首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4724篇
  免费   386篇
  国内免费   297篇
  2024年   2篇
  2023年   57篇
  2022年   46篇
  2021年   100篇
  2020年   69篇
  2019年   105篇
  2018年   109篇
  2017年   123篇
  2016年   154篇
  2015年   138篇
  2014年   180篇
  2013年   207篇
  2012年   137篇
  2011年   143篇
  2010年   114篇
  2009年   204篇
  2008年   185篇
  2007年   254篇
  2006年   258篇
  2005年   209篇
  2004年   182篇
  2003年   177篇
  2002年   186篇
  2001年   180篇
  2000年   177篇
  1999年   152篇
  1998年   166篇
  1997年   174篇
  1996年   141篇
  1995年   127篇
  1994年   130篇
  1993年   106篇
  1992年   77篇
  1991年   75篇
  1990年   75篇
  1989年   51篇
  1988年   55篇
  1987年   48篇
  1986年   51篇
  1985年   44篇
  1984年   49篇
  1983年   24篇
  1982年   49篇
  1981年   32篇
  1980年   30篇
  1979年   21篇
  1978年   14篇
  1977年   4篇
  1976年   12篇
  1974年   3篇
排序方式: 共有5407条查询结果,搜索用时 15 毫秒
991.
A Fire Risk Index (FIRISK) based on an original land cover/land use nomenclature has been developed in the framework of EU-funded MEDALUS projects and integrated into a composite index of sensitivity to desertification (the so called environmentally sensitive area index: ESAI). The objective of the present study is to introduce a validation approach assessing the correlation between the FIRISK and seven independent fire indicators and quantifying the similarity in their spatial distribution in a study area (Attica, Greece) characterized by high fire risk and fragmented land-use structure. The FIRISK correlated positively with four out of seven fire indicators (average fire size, percentage of burnt area on the total municipal area, percentage of cropland and pastures burnt in each fire) in a non-linear fashion. Cluster analysis indicates that FIRISK spatial distribution was coherent with variables describing both fire size and the use of land endangered by fire. Results of the present study demonstrate that the FIRISK is a reliable indicator of fire risk within the ESAI framework. Moreover, the index is considered a key component of multi-criteria decision support systems classifying land according to the level of fire sensitivity.  相似文献   
992.
993.
994.
At least 10% of the wetlands in Mexico are situated in the State of Veracruz on the central Gulf coast. The region has been intensively used for agriculture by humans for the last thousand years and today pastureland for cattle ranching is the dominant element of the landscape. We present a study of the wetland vegetation in a series of representative and internationally significant freshwater wetlands near the coastal lagoon La Mancha in Veracruz State. We obtained eleven wetland types formed by three wetland communities dominated by trees (mangrove or swamps), eight dominated by herbaceous species which we classified as marsh, five of which were native emergent marsh, one was a non-native emergent marsh and two were floating-leaved marsh. The highest diversity values were found in the Annona swamp, followed by the Sagittaria marsh and the Rhizophora–Laguncularia–Avicennia mangrove. CCA ordination showed that water level, disturbance and species number were the major factors explaining variation along axis 1, and on axis 2 conductivity, salinity, and bulk density. Wetland size was a major factor in both axes. We examined the interplay of intrinsic and extrinsic factors that give rise to the nature and diversity of wetland types that characterize the important wetland landscapes in this part of Mexico, being disturbance by human activities an important factor that favors the appearance of diverse species combinations.  相似文献   
995.
The molecular interactions between macrophage colony-stimulating factor (M-CSF) and the tyrosine kinase receptor c-FMS play a key role in the immune response, bone metabolism, and the development of some cancers. Because no x-ray structure is available for the human M-CSF·c-FMS complex, the binding epitope for this complex is largely unknown. Our goal was to identify the residues that are essential for binding of the human M-CSF to c-FMS. For this purpose, we used a yeast surface display (YSD) approach. We expressed a combinatorial library of monomeric M-CSF (M-CSFM) single mutants and screened this library to isolate variants with reduced affinity for c-FMS using FACS. Sequencing yielded a number of single M-CSFM variants with mutations both in the direct binding interface and distant from the binding site. In addition, we used computational modeling to map the identified mutations onto the M-CSFM structure and to classify the mutations into three groups as follows: those that significantly decrease protein stability; those that destroy favorable intermolecular interactions; and those that decrease affinity through allosteric effects. To validate the YSD and computational data, M-CSFM and three variants were produced as soluble proteins; their affinity and structure were analyzed; and very good correlations with both YSD data and computational predictions were obtained. By identifying the M-CSFM residues critical for M-CSF·c-FMS interactions, we have laid down the basis for a deeper understanding of the M-CSF·c-FMS signaling mechanism and for the development of target-specific therapeutic agents with the ability to sterically occlude the M-CSF·c-FMS binding interface.  相似文献   
996.
Rangelands are among the most extensive anthropogenic landscapes on earth, supporting nearly 500 million people. Disagreements over the extent and severity of rangeland degradation affect pastoralist livelihoods, especially when impacts of drought and over-grazing are confounded. While vegetation indices (such as NDVI, or Normalized Difference Vegetation Index) derived from remotely sensed imagery are often used to monitor rangelands, their strategic integration with local ecological knowledge (LEK) is under-appreciated. Here, we explore these complementary approaches in Kyrgyzstan’s pasture-rich province of Naryn, where disagreements regarding pasture degradation could greatly benefit from additional information. We examine a time series of MODIS satellite imagery (2000–2015) to characterize browning trends in vegetation as well as to distinguish between climate- and grazing-induced trends. We also compare and contrast measured trends with LEK perceptions of pasture degradation. To do so, we first examine statistical trends in NDVI as well as in NDVI residuals after de-trending with meteorological data. Second, we use participatory mapping to identify areas local pasture managers believe are overgrazed, a particularly useful approach in lieu of reliable historical stocking rates for livestock in this region. Lastly, we compare the strengths and weaknesses of LEK and remote sensing for landscape monitoring.Browning trends were widespread as declining trends in NDVI (and NDVI residuals) covered 24% (and 9%) of the landscape, respectively. Local managers’ perceptions of pasture degradation better reflected trends seen in NDVI than in climate-controlled NDVI residuals, suggesting patterns in the latter are less apparent to managers. Our approach demonstrated great potential for the integration of two inexpensive and effective methods of rangeland monitoring well-suited to the country’s needs. Despite limitations due to terrain, our approach was most successful within the semi-arid steppe where pasture degradation is believed to be most severe. In many parts of the world, sources of long-term spatially extensive data are rare or even non-existent. Thus, paired LEK and remote sensing can contribute to comprehensive and informative assessments of land degradation, especially where contentious management issues intersect with sparse data availability. LEK is a valuable source of complementary information to remote sensing and should be integrated more routinely and formally into landscape monitoring. To aid this endeavor, we synthesize advice for linking LEK and remote sensing across diverse landscape situations.  相似文献   
997.
998.
Linkage mapping and genome length in eastern white pine (Pinus strobus L.)   总被引:2,自引:0,他引:2  
 Haploid linkage analysis of eastern white pine, Pinus strobus L., was carried out using mainly RAPD markers and microsatellite, or simple-sequence-repeat, markers. Ninety one loci mapped to 12 linkage groups of three or more markers. The resulting framework genome map, the first for a soft pine species, contained 69 markers. The map covered 58% of the estimated genome length of 2071 cM(K), with a 95% confidence interval of 1828–2242 cM(K). A systematic comparison of linkage data from eastern white pine, longleaf pine (P. palustris Mill.) and maritime pine (P. pinaster Ait.), gave genome-length estimates for all three species very close to either 2000 cM(K) or 2600 cM(H), depending on whether the Kosambi(K) or Haldane(H) map functions, respectively, were employed. Differences among previous pine genome-length estimates were attributed to the divergent criteria used in the methods of estimation, and indicate the need for the adoption of uniform criteria when performing genome-length estimates. Current data suggest that members of the two pine subgenera, which diverged during the late Mesozoic era, have highly conserved rates of recombination. Received: 5 January 1997/Accepted: 24 January 1997  相似文献   
999.
J. S. Song 《Plant Ecology》1992,98(2):175-186
Previous classification systems of the subalpine coniferous forests, belonging to the class Vaccinio-Piceetea Br.-Bl. 1939, in northeastern Asia were reviewed based on phytosociological data of Korea and Japan. A new order, the Abieti nephrolepidis-Piceetalia jezoensis was proposed for the subalpine coniferous forests in the continental part of northeastern Asia. Its distribution range and subordinate vegetation units are as follows:Order Abieti nephrolepidis-Piceetalia jezoensis ordo nov.In Korea:Alliance Abieti nephrolepidis-Piceion jezoensis Song 1991Association Taxo-Pinetum pumilae Song et Nakanishi 1985Association Thujo-Abietetum nephrolepidis Song 1991Association Abieti koreanae-Piceetum jezoensis Song 1991Alliance Abietion koreanae Song 1991Association Saso-Abietetum koreanae Song et Nakanishi 1985Association Betulo saitoanae-Abietetum koreanae (Song et Nakanishi 1985) em. Song 1991In northeast China and Maritime Province of Siberia, U.S.S.R.: (one separate alliance level or the same alliance as Abieti nephrolepidis-Piceion jezoensis)Formation Abies nephrolepis form. (Wu 1980)Formation Picea jezoensis-Abies nephrolepis form. (Wu 1980)Generally the Abieti nephrolepidis-Piceetalia jezoensis develops under the dry and cool continental climate and on the monophylogenic Dark Brown Conifer Soil Group derived mostly from granite and granitic gneiss. The subalpine or subarctic coniferous forests in Hokkaido, Sakhalin and Southern Kuriles may also belong to the Abieti nephrolepidis-Piceetalia jezoensis because of their floristic composition and the historical relevance of plant geography. The subordinate vegetation units are as follows:Alliance Piceion jezoensis Suz.-Tok. ex Jinno et Suzuki 1973Association Piceo-Abietetum sachalinensis Ohba 1967Association Piceetum glehnii Suz.-Tok. ex Miyawaki 1988The Abieti veitchii-Piceetalia jezoensis hondoensis Miyawaki et al. 1968 seems to fit only to the subalpine coniferous forests of Honshu and Shikoku of the Japanese Archipelago.  相似文献   
1000.
A bovine bacterial artificial chromosome (BAC) library was screened for the presence of eight type I anchor loci previously used within hybrid somatic cells and an interspecies hybrid backcross to construct a genome map of bovine chromosome 19 (BTA19). Six out of eight loci were identified in the BAC library ( NF1, CRYB1, CHRNB1, TP53, GH1 and P4HB ). The BACs were then used in single-colour fluorescence in situ hybridization (FISH) to assign these genes to BTA19 band locations. Gene order was determined by single-colour FISH, and was confirmed by dual-colour FISH to mitotic and meiotic chromosomes. The order, centromere- NF1-CRYB1-CHRNB1-TP53-GH1-P4HB , was in agreement with the order determined by linkage analyses. In addition, the order of CHRNB1 and TP53 , previously unresolved by linkage analyses, was established. These data provide high-resolution cytogenetic anchorage of the BTA19 genome map from chromosome bands 14–22.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号