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   1097篇
  免费   56篇
  国内免费   84篇
测绘学   33篇
大气科学   4篇
地球物理   53篇
地质学   133篇
海洋学   357篇
天文学   1篇
综合类   152篇
自然地理   504篇
  2024年   5篇
  2023年   17篇
  2022年   27篇
  2021年   24篇
  2020年   35篇
  2019年   34篇
  2018年   33篇
  2017年   27篇
  2016年   30篇
  2015年   23篇
  2014年   51篇
  2013年   96篇
  2012年   62篇
  2011年   45篇
  2010年   42篇
  2009年   57篇
  2008年   50篇
  2007年   56篇
  2006年   65篇
  2005年   59篇
  2004年   60篇
  2003年   33篇
  2002年   50篇
  2001年   39篇
  2000年   37篇
  1999年   30篇
  1998年   26篇
  1997年   15篇
  1996年   19篇
  1995年   17篇
  1994年   11篇
  1993年   12篇
  1992年   13篇
  1991年   9篇
  1990年   4篇
  1989年   4篇
  1988年   4篇
  1987年   5篇
  1986年   2篇
  1985年   3篇
  1984年   2篇
  1983年   2篇
  1982年   2篇
排序方式: 共有1237条查询结果,搜索用时 15 毫秒
831.
以合浦珠母贝Pinctadamartensii(Dunker)作为珠母贝进行插核育珠试验,分别以其左、右边外套膜作为小片,并用珍珠生长激素浸泡小片。两个试验组用珍珠生长激素浸泡的小片各插核育珠贝200个,空白对照组用海水浸泡小片括核育珠贝207个,育珠时间为6个多月。试验结束时,实验组珍珠的正圆珠率、白色珍珠比例以及珠层厚度比空白对照组分别提高了9.28%,22.57%和52μm。  相似文献   
832.
INTRODUCTIONSomeω3andω6long chainedpolyunsaturatedfattyacids (PUFAs)havebeenrecognizedasanessentialcomponentsinhumannutrientsduetotheirtherapeuticeffectsandrolesinmaintainingcellmembranesstructure ,fluidityandfunction .AmongthesePUFAs,arachidonicacid (AA ,2 0…  相似文献   
833.
在室内水池内以五层笼摞叠养殖方式,试验了入笼时平均壳长21.2 mm(平均体重1.1 7 g)、37.0 mm(平均体重5.43 g)、51.8 mm(平均体重16.08 g)小、中、大3种规格的九孔鲍的不同放养密度对其生长的影响.笼子大小为40 cm×30 cm×12 cm.小规格设每笼放养量为25、30、35、40、45与50只6种密度;中为25、30、35、40、与45只5种密度;大为20、25、30与40只4种密度.经60 d试验,结果表明(1)小个体组以30只/笼生长最快,平均160μm/d,33 mg/d;(2)中个体组以25只/笼生长最快,平均123um/d,62 mg/d;(3)大个体组以20只/笼生长最快,平均95 μm/d,116 mg/d.  相似文献   
834.
通过对《源氏物语》文化背景的探讨,论证了其主题并非“物哀”文化所能范围。同时,通过对日本平安文化的代表——物语文化及其特色的考述,指出日本古典文学中《源氏物语》以特有的文学样式集中地表现了“唐风文化”向“国风文化”的转变,并充分反映了日本文化在传承中国文化的同时,所创造出的颇具日本特色的“国风文化”及其派生出来的其它文化。  相似文献   
835.
The Ecuadorian Amazon, one of the richest reserves of biodiversity in the world, has faced one of the highest rates of deforestation of any Amazonian nation. Most of this forest elimination has been caused by agricultural colonization that followed the discovery of oil fields in 1967. Since the 1990s, an increasing process of urbanization has also engendered new patterns of population mobility within the Amazon, along with traditional ways by which rural settlers make their living. However, while very significant in its effects on deforestation, urbanization and regional development, population mobility within the Amazon has hardly been studied at all, as well as the distinct migration patterns between men and women. This paper uses a longitudinal dataset of 250 farm households in the Northern Ecuadorian Amazon to understand differentials between men and women migrants to urban and rural destinations and between men and women non-migrants. First, we use hazard analysis based on the Kaplan–Meier (KM) estimator to obtain the cumulative probability that an individual living in the study area in 1990 or at time t, will out-migrated at some time, t+n, before 1999. Results indicate that out-migration to other rural areas in the Amazon, especially pristine areas is considerably greater than out-migration to the growing, but still incipient, Amazonian urban areas. Furthermore, men are more likely to out-migrate to rural areas than women, while the reverse occurs for urban areas. Difference-of-means tests were employed to examine potential factors accounting for differentials between male and female out-migration to urban and rural areas. Among the key results, relative to men younger women are more likely to out-migrate to urban areas; more difficult access from farms to towns and roads constrains women's migration; and access to new lands in the Amazon–an important cause of further deforestation–is more associated with male out-migration. Economic factors such as engagement in on-farm work, increasing resource scarcity–measured by higher population density at the farm and reduction in farm land on forest and crops–and increase in pasture land are more associated with male out-migration to rural areas. On the other hand, increasing resource scarcity, higher population density and weaker migration networks are more associated with female out-migration to urban areas. Thus, a “vicious cycle” is created: Pressure over land leads to deforestation in most or all farm forest areas and reduces the possibilities for further agricultural extensification (deforestation); out-migration, especially male out-migration, occurs to other rural or forest areas in the Amazon (with women being more likely to choose urban destinations); and, giving continuing population growth and pressures in the new settled areas, new pressures promote further out-migration to rural destinations and unabated deforestation.  相似文献   
836.
As seedling culture is a crucial factor for successful cultivation ofGelidium, the authors researched tissue culture technology for producing seedlings. The morphogeny and experimental ecology were observed and studied fully in 2–5 mm isolated tissue fragments. Regeneration, appearance of branching creepers and attaching structure and new erect seedlings production and development were studied. Fragments were sown on bamboo slice and vinylon rope. The seedlings were cultured 20–30 days indoor, then cultured in the sea, where the density of erect seedlings was 3–19 seedlings/cm2, growth rate was 3.84% day. The frond arising from seedlings directly was up to 10 cm per year. The ecological conditions for regenerated seedlings are similar to the natural ones. The regenerated seedlings are suitable for raft culture in various sea areas.  相似文献   
837.
丁华  丁辉  陈鑫源  陈姗姗  廖文强 《地质论评》2022,68(1):2022020002-2022020002
地质文化村建设是积极响应“乡村振兴”等国家战略的地质实践,其理论体系构建与实践模式探索具有重要意义。本文基于乡村振兴视角,以“生态振兴”、“文化振兴”、“产业振兴”、“人才振兴”和“组织振兴”五个方面为总纲,以“两山理论”、“乡村治理理论”等为指导,提出地质文化村建设应以“地质保护优先”+“科学合理利用”为核心宗旨,以“讲好地质故事”+“传承乡村文化”为发展动力,以“产业体系构建”+“村民增收致富”为重点内容,以“人才队伍建设”+“共建共享共赢”为建设方式,以“地质价值认同”+“基层组织建设”为根本保证,形成地质文化村的理论框架体系。在实践模式方面,体现了“三生一体”、“近景、近园”、“多元融合发展”、“村支两委+村民”主导等模式。在未来发展中,应大力开展全国创建试点,加大地质文化村理论研究的广度和深度,构建服务地方工作机制。  相似文献   
838.
A major problem associated with marine spatial planning (MSP) involves the difficult and time-consuming practice of creating a scenario that encompasses complex datasets in near real time via the use of a simple spatial analysis method. Moreover, decision-makers require a reliable, user-friendly system to quickly and accessibly acquire accurate spatial planning information. The development of national spatial data infrastructure (NSDI), which links the spatial data of a nation’s many diverse institutions, may pave the way for the development of a tool that can better utilize spatial datasets, such as a spatial decision support system (SDSS). Thus, this project aimed to develop an SDSS for MSP and to evaluate the feasibility of its integration within the NSDI framework. The seaweed culture was selected as an example due to its economic and technological acceptance by traditional fishers. Additionally, a multi-criteria analysis was used to develop the tool. Furthermore, a feasibility evaluation of its implementation within the NSDI framework was conducted based on the Delphi method. The results of the assessment indicated that the SDSS can be incorporated into the NSDI framework by addressing the policy issue – one map policy, updating custodians’ decree and data, and improve the standard and protocol.  相似文献   
839.
基于福建省流动人口问卷调查数据,使用多种流动指标分析和生存分析方法,探讨不同性别流动人口城市间流动的时空特征和规律。研究发现:流动人口的城市间流动是一种常见的现象,在空间特征上,遵循随着流动次数的增加在距离上由近及远、在城市等级上由小到大的规律,且流动距离相对于初次流动趋于缩小,流动的就近性特征显著;在时间特征上,大多数城市间流动发生在流动生涯的前期,随着流动时间的延长,流动状态趋于稳定;在性别差异上,女性城市间流动的强度大于男性,地域选择具有明显的大城市导向,而男性的流动空间活动范围大,地域选择则是大城市和小城市同时兼顾并重。  相似文献   
840.
Food web structure and trophic levels in polyculture rice-crab fields   总被引:1,自引:0,他引:1  
Stable carbon and nitrogen isotopes were used to investigate nutrient pathways and trophic relationships from the rice-crab system in Panjin,Liaoning Province,China. Values of δ 13 C ranged from-27.38‰±0.44‰ to-18.34‰±0.26‰ and δ 15 N ranged from 1.10‰±0.88‰ to 9.33‰±0.57‰. Pseudorasbora parva(Stone moroko) had the highest δ 13 C and δ 15 N values. The lowest δ 13 C values were obtained for the macrophytes and the lowest δ 15 N value was found in sediments. Stable carbon and nitrogen isotopes were used to determine the contribution of different food items to the diets of crabs. The δ 13 C results indicated that the P seudorasbora parva made the greatest contribution to the diet of E riocheir sinensis(Chinese mitten crab),while the δ 15 N results indicated that most food items contributed more than 10% to the diet of the crab. There were three trophic levels identified in the system(Levels 0–2). The crab Eriocheir sinensis,fish Pseudorasbora parva and Misgurnus anguillicaudatus(Oriental weatherfish),and the oligochaete L imnodrilus hoffmeisteri(Limnodrilus worm),were at the second level,zooplankton were at the first level and suspended particulate matter and macrophytes were at trophic position 0.  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