全文获取类型
收费全文 | 91326篇 |
免费 | 29347篇 |
国内免费 | 55404篇 |
专业分类
测绘学 | 14803篇 |
大气科学 | 26197篇 |
地球物理 | 21663篇 |
地质学 | 66789篇 |
海洋学 | 24307篇 |
天文学 | 1554篇 |
综合类 | 8707篇 |
自然地理 | 12057篇 |
出版年
2024年 | 655篇 |
2023年 | 1710篇 |
2022年 | 4282篇 |
2021年 | 5497篇 |
2020年 | 5158篇 |
2019年 | 8287篇 |
2018年 | 7784篇 |
2017年 | 7339篇 |
2016年 | 7383篇 |
2015年 | 7967篇 |
2014年 | 7324篇 |
2013年 | 8857篇 |
2012年 | 9457篇 |
2011年 | 9406篇 |
2010年 | 9395篇 |
2009年 | 8411篇 |
2008年 | 8043篇 |
2007年 | 7745篇 |
2006年 | 7185篇 |
2005年 | 6164篇 |
2004年 | 5396篇 |
2003年 | 4274篇 |
2002年 | 4148篇 |
2001年 | 3727篇 |
2000年 | 3122篇 |
1999年 | 2186篇 |
1998年 | 1848篇 |
1997年 | 1874篇 |
1996年 | 1442篇 |
1995年 | 1347篇 |
1994年 | 1204篇 |
1993年 | 1179篇 |
1992年 | 1028篇 |
1991年 | 743篇 |
1990年 | 729篇 |
1989年 | 588篇 |
1988年 | 498篇 |
1987年 | 403篇 |
1986年 | 327篇 |
1985年 | 261篇 |
1984年 | 288篇 |
1983年 | 165篇 |
1982年 | 213篇 |
1981年 | 152篇 |
1980年 | 110篇 |
1979年 | 138篇 |
1978年 | 49篇 |
1977年 | 45篇 |
1971年 | 53篇 |
1954年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Diagnosis of iridovirus in large yellow croaker by PCR 总被引:1,自引:0,他引:1
A rapid and sensitive PCR-based method for the detection of the large yellow croaker iridovirus(LYCIV) is described,which involves the amplification of a 295 bp fragment of the LYCIV ATPase gene from DNA isolated from naturally infected fish spleen.Sequencing of LYCIV ATPase gene fragment showed it shared 100% nucleotide sequence homology with the corresponding region of the ATPase gene of red sea bream iridovirus(RSIV) and sea bass iridovirus(SBIV),suggesting that LYCIV was homologous with RSIV and SBIV at least in part of the gemone.The specificity and sensitivity of the PCR procedure were tested on the iridovirus-infected fishes,the expected fragment was detected from spleen DNA samples of infected fishes,whereas no fragments were amplified from healthy fish spleen DNA,white spot syndrome baculoviruses(WSBV) DNA and pseudorabies virus(PRV) DNA.Detection limit of this method was 10-7 ng positive plasmid DNA containing target sequence,equal to about 100 virions.In the infected experiment,first positive detection(1/4) appeared at Day 3 post-infection,all fish(4/4) tested positive at Day 7,however obvious symptoms were observed at Day 8,so LYCIV infection could be detected prior to the appearance of obvious symptoms.These results indicate that this PCR method could be used for early,rapid and specific detection of LYCIV infection. 相似文献
952.
水成沉积与风成沉积及古土壤的磁组构特征 总被引:14,自引:2,他引:14
晚第四纪水成沉积、风成沉积、古土壤等的磁组构研究表明,不同成因的沉积物具有明显不同的磁组构特征。一般地,水成沉积的体积磁化率K受物源和水动力因素控制,P和F均>1.02,q<0.5;风成沉积的K受物源控制,内陆黄土的K明显高于沿海黄土及海岸风沙K,P和F均<1.02,q的平均值>0.55;古土壤的K受母质沉积类型和成土环境的氧化还原条件控制,内陆古土壤的K偏高,沿海古土壤的K偏低,内陆古土壤的K远高于沿海古土壤的K,P和F均<1.02,q的平均值在0.5左右。运用典型沉积的磁组构特征对比分析未知沉积物,可以得到未知沉积物的物源、沉积过程和环境信息。 相似文献
953.
小球藻紫外线诱变及高产藻株筛选 总被引:1,自引:0,他引:1
利用单细胞分离技术和紫外诱变育种技术,筛选高产小球藻株。通过单细胞分离技术获得的藻株Chlorella vul-garisQ7,生长速度和蛋白含量均优于其它藻株。以C.vulgarisQ7为出发株,进行紫外诱变,从120株诱变单克隆中筛选出3个株系:M51,M59,M73。与出发株相比,突变株M51,M59,M73的生长速率分别提高了6.23%,3.8%,5.92%。蛋白含量分别提高了2.5%,3.1%,1.9%。研究了诱变株的最适生长条件,结果表明在140μmol.m-2.s-1,25℃时,诱变株具有最大的生长速率和生物量。探讨了诱变株的遗传稳定性,结果表明诱变株可稳定遗传。 相似文献
954.
955.
Observations of fluid mud were made in the lower North Passage of the Yangtze Estuary in February 2000, on 10 -11 August 2000, on 30 - 31 August 2000 (after two strong typhoons), on 21 - 24 August 2000 (neap tide) and on 3 -6 September 2000 (mean tide) respectively. In situ data show that the fluid mud in this area consists of fine cohesive sediment (median size 7.23 μm). The formation and movement of fluid mud varied during the neap-spring and flood-ebb tidal cycle. Observations suggest that fluid mud phenomena in this area may be categorised in a three-fold manner as slack water, storm and saltwedge features. The thickness of the fluid mud layer of slack water during the neap tide ranged from 0.2 to 0.96 m, whereas during the mean tide, the thickness ranged from 0.17 to 0.73 m, and the thickness of the fluid mud layer was larger during slack water than at the flood peak. Shoals cover an area of 800 km^2 with a water depth smaller than 5 m. Erosion of these extensive intertidal mudflats due to storm action provides an abundant sediment source. This is particularly significant in this estuary when the tidal level is lower than 5 m. The lower North Passage is a typical zone of saltwater wedging, so the saltwedge fluid mud has the most extensive spatial range in the estuary. 相似文献
956.
957.
958.
959.
双壳类软体动物精子发生及其在系统演化研究中的应用前景 总被引:11,自引:0,他引:11
综述了双壳类软体动物精子发生及精子形态结构方面的研究进展,指出双壳类软体动物精子发生过程中精细胞分化具有种、属特异性,精细胞分化的细节是区别个体外部形态很近的物种的依据;精子的形态结构具有种的特征,精子结构的比较研究将成为双壳类软体动物系统演化研究的重要方法之一。 相似文献
960.
The Barra Nova inlet, in south Portugal, is known to migrate progressively southeastwards under wave action. The morphodynamics
of this system during a representative year suggests that this long-term evolution is dependent on a seasonal behavior of
the tidal inlet which can be described through a three-stage model of post-storm, transition and extended fair-weather conditions.
Processes involved in this evolution indicate that the historical migration of the Barra Nova is not dependent on the longshore
drift forcing constricting the channel on the updrift coast, but rather on the adjustment of the system to a major erosion
of the downdrift coast during short storm events. 相似文献