全文获取类型
收费全文 | 13185篇 |
免费 | 962篇 |
国内免费 | 7篇 |
专业分类
14154篇 |
出版年
2023年 | 100篇 |
2022年 | 188篇 |
2021年 | 332篇 |
2020年 | 271篇 |
2019年 | 333篇 |
2018年 | 408篇 |
2017年 | 340篇 |
2016年 | 500篇 |
2015年 | 802篇 |
2014年 | 811篇 |
2013年 | 976篇 |
2012年 | 1162篇 |
2011年 | 1071篇 |
2010年 | 683篇 |
2009年 | 593篇 |
2008年 | 723篇 |
2007年 | 716篇 |
2006年 | 652篇 |
2005年 | 587篇 |
2004年 | 528篇 |
2003年 | 432篇 |
2002年 | 416篇 |
2001年 | 129篇 |
2000年 | 102篇 |
1999年 | 116篇 |
1998年 | 131篇 |
1997年 | 103篇 |
1996年 | 76篇 |
1995年 | 73篇 |
1994年 | 71篇 |
1993年 | 72篇 |
1992年 | 56篇 |
1991年 | 53篇 |
1990年 | 51篇 |
1989年 | 46篇 |
1988年 | 40篇 |
1987年 | 31篇 |
1986年 | 30篇 |
1985年 | 37篇 |
1984年 | 42篇 |
1983年 | 25篇 |
1982年 | 29篇 |
1981年 | 26篇 |
1980年 | 17篇 |
1979年 | 15篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1975年 | 20篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Retention of habitat fragments within the urban matrix can provide critical resources for the maintenance of regional biodiversity while still providing socio-economic value. Euglossini bees are important components in a community as they are important pollinators for economically valuable plants as well as hundreds of orchid species. However, some species are very sensitive to environmental impacts like urbanization. This study presents the role of antique urban fragments in a historical city in Brazil and compares it with a conservation area on the aspects of orchid bee assemblage, such as richness, composition, and abundance. Four fragments inside the city of Ouro Preto and three inside Parque Estadual do Itacolomi (PEIT) were sampled for Euglossini bees. Sorensen similarity index was used to compare community composition. The Mantel test was applied to verify the hypothesis that an urban center is a barrier for the mobility of the individuals. Fourteen Euglossini species from the region were registered. Close to 75% of the sampled bees were collected from the PEIT sampling areas. The fragments presented differences in Euglossini richness and abundance. A majority of the sampled fragments were dominated by the Eulaema cingulata Fabricius, Eulaema nigrita Lepeletier, and Euglossa securigera Dressler species. We found differences on community composition between the fragments localized in PEIT and those located in the urban center. The data suggest that there is a possible flux of individuals between the sampled fragments. The various small forest fragments in Ouro Preto, primarily in backyards, may also serve as stepping stones between sampled fragments. 相似文献
2.
3.
Francisco Gutirrez‐Corchero M. Victoria Arruga Lourdes Sanz Cristina García M. Angeles Hernndez Francisco Campos 《Molecular ecology resources》2002,2(1):75-77
FTA® cards were used for long‐term storage of avian blood samples. Blood DNA was extracted by a simple method and used in PCR for sex identification of adult and nestling Great Grey Shrikes Lanius excubitor. 相似文献
4.
5.
6.
Francisco E. Fontúrbel 《Mammalian Biology》2010,75(4):294-301
Assessing small mammal diversity is a common procedure, which usually employs widespread standard techniques, for gathering information for a wide range of studies. Traditional methods, however, may be biased against capturing arboreal marsupials, such as Dromiciops gliroides, an endemic marsupial currently considered a rare species in the Patagonian temperate rainforest due to the low abundances reported previously. I tested a new capturing methodology to assess the small mammal diversity of an old-growth forest in Patagonia, based on a randomized and balanced design, which incorporated a combination of different trap types, bait types, and placement heights. The proposed methodology included four trap types (two for live-capturing: wire-mesh and Sherman traps, and two sign-recording traps for tracks and hair), two types of bait (banana and rolled oats), and two trap placements (ground level and 1.5–2.5 m above the ground). Trap type, bait type, and height of placement all had significantly different effects on capturing and detecting rodents or marsupials; environmental variables at the trap location also affected the ability to detect rodents and marsupials. Traditional methods used for sampling small mammals performed well for rodents but are not effective for capturing marsupials and vice versa, showing species-specific sampling protocols. There is no single combination of trap-bait-height capable to assess the entire small mammal community, but the combination of the most effective protocol for rodents and the most effective protocol for marsupials guarantee better results. 相似文献
7.
8.
9.
A Shafiee M Vosooghi C G Francisco R Freire R Hernandez J A Salazar E Suarez S Sotheeswaran A A Gunatilaka 《Steroids》1987,49(4-5):397-402
The chemical synthesis and physical data of several new esters of testosterone (17 beta-hydroxyandrost-4-en-3-one), which contain either a halogeno or an alkoxy substituent in the acid chain, are reported. 相似文献
10.
Pablo Rodriguez-Palenzuela Joaquin Royo Luis Gómez Rosa Sánchez-Monge Gabriel Salcedo José Luis Molina-Cano Francisco Garcia-Olmedo Pilar Carbonero 《Molecular & general genetics : MGG》1989,219(3):474-479
Summary A cDNA encoding trypsin inhibitor CMe from barley endosperm has been cloned and characterized. The longest open reading frame of the cloned cDNA codes for a typical signal peptide of 24 residues followed by a sequence which is identical to the known amino acid sequence of the inhibitor, except for an Ile/Leu substitution at position 59. Southern blot analysis of wheat-barley addition lines has shown that chromosome 3H of barley carries the gene for CMe. This protein is present at less than 2%–3% of the wild-type amount in the mature endosperm of the mutant Risø 1508 with respect to Bomi barley, from which it has been derived, and the corresponding steady state levels of the CMe mRNA are about I%. One or two copies of the CMe gene (synonym Itc1) per haploid genome have been estimated both in the wild type and in the mutant, and DNA restriction patterns are identical in both stocks, so neither a change in copy number nor a major rearrangement of the structural gene account for the markedly decreased expression. The mutation at the lys 3a locus in Risø 1508 has been previously mapped in chromosome 7 (synonym 5H). A single dose of the wild-type allele at this locus (Lys 3a) restores the expression of gene CMe (allele CMe-1) in chromosome 3H to normal levels. 相似文献