全文获取类型
收费全文 | 6114篇 |
免费 | 529篇 |
国内免费 | 3篇 |
出版年
2023年 | 33篇 |
2022年 | 50篇 |
2021年 | 111篇 |
2020年 | 80篇 |
2019年 | 94篇 |
2018年 | 117篇 |
2017年 | 106篇 |
2016年 | 157篇 |
2015年 | 260篇 |
2014年 | 262篇 |
2013年 | 340篇 |
2012年 | 439篇 |
2011年 | 413篇 |
2010年 | 307篇 |
2009年 | 249篇 |
2008年 | 367篇 |
2007年 | 345篇 |
2006年 | 299篇 |
2005年 | 322篇 |
2004年 | 275篇 |
2003年 | 244篇 |
2002年 | 254篇 |
2001年 | 93篇 |
2000年 | 66篇 |
1999年 | 89篇 |
1998年 | 86篇 |
1997年 | 56篇 |
1996年 | 71篇 |
1995年 | 52篇 |
1994年 | 48篇 |
1993年 | 49篇 |
1992年 | 56篇 |
1991年 | 64篇 |
1990年 | 43篇 |
1989年 | 43篇 |
1988年 | 40篇 |
1987年 | 53篇 |
1986年 | 29篇 |
1985年 | 45篇 |
1984年 | 42篇 |
1983年 | 30篇 |
1982年 | 34篇 |
1981年 | 33篇 |
1980年 | 40篇 |
1979年 | 34篇 |
1978年 | 24篇 |
1977年 | 25篇 |
1976年 | 32篇 |
1973年 | 28篇 |
1972年 | 28篇 |
排序方式: 共有6646条查询结果,搜索用时 156 毫秒
1.
2.
3.
The Asian Tree Toad genus Pedostibes, as currently understood, exhibits a conspicuously disjunct distribution, posing several immediate questions relating to the biogeography and taxonomy of this poorly known group. The type species, P. tuberculosus and P. kempi, are known only from India, whereas P. hosii, P. rugosus, and P. everetti are restricted to Southeast Asia. Several studies have shown that these allopatric groups are polyphyletic, with the Indian Pedostibes embedded within a primarily South Asian clade of toads, containing the genera Adenomus, Xanthophryne, and Duttaphrynus. Southeast Asian Pedostibes on the other hand, are nested within a Southeast Asian clade, which is the sister lineage to the Southeast Asian river toad genus Phrynoidis. We demonstrate that Indian and Southeast Asian Pedostibes are not only allopatric and polyphyletic, but also exhibit significant differences in morphology and reproductive mode, indicating that the Southeast Asian species’ are not congeneric with the true Pedostibes of India. As a taxonomic solution, we describe a new genus, Rentapia
gen. nov. to accommodate the Southeast Asian species. 相似文献
4.
Genes for the biosynthesis of spinosyns: applications for yield improvement in Saccharopolyspora spinosa 总被引:2,自引:0,他引:2
K Madduri C Waldron P Matsushima M C Broughton K Crawford D J Merlo R H Baltz 《Journal of industrial microbiology & biotechnology》2001,27(6):399-402
Spinosyns A and D are the active ingredients in an insect control agent produced by fermentation of Saccharopolyspora spinosa. Spinosyns are macrolides with a 21-carbon, tetracyclic lactone backbone to which the deoxysugars forosamine and tri-O-methylrhamnose are attached. The spinosyn biosynthesis genes, except for the rhamnose genes, are located in a cluster that
spans 74 kb of the S. spinosa genome. DNA sequence analysis, targeted gene disruptions and bioconversion studies identified five large genes encoding type
I polyketide synthase subunits, and 14 genes involved in sugar biosynthesis, sugar attachment to the polyketide or cross-bridging
of the polyketide. Four rhamnose biosynthetic genes, two of which are also necessary for forosamine biosynthesis, are located
outside the spinosyn gene cluster. Duplication of the spinosyn genes linked to the polyketide synthase genes stimulated the
final step in the biosynthesis — the conversion of the forosamine-less pseudoaglycones to endproducts. Duplication of genes
involved in the early steps of deoxysugar biosynthesis increased spinosyn yield significantly. Journal of Industrial Microbiology & Biotechnology (2001) 27, 399–402.
Received 31 May 2001/ Accepted in revised form 09 July 2001 相似文献
5.
The distribution of grooming and touching behaviours was recorded in a group of captive ring-tailed lemurs. Grooming was found
to be performed chiefly by older, higher ranking animals; touching (i.e., “reach out and touch” behaviour) was directed primarily
by younger, low ranking animals to older, high ranking individuals. It is suggested that such touching is a submissive gesture
in this species. 相似文献
6.
7.
8.
Sandra L. Almeyda Zambrano Eben N. Broadbent Sam Shanee Noga Shanee Anneke Deluycker Michael Steinberg Scott A. Ford Alma Hernndez Jaramillo Robin Fernandez‐Hilario Carolina Lagos Castillo Angelica M. Almeyda Zambrano 《American journal of primatology》2019,81(8)
Habitat loss is one of the main threats to wildlife. Therefore, knowledge of habitat use and preference is essential for the design of conservation strategies and identification of priority sites for the protection of endangered species. The yellow‐tailed woolly monkey (Lagothrix flavicauda Humboldt, 1812), categorized as Critically Endangered on the IUCN Red List, is endemic to montane forests in northern Peru where its habitat is greatly threatened. We assessed how habitat use and preference in L. flavicauda are linked to forest structure and composition. The study took place near La Esperanza, in the Amazonas region, Peru. Our objective was to identify characteristics of habitat most utilized by L. flavicauda to provide information that will be useful for the selection of priority sites for conservation measures. Using presence records collected from May 2013 to February 2014 for one group of L. flavicauda, we classified the study site into three different use zones: low‐use, medium‐use, and high‐use. We assessed forest structure and composition for all use zones using 0.1 ha Gentry vegetation transects. Results show high levels of variation in plant species composition across the three use zones. Plants used as food resources had considerably greater density, dominance, and ecological importance in high‐use zones. High‐use zones presented similar structure to medium‐ and low‐use zones; thus it remains difficult to assess the influence of forest structure on habitat preference. We recommend focusing conservation efforts on areas with a similar floristic composition to the high‐use zones recorded in this study and suggest utilizing key alimentation species for reforestation efforts. 相似文献
9.
Abstract Broad host-range RK2-based cosmid vectors ('costramids') are increasingly used in molecular genetic studies of Gram-negative soil bacteria such as Rhizobium spp. we describe a simple modification of existing methods, whereby a genomic library constructed in a stringently replicated vector can be screened for genes which are undetectable by colony hybridization due to background cross-hybridization. This method allows the use of 'heterologous' probes (interspecies hybridization) to isolate several presumptive genes of interest from a gene bank of Rhizobium sp. NGR234 made in the costramid pRK7813. These are a gene with homology to the citrate synthase gene ( gltA ) or Escherichia coli , the gene encoding δ-aminol evulinic acid synthase ( hemA ), and a gene or genes regulating dicarboxylate transport. 相似文献
10.