全文获取类型
收费全文 | 2433篇 |
免费 | 43篇 |
国内免费 | 18篇 |
专业分类
系统科学 | 39篇 |
丛书文集 | 4篇 |
教育与普及 | 2篇 |
理论与方法论 | 5篇 |
现状及发展 | 1149篇 |
研究方法 | 162篇 |
综合类 | 1122篇 |
自然研究 | 11篇 |
出版年
2018年 | 23篇 |
2017年 | 26篇 |
2016年 | 35篇 |
2015年 | 18篇 |
2014年 | 29篇 |
2013年 | 56篇 |
2012年 | 115篇 |
2011年 | 116篇 |
2010年 | 83篇 |
2009年 | 36篇 |
2008年 | 89篇 |
2007年 | 89篇 |
2006年 | 96篇 |
2005年 | 106篇 |
2004年 | 67篇 |
2003年 | 74篇 |
2002年 | 68篇 |
2001年 | 47篇 |
2000年 | 68篇 |
1999年 | 37篇 |
1994年 | 20篇 |
1992年 | 35篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 34篇 |
1988年 | 30篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 30篇 |
1984年 | 30篇 |
1983年 | 24篇 |
1982年 | 23篇 |
1981年 | 18篇 |
1980年 | 20篇 |
1979年 | 50篇 |
1978年 | 32篇 |
1977年 | 50篇 |
1976年 | 29篇 |
1975年 | 27篇 |
1974年 | 46篇 |
1973年 | 41篇 |
1972年 | 58篇 |
1971年 | 39篇 |
1970年 | 47篇 |
1969年 | 45篇 |
1968年 | 50篇 |
1967年 | 39篇 |
1966年 | 36篇 |
1965年 | 28篇 |
1964年 | 26篇 |
排序方式: 共有2494条查询结果,搜索用时 140 毫秒
161.
A highly unsaturated fatty acid predicts carbon transfer between primary producers and consumers 总被引:19,自引:0,他引:19
The factors that regulate energy transfer between primary producers and consumers in aquatic ecosystems have been investigated for more than 50 years. Among all levels of the food web (plants, herbivores, carnivores), the plant-animal interface is the most variable and least predictable link. In hypereutrophic lakes, for example, biomass and energy transfer is often inhibited at the phytoplankton-zooplankton link, resulting in an accumulation of phytoplankton biomass instead of sustaining production at higher trophic levels, such as fish. Accumulation of phytoplankton (especially cyanobacteria) results in severe deterioration of water quality, with detrimental effects on the health of humans and domestic animals, and diminished recreational value of water bodies. We show here that low transfer efficiencies between primary producers and consumers during cyanobacteria bloom conditions are related to low relative eicosapentaenoic acid (20:5omega3) content of the primary producer community. Zooplankton growth and egg production were strongly related to the primary producer 20:5omega3 to carbon ratio. This indicates that limitation of zooplankton production by this essential fatty acid is of central importance at the pelagic producer-consumer interface. 相似文献
162.
Imamizu H Miyauchi S Tamada T Sasaki Y Takino R Pütz B Yoshioka T Kawato M 《Nature》2000,403(6766):192-195
Theories of motor control postulate that the brain uses internal models of the body to control movements accurately. Internal models are neural representations of how, for instance, the arm would respond to a neural command, given its current position and velocity. Previous studies have shown that the cerebellar cortex can acquire internal models through motor learning. Because the human cerebellum is involved in higher cognitive function as well as in motor control, we propose a coherent computational theory in which the phylogenetically newer part of the cerebellum similarly acquires internal models of objects in the external world. While human subjects learned to use a new tool (a computer mouse with a novel rotational transformation), cerebellar activity was measured by functional magnetic resonance imaging. As predicted by our theory, two types of activity were observed. One was spread over wide areas of the cerebellum and was precisely proportional to the error signal that guides the acquisition of internal models during learning. The other was confined to the area near the posterior superior fissure and remained even after learning, when the error levels had been equalized, thus probably reflecting an acquired internal model of the new tool. 相似文献
163.
A self-extending paediatric leg implant 总被引:1,自引:0,他引:1
164.
Detection of preinvasive cancer cells 总被引:12,自引:0,他引:12
165.
The most striking feature of the Sun's magnetic field is its cyclic behaviour. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed on this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth century, when there were very few sunspots. A part of the Sun's magnetic field reaches out from the surface into interplanetary space, and it was recently discovered that the average strength of this interplanetary field has doubled in the past 100 years. There has hitherto been no clear explanation for this doubling. Here we present a model describing the long-term evolution of the Sun's large-scale magnetic field, which reproduces the doubling of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variations. 相似文献
166.
167.
^226Ra evidence for the ecosystem shift over the past 40 years in the North Pacific Subtropical Gyre 总被引:1,自引:0,他引:1
YANG JunHong CHEN Min QIU YuSheng LI YanPing MA Qiang LUE E ZHANG Run HUANG YiPu 《科学通报(英文版)》2007,52(6):832-838
Surface seawater was collected for ~(226)Ra measurement in the North Pacific Subtropical Gyre from July to October, 1999 and October to December, 2003. Combined with the historical data reported for this sea area, a declined trend of surface ~(226)Ra concentrations was observed since 1960s, indicating the ecosystem shift in response to global warming. On one side, the enhanced stratification of the upper water column resulting from global warming reduced the ~(226)Ra input from the depth, on the other, the temporal increase of biological production resulting from the climate-related ecosystem structure change strengthened the ~(226)Ra removal from the surface ocean. Both the physical and biological processes resulted in the decrease of surface ~(226)Ra concentrations in the North Pacific Subtropical Gyre. The temporal trend of surface ~(226)Ra concentrations was consistent with the trends of chlorophyll a, silicate, phosphate and primary production previously reported. This study provided ~(226)Ra evidence for the ecosystem shift under global change. 相似文献
168.
以拟穴青蟹(Scylla paramamosain)的血淋巴细胞为材料,刀额新对虾(Metapenaeus ensis)的血淋巴细胞为参照系,利用倍性分析仪结合外标定法测定其细胞核DNA含量.拟穴青蟹的血淋巴的细胞核DNA含量是2.677 pg/2c.此外,本文作者还发现拟穴青蟹血淋巴与肌肉组织细胞核DNA含量存在差异.报道了我国拟穴青蟹细胞核DNA含量,并且提供了拟穴青蟹的细胞遗传学特征,为蟹类系统重建与演化过程分析提供新资料. 相似文献
169.
Phylogenetic position of sponges in early metazoan evolution and bionic applications of siliceous sponge spicules 总被引:1,自引:0,他引:1
Werner E G MULLER WANG XiaoHong ZENG LingSen Heinz C SCHRODER 《科学通报(英文版)》2007,52(22):3029-3040
Sponges are the oldest and the simplest but not primitive multicellular animals. They represent the earliest evolutionary metazoan phylum still extant. It was a long and painful scientific process to posi-tion the most enigmatic and mysterious metazoan,the Porifera,into their correct phylogenetic place among the eukaryotes in general and multicellular animals in particular. As living fossils,sponges provide the best evidence for the early evolution of Metazoa. More recently,interest has been focused on the bionic applications of sponges' siliceous spicules,after the discovery of their unique structure and high fiber performance. In this review,the emergence of sponges,evolutionary novelties found in sponges,and the phylogenetic position of sponges in early metazoan evolution are highlighted. In ad-dition,the present state of knowledge on silicatein-mediated "biosilica" formation in marine sponges,including the involvement of other molecules in silica metabolism and their potential application in nanobiotechnology and medicine,is given. 相似文献
170.
LI Huili LUE Bingnan WANG Dan LIU Xinyue 《武汉大学学报:自然科学英文版》2007,12(3):548-552
An expanded granular sludge bed (EGSB) reactor was adopted to study the influence factors and rule of enhancing granular sludge concentration and performance. The experiment was performed at 33 ℃, pH 6.0-8.0 with continuous flow by adding proper quantity of nutritional trace elements. The results show that SLR was the key of steady operation of EGSB reactor. The increment of the granular sludge was influenced by volume loading rate (VLR), liquid up-flow velocity and sludge loading rate (SLR). Concentration of granular sludge increased rapidly when liquid up-flow velocity was over 0.94 m · h^-1 with SLR being at 1.0-2.0 d ^-1. With the propriety parameters: liquid up-flow velocity 2.52 m · h^-1, SLR 1.0-2.2 d^-1 and VLR 8.2-13.1 kg · m ^3 · d^-1, 23 days' continuous operation resulted in an increment by over 80% of granular sludge concentration in the EGSB reactor, plus good granular sludge property. 相似文献