studying present-day and ancient human populations should be considered case-by-case. Nevertheless, the discussions have focused on US and European perspectives. To contribute from a local and Latin American position to the problem, we present the history of consensus and disagreement of the relationships between scientists and Indigenous communities of the Atlantic coast of the central Argentinian Patagonia. We describe how these relationships resulted in the approval of a groundbreaking provincial law that acknowledges the Indigenous community's right to be involved in decision-making concerning their Ancestors. In addition, we emphasize how these established relationships allowed the development of aDNA studies. With this background, we address the main ethical concerns of genomic studies of Ancestors identified in the reference literature and commit to applying some of the recommendations suggested in those ethical guidelines. Then, we reflect on possible negative consequences of ongoing research and propose some suggestions based on personal experiences that will contribute to moving the ethical field towards a more contextualized science with a local perspective. 相似文献
74.
As urban areas expand due to increasing human populations, natural habitats are diminishing in quantity and quality. We conducted a study to examine the response of diurnal acoustic communities along a gradient of environments, ranging from native to dense urban areas. We hypothesized that acoustic indices would decline with urbanization, transitioning from natural and rural areas to urban environments. We conducted the research in and around Brasília, Central Brazil. We deployed 24 digital recorders in native, rural, low-density and high-density urban areas. We employed five commonly used acoustic indices (ADI, ACI, NDSI, H and BI) to characterize the acoustic communities and represent the existing biodiversity numerically. We initially compared the index values across different land use types using a non-parametric rank sum test. Subsequently, we selected eight landscape metrics and performed a principal component analysis to summarize the surrounding matrix at each sampling point. We then employed generalized linear models to determine if the acoustic indices exhibited the anticipated variations. The results indicated significant variations in all indices among the different land use types, indicating their responsiveness to distinct acoustic communities and environmental characteristics. ADI, H and NDSI exhibited declining values from native to urban areas, while ACI and BI displayed the opposite trend. These findings underscore the influence of landscape structure on acoustic indices. Consequently, we concluded that adopting appropriate landscape planning, mainly through integrating natural and urban areas, could help preserve biodiversity in tropical urban regions. 相似文献
75.
The topography of the Sado estuary, the second largest of Portugal, comprises the outer estuary inside the entrance channel
and the inner estuary, on the inward side of which begins the tidal mudflats. The outer estuary subtidal area covers approximately
70 km2 and presents a series of longitudinal intertidal sandbanks, separating a northern and a southern channel.
A benthic survey was undertaken in the outer estuary during June 1986, in which superficial sediments and macrofauna were
sampled at 133 locations. The environmental variables measured in the superficial sediments were the temperature, the granulometric
structure, the silt, sand and the gravel content, and the total organic matter content. The primary macrofauna biological
variables studied were the species composition, abundance and biomass, calculated on wet, dry and ash-free dry weight.
The granulometry and the organic content of superficial sediments agreed with the transient and the residual currents velocity
field, simulated in a 2-D hydrodynamic model previously elaborated for the outer estuary. The northern channel superficial
sediments showed higher silt and total organic matter content, while the model also suggested lower transient and residual
velocities, water flow and shear stress in this channel.
The distribution patterns of the subtidal macrofauna were separated into two main groups of species, one comprising taxa essentially
settled near the estuarine mouth and the other inwards. Biological primary variables also showed consistent patterns, comparable
to other Portuguese estuaries. The major subtidal benthic biotopes were obtained through classification analysis and related
to the prevailing hydrophysical and sedimentary conditions in the outer estuary. 相似文献
76.
77.
Edna De la Llata Quiroga Randall Arauz Arturo Tripp Valdez Laura Porras Murillo Manuel Spinola Parallada Ricardo Sánchez-Murillo Elpis J. Chávez 《Journal of fish biology》2023,102(3):669-679
Bull shark (Carcharhinus leucas) is a near-threatened elasmobranch species capable of moving between the fresh and salty waters of tropical and subtropical coastal areas, for which we still lack important ecological information. During their first years of life, bull sharks use estuarine systems as nursery areas, making them highly susceptible to environmental and anthropogenic pressures. We studied the trophic ecology of juveniles found in the Coyote estuary, a potential nursery area in Costa Rica, to understand the potential impact of further bull shark declines and gain knowledge that could aid in their conservation. We analysed the trophic ecology of juvenile bull sharks [81–103 cm total length (TL)] in the Coyote estuary, Costa Rica, using stable isotopes of δ15N and δ13C. Since one problem using this technique in juveniles is the confounding effect of the maternal signature, we sampled different tissues (muscle and plasma), verified the status of the shark's umbilical scar and identified the size at which the isotope signature is a result of the animal's current diet. The isotopic values of the muscle tissue reflected the maternal isotopic signature. In contrast, plasma values reflected the diet of juvenile bull sharks >95 cm TL and with a closed umbilical scar. Juvenile bull sharks fed primarily on teleost fishes of the order Anguilliformes and Siluriformes, and have a high trophic position (≥4.0) in the Coyote estuary. Our findings suggest that this estuary is an important feeding site for juvenile bull sharks of the Pacific of Costa Rica. Thus, the protection of essential habitats such as the Coyote estuary will benefit not only bull shark conservation, but also the conservation of an array of fish species that also use this habitat as a rookery, many of which are of commercial interest. 相似文献
78.
79.
Marta Clariano Vanda Marques João Vaz Salma Awam Marta B. Afonso Maria Jesus Perry Cecília MP Rodrigues 《化学与生物多样性》2023,20(3):e202300222
Curcumin has a plethora of biological properties, making this compound potentially effective in the treatment of several diseases, including cancer. However, curcumin clinical use is compromised by its poor pharmacokinetics, being crucial to find novel analogs with better pharmacokinetic and pharmacological properties. Here, we aimed to evaluate the stability, bioavailability and pharmacokinetic profiles of monocarbonyl analogs of curcumin. A small library of monocarbonyl analogs of curcumin 1a–q was synthesized. Lipophilicity and stability in physiological conditions were both assessed by HPLC-UV, while two different methods assessed the electrophilic character of each compound monitored by NMR and by UV-spectroscopy. The potential therapeutic effect of the analogs 1a–q was evaluated in human colon carcinoma cells and toxicity in immortalized hepatocytes. Our results showed that the curcumin analog 1e is a promising agent against colorectal cancer, with improved stability and efficacy/safety profile. 相似文献
80.
João Ricardo Sato Claudinei Eduardo Biazoli Ana Paula Arantes Bueno Arthur Caye Pedro Mario Pan Marcos Santoro Jessica Honorato-Mauer Giovanni Abrahão Salum Marcelo Queiroz Hoexter Rodrigo Affonseca Bressan Andrea Parolin Jackowski Euripedes Constantino Miguel Sintia Belangero Luis Augusto Rohde 《Genes, Brain & Behavior》2023,22(2):e12838