全文获取类型
收费全文 | 3355篇 |
免费 | 470篇 |
国内免费 | 378篇 |
专业分类
4203篇 |
出版年
2024年 | 19篇 |
2023年 | 78篇 |
2022年 | 63篇 |
2021年 | 91篇 |
2020年 | 175篇 |
2019年 | 177篇 |
2018年 | 163篇 |
2017年 | 153篇 |
2016年 | 164篇 |
2015年 | 159篇 |
2014年 | 157篇 |
2013年 | 199篇 |
2012年 | 158篇 |
2011年 | 165篇 |
2010年 | 141篇 |
2009年 | 193篇 |
2008年 | 187篇 |
2007年 | 189篇 |
2006年 | 173篇 |
2005年 | 155篇 |
2004年 | 125篇 |
2003年 | 107篇 |
2002年 | 101篇 |
2001年 | 78篇 |
2000年 | 76篇 |
1999年 | 61篇 |
1998年 | 58篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 44篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 25篇 |
1990年 | 33篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1987年 | 27篇 |
1986年 | 28篇 |
1985年 | 23篇 |
1984年 | 46篇 |
1983年 | 40篇 |
1982年 | 40篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 19篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有4203条查询结果,搜索用时 15 毫秒
41.
B. Labedan K.B. Heller A.A. Jasaitis T.H. Wilson E.B. Goldberg 《Biochemical and biophysical research communications》1980,93(2):625-630
DNA penetration from T4 phage adsorbed to was measured at different membrane potentials. There was a precipitous reduction in DNA penetration between 110 mV and 60 mV. This threshold of membrane potential for DNA penetration is independent of ΔpH and rather insensitive to external pH between 6 and 8. 相似文献
42.
The determination of enzymatic activities in cell-free extracts of Acidaminococcus fermentans and Peptostreptococcus asaccharolyticus led to a refined scheme for the pathway of glutamate fermentation via (R)-2-hydroxyglutarate to acetate and butyrate. From the ratio of these products the amount of ATP generated by substrate level phosphorylation was calculated. Growth experiments with the organisms including Clostridium symbiosum and Clostridium tetanomorphum indicated that a sodium gradient contributed additional energy for growth. The high growth yields found in organisms containing the biotin dependent sodium pump glutaconyl-CoA decarboxylase could be reduced by the sodium ionophor monensin. In P. asaccharolyticus energy equivalent up to 0.6 mol ATP per mol of glutaconyl-CoA decarboxylated was conserved via the Na+ gradient. The data may explain the growth promoting effects of monensin in cattle. 相似文献
43.
植物种间相互作用及其对环境胁迫的响应一直是物种共存和生物多样性维持研究的一个热点, 从地下根系入手来探讨混生群落植物种间关系及其对环境胁迫响应的研究少见报道。该文以荒漠草原区(灵武)、典型荒漠区(张掖)和极端荒漠区(酒泉) 3个不同生境条件下单生与混生红砂(Reaumuria soongarica)和珍珠猪毛菜(Salsola passerina)为实验材料, 采用分层取样法对其垂直根系参数进行测定和分析, 探讨了两种植物根系分布对混生及荒漠环境梯度的响应。结果表明: 同一生境条件下, 混生红砂和珍珠猪毛菜比根长和比表面积均高于单生, 说明红砂、珍珠猪毛菜混生后其根系相互作用关系表现为互惠, 促进了植株对土壤养分和水分的吸收。不同生境条件下, 同一生长方式的红砂根系分布深度均大于珍珠猪毛菜, 且根系消弱系数也普遍高于珍珠猪毛菜, 说明二者在不同生境条件下占据不同生态位, 红砂表现为深根性, 根系位于土壤深层, 珍珠猪毛菜表现为浅根性, 根系分布于土壤浅层。随着荒漠环境胁迫增强, 单生和混生红砂与珍珠猪毛菜的比根长和比表面积均呈现出极端荒漠区>典型荒漠区>草原荒漠区的规律, 且生境越干旱, 混生群落根系分离越明显; 单生与混生红砂根系消弱系数也逐渐增大, 在极端干旱区达到最大值, 珍珠猪毛菜变化不大, 表明红砂-珍珠猪毛菜混生群落根系生态位分离随荒漠环境胁迫增强而加大, 验证了环境胁迫梯度假说。可见“地上聚生, 地下分离”的混生方式可能是红砂-珍珠猪毛菜混生群落适应干旱胁迫环境的生长策略。 相似文献
44.
James Richardson Alexander C. Lees Eliot T. Miller Stuart J. Marsden 《Ecology letters》2023,26(8):1301-1313
Understanding the composition of urban wildlife communities is crucial to promote biodiversity, ecosystem function and links between nature and people. Using crowdsourced data from over five million eBird checklists, we examined the influence of urban characteristics on avian richness and function at 8443 sites within and across 137 global cities. Under half of the species from regional pools were recorded in cities, and we found a significant phylogenetic signal for urban tolerance. Site-level avian richness was positively influenced by the extent of open forest, cultivation and wetlands and avian functional diversity by wetlands. Functional diversity co-declined with richness, but groups including granivores and aquatic birds occurred even at species-poor sites. Cities in arid areas held a higher percentage of regional species richness. Our results indicate commonalities in the influence of habitat on richness and function, as well as lower niche availability, and phylogenetic diversity across the world's cities. 相似文献
45.
An improved method for the isolation of rubella virus-like particles (RVLP) from cell culture supernatant of transfected Chinese hamster ovary (CHO24S) cells is described. It employs a combination of membrane filtration with sucrose gradient ultracentrifugation. It was found that staining the RVLP band with Coomassie brilliant blue G (CBB) resulted in the CBB crystals adsorbing RVLP. After ultracentrifugation (25,000 rpm, 3h, 4 degrees C) a sharp blue band with crystals (diameter 30-40 microm) was observed (at a density of 1.250 g/ml at 25 degrees C) in a 30-60% sucrose gradient. Using a combination of SDS-PAGE and Western blotting techniques, E1 rubella virus structural protein was detected only in the solutions derived from the sharp blue band. A decrease in crystal concentration a few millimeters above or below the main band was associated with a decrease in protein concentration. By dilution with a saturated ice-cold 30% sucrose solution it was possible to pellet the crystals by centrifugation (15,000 rpm, 10 min). SDS-PAGE showed a much higher concentration of RVLP structural protein in the pellet than in the supernatant. This RVLP-containing material is especially suitable for the preparation of rubella virus immunoblot stripes. 相似文献
46.
Scott Bennett Thomas Wernberg Thibaut de Bettignies Gary A. Kendrick Robert J. Anderson John J. Bolton Kirsten L. Rodgers Nick T. Shears Jean‐Charles Leclerc Laurent Lévêque Dominique Davoult Hartvig C. Christie 《Ecology letters》2015,18(7):677-686
Species interactions are integral drivers of community structure and can change from competitive to facilitative with increasing environmental stress. In subtidal marine ecosystems, however, interactions along physical stress gradients have seldom been tested. We observed seaweed canopy interactions across depth and latitudinal gradients to test whether light and temperature stress structured interaction patterns. We also quantified interspecific and intraspecific interactions among nine subtidal canopy seaweed species across three continents to examine the general nature of interactions in subtidal systems under low consumer pressure. We reveal that positive and neutral interactions are widespread throughout global seaweed communities and the nature of interactions can change from competitive to facilitative with increasing light stress in shallow marine systems. These findings provide support for the stress gradient hypothesis within subtidal seaweed communities and highlight the importance of canopy interactions for the maintenance of subtidal marine habitats experiencing environmental stress. 相似文献
47.
The anaerobic ammonium oxidation process is a new process for ammonia removal from wastewater. It is also a new microbial
physiology that was previously believed to be impossible. The identification of Candidatus Brocadia anammoxidans and its relatives as the responsible bacteria was only possible with the development of a new experimental
approach. That approach is the focus of this paper. The approach is a modernisation of the Winogradsky/Beyerinck strategy
of selective enrichment and is based on the introduction of the molecular toolbox and modern bioreactor engineering to microbial
ecology. It consists of five steps: (1) postulation of an ecological niche based on thermodynamic considerations and macro-ecological
field data; (2) engineering of this niche into a laboratory bioreactor for enrichment culture; (3) black-box physiological
characterisation of the enrichment culture as a whole; (4) phylogenetic characterisation of the enriched community using molecular
tools; (5) physical separation of the dominant members of the enrichment culture using gradient centrifugation and the identification
of the species of interest in accordance with Koch's postulates; (6) verification of the in situ importance of these species in the actual ecosystems. The power of this approach is illustrated with a case study: the identification
of the planctomycetes responsible for anaerobic ammonium oxidation. We argue that this was impossible using molecular ecology
or conventional ‘cultivation based techniques’ alone. We suggest that the approach might also be used for the microbiological
study of many interesting microbes such as anaerobic methane oxidisers.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
48.
49.
50.
The ability to germinate under a variety of environmental conditions is essential for plant species inhabiting a wide range of altitudes and latitudes. Phacelia secunda J. F. Gmel. (Hydrophyllaceae) is a perennial herb with wide latitudinal and altitudinal distributional ranges. In the central Chilean Andes (33 °S) P. secunda can be found from 1600 m sealevel up to the vegetation limit at 3400 m. It has been suggested that seeds from populations encountering long periods with snow cover and adverse winter conditions would require longer periods of cold stratification for germination than those from populations exposed to milder winters. Given that the snow-free period decreases with elevation, seeds from high elevation populations could require longer period of cold stratification to germinate. Moreover, it has been shown that seeds from arctic and higher elevations environments are adapted to germinate better under high temperature conditions. Germination response with increasing periods of cold stratification (0–6 mo.) and under two contrasting thermoperiods (20 °/1O °C; 10 °/5 °C; 12 h day/night), were studied for 4 populations of P. secunda located at 1600, 2100, 2900 and 3400 m a.s.l. Initiation of germination required increasingly longer periods of stratification with elevation, and proportionately fewer seeds germinated for any one stratification treatment at the higher elevations. Seeds from higher elevations germinated to a higher percentage under the high than the low temperature thermoperiods. These results illustrates a significant variation in germination characteristics over a spatially short environmental gradient. 相似文献