全文获取类型
收费全文 | 5109篇 |
免费 | 676篇 |
国内免费 | 1064篇 |
专业分类
测绘学 | 503篇 |
大气科学 | 565篇 |
地球物理 | 1172篇 |
地质学 | 2826篇 |
海洋学 | 747篇 |
天文学 | 122篇 |
综合类 | 359篇 |
自然地理 | 555篇 |
出版年
2024年 | 42篇 |
2023年 | 113篇 |
2022年 | 131篇 |
2021年 | 146篇 |
2020年 | 87篇 |
2019年 | 143篇 |
2018年 | 150篇 |
2017年 | 100篇 |
2016年 | 132篇 |
2015年 | 140篇 |
2014年 | 266篇 |
2013年 | 181篇 |
2012年 | 217篇 |
2011年 | 235篇 |
2010年 | 160篇 |
2009年 | 201篇 |
2008年 | 243篇 |
2007年 | 266篇 |
2006年 | 229篇 |
2005年 | 264篇 |
2004年 | 248篇 |
2003年 | 199篇 |
2002年 | 180篇 |
2001年 | 156篇 |
2000年 | 180篇 |
1999年 | 172篇 |
1998年 | 175篇 |
1997年 | 196篇 |
1996年 | 200篇 |
1995年 | 203篇 |
1994年 | 169篇 |
1993年 | 164篇 |
1992年 | 144篇 |
1991年 | 182篇 |
1990年 | 160篇 |
1989年 | 131篇 |
1988年 | 77篇 |
1987年 | 83篇 |
1986年 | 53篇 |
1985年 | 70篇 |
1984年 | 37篇 |
1983年 | 42篇 |
1982年 | 36篇 |
1981年 | 31篇 |
1980年 | 37篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1964年 | 5篇 |
1960年 | 6篇 |
1958年 | 8篇 |
排序方式: 共有6849条查询结果,搜索用时 15 毫秒
61.
62.
We find that early sea urchin embryos have the capability to induce programmed cell death, or apoptosis, in response to chemical and physical stress. Strongylocentrotus purpuratus embryos (fertilized, 4 cell, 16 cell, 64 cell, and early blastula) were exposed to known cytotoxins, in order to determine when apoptosis occurs naturally and in response to stress. Using cell permeability as an indicator of early stage apoptosis, caspase activation as a mid-stage indicator, and DNA fragmentation as a late stage indicator, we find that during the cleavage stage of embryogenesis apoptosis is almost completely absent. However, a statistically significant (p<0.001) rise in apoptosis in stressed embryos is evident around 24 h after fertilization, during the early blastula stage and shortly after hatching. Before this stage, exposed embryos show no statistically significant increases in apoptosis in comparison to the controls. This pattern of apoptosis in development is similar to that seen in lower vertebrate models in which stress-induced apoptosis occurs only around the mid-blastula transition. We conclude that apoptosis may be used to rid embryos of aberrant or damaged cells in early development, but this response is stage-dependant. Repair, rather than apoptosis, may be utilized during earlier stages, or alternatively, embryos exposed to such stressors may continue development with damaged cells and perhaps damaged DNA. Our continued studies will focus on these alternative hypotheses. 相似文献
63.
64.
65.
大西洋扇贝贝壳生长年轮的氧同位素研究 总被引:2,自引:0,他引:2
两只活的大洋扇贝Placopecten magellanicus分别于1983和1985年采自加拿大诺瓦·斯科舍省布朗斯浅滩,其氧同位素记录展示出一种年度性的循环,并与由环境海水的实测温度和同位素组成所预计的贝壳同位素组成十分接近。外生长线的位置与这些循环中δ~(18)O的最大值吻合,这说明外生长线于底层海水温度最低时(春季)形成。据测定的δ~(18)O的结果证明,外生长线是年度性形成的,这与生物学证据一致。 相似文献
66.
Todorov JR Elskus AA Schlenk D Ferguson PL Brownawell BJ McElroy AE 《Marine environmental research》2002,54(3-5):691-695
To determine the estrogenicity of effluents from sewage treatment plants (STPs) to larval fish, 2-day-old sunshine bass were exposed to effluents from three STPs serving New York City (NYC), varying in size and treatment level. Estrogenic response was evaluated by measuring vitellogenin (VTG) and estrogen receptor (ER) expression in cytosolic fractions of whole body homogenates. Concentrations of the presumptive endocrine disruptors in the effluents were also measured. VTG and ER levels in sewage-exposed fish were 3-5 times that observed in controls. Combined concentrations of estradiol and estrone ranged from 5 to 13 ng/l and nonylphenol-ethoxylate metabolites (NPEOs: 4-nonylphenol, and 1-, 2-, and 3-nonylphenol-ethoxylates) ranged from 180 to 470 microg/l in chlorinated effluent. Results indicate that both ER and VTG can be used as biomarkers for endocrine disruption in larval fish, and that 4-day exposure to sewage effluent is sufficient to elicit significant expression of these markers in sunshine bass larvae. The extremely higher concentrations of NPEOs found in effluent relative to hormones (approximately 40,000-fold) indicates that surfactant metabolites may be contributing significantly to the estrogenic effects observed. 相似文献
67.
68.
珠江(广州河段)沉积物中砷形态的存在及迁移特征 总被引:4,自引:0,他引:4
根据珠江(广州河段)沉积物中As(T),As(V),MMA,DMA含量与Fe,Al,K,Ca,Mg及有机碳含量,研究了南亚热带近河口区沉积物中As形态的存在特征,并计算As形态间及与其它元素间的相关性,从而说明As形态河口区的存在及迁移特征。 相似文献
69.
Organochlorine concentrations were measured in white-sided dolphins, pilot whales, and their prey from the Gulf of Maine and used to identify species, tissue, and gender differences, and trophic transfer trends, in bioaccumulation. Polychlorinated biphenyl concentrations ([PCB]) in dolphin blubber (13 +/- 7.1 micrograms/g fresh wt.) were twice those in pilot whales, but pesticide concentrations (20 +/- 13 micrograms/g fresh) were similar between species. 4,4'-DDE, trans-non-achlor, Cl6(153) and Cl6(138) concentrations were highest. Skin tissues had more recalcitrant organochlorines than the internal organs. Male dolphins bioaccumulated higher concentrations of nonmetabolizable PCBs and hexachlorocyclohexane (HCH) isomers, whereas pilot whales had no gender-related differences in bioaccumulation. Pilot whales, mackerel, and herring had proportionately higher concentrations of DDTs, whereas [PCB] were higher in dolphins and squid. Although these odontocetes feed at the same trophic level and store a similar suite of contaminants, dolphins bioaccumulated higher and potentially hazardous 4,4'-DDE and PCB concentrations from food in their more geographically restricted range. 相似文献
70.
海涂—一种潜在的土地资源 总被引:1,自引:0,他引:1
海涂作为一种潜在的土地资源,主要分布于海岸、海湾和河口三角洲,按成因可分为平直海岸、泻湖海湾和河口三角洲三种类型。根据经济发展的需要与海涂自然条件的不同,海涂围垦可以分为农业围垦、协业围垦、水产养殖业围垦和滨海新兴城镇、港口、开发区用地围垦等。海涂的开发利用应从全局的需要和资源的自然属性出发,进行合理开发,综合利用,以利于社会经济和环境的可持续发展。 相似文献