全文获取类型
收费全文 | 9572篇 |
免费 | 862篇 |
国内免费 | 705篇 |
专业分类
11139篇 |
出版年
2024年 | 32篇 |
2023年 | 118篇 |
2022年 | 302篇 |
2021年 | 487篇 |
2020年 | 299篇 |
2019年 | 411篇 |
2018年 | 406篇 |
2017年 | 293篇 |
2016年 | 406篇 |
2015年 | 598篇 |
2014年 | 752篇 |
2013年 | 737篇 |
2012年 | 873篇 |
2011年 | 758篇 |
2010年 | 463篇 |
2009年 | 420篇 |
2008年 | 401篇 |
2007年 | 408篇 |
2006年 | 342篇 |
2005年 | 291篇 |
2004年 | 233篇 |
2003年 | 251篇 |
2002年 | 226篇 |
2001年 | 194篇 |
2000年 | 182篇 |
1999年 | 190篇 |
1998年 | 103篇 |
1997年 | 103篇 |
1996年 | 107篇 |
1995年 | 74篇 |
1994年 | 82篇 |
1993年 | 56篇 |
1992年 | 81篇 |
1991年 | 71篇 |
1990年 | 60篇 |
1989年 | 62篇 |
1988年 | 47篇 |
1987年 | 40篇 |
1986年 | 18篇 |
1985年 | 29篇 |
1984年 | 12篇 |
1983年 | 23篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 16篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Yihe Lu Bojie Fu Liding Chen Zhiyun Ouyang Jianying Xu 《Biodiversity and Conservation》2006,15(8):2813-2827
Resolving the conflicts between biodiversity conservation and socioeconomic development is a global pursuit for the long-run
prospects of the human species. Based on Wenchuan County, a typical county in southwestern China, a group of 20 indicators
quantifying regional biodiversity and socioeconomic development was established to classify and evaluate the county area spatially.
A fuzzy c-means clustering (FCM) algorithm was used as the classification method. Three indices including BD, DL and DR characterizing
the value of biodiversity, the level and rate of socioeconomic development of the delineated regions were formulated. The
results indicated that Wenchuan County was optimally classified into 4 types of regions (region I to IV). The area percentages
of the regions vary widely from 4.3 to 65.7%. The sequences of the regions on biodiversity, socioeconomic development level,
and socioeconomic development rate were, respectively, IV > II > III > I, I > III > II > IV and III >I >II >IV. The spatial
strategy on coordinating biodiversity conservation and regional development is to develop mainly from the east(I, II, III)
and to conserve mainly in the west(IV). Eco-industry, such as eco-tourism and eco-agriculture, need to be emphasized in the
process of regional development. The quantitative methods used here may have a wide applicability. 相似文献
2.
The mechanism of mass collaboration in risk management was studied during the Sichuan earthquake under a Web-based “PeopleFinder” project, where information is contributed and shared among mass contributors. The case study is provided by a great earthquake that happened in Wenchuan County, Sichuan Province, of southwestern China at 2:28 p.m. on May 12, 2008. We witnessed and experienced the rescue and relief efforts for the great earthquake. In this article, two fundamental frameworks are developed to study the mechanism of mass collaboration. Mass collaboration is proven to be effective in a big public crisis such as the Sichuan earthquake. 相似文献
3.
4.
5.
Prognostic significance of DNA ploidy and morphometric analyses of adenocarcinoma of the uterine cervix 总被引:1,自引:0,他引:1
Y S Fu T L Hall J S Berek N F Hacker J W Reagen 《Analytical and quantitative cytology and histology / the International Academy of Cytology [and] American Society of Cytology》1987,9(1):17-24
In an effort to improve the prognostic accuracy of the histologic criteria used for cervical adenocarcinomas, the nuclear DNA ploidy levels, means and standard deviations of nuclear areas and amounts of lumen and neoplastic tissue were quantitated. Useful thresholds in discriminating recurrent disease, as identified by logistic regression analysis, included a DNA ploidy level of 3.0 N, a percent of lumen of 34.6% and nuclear area mean and standard deviation of 53.1 sq micron and 20.1 sq micron, respectively. These parameters should provide useful guidelines in the visual assessment of histologic features that have prognostic significance. 相似文献
6.
8.
Many N2-fixing organisms can turn off nitrogenase activity in the presence of NH4
+ and turn it on again when the NH4
+ is exhausted. One of the most interesting systems for accomplishing this is by covalent modification of one subunit of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase (DRAT). The system can be reactivated when NH4
+ is exhausted, by dinitrogenase reductase activating glycohydrolase (DRAG) which removes the inactivating group. It is fascinating that some species of the genusAzospirillum possess the DRAT and DRAG systems (A. lipoferum andA. brasilense), whereasA. amazonense in the same genus lacks DRAT and DRAG.A. amazonense responds to NH4
+ but does not exhibit modification of dinitrogenase reductase characteristic of the action of DRAT. However, it has been possible to clone DRAT and DRAG and to introduce them intoA. amazonense, whereupon they become functional in this organism. The DRAT and DRAG system does not appear to function inAcetobacter diazotrophicus, an organism isolated from sugar cane, that fixes N2 at a pH as low as 3.0.A. diazotrophicus does show a rather sluggish response to NH4
+. A level of about 10 M NH4
+ is required to switch off the system. The response to NH4
+ is influenced by the dissolved oxygen concentration (DOC) as has been reported forAzospirillum sp. A DOC in equilibrium with 0.1 to 0.2 kPa O2 seems optimal for the response inA. diazotrophicus. 相似文献
9.
青甘边区黑河流域森林植被带及其昆虫区系的组成 总被引:1,自引:0,他引:1
黑河发源于青海、甘肃交界的祁连山区,是甘肃河西走廊最大的内陆河水系之一,水源丰富,流量稳定,这不仅取决于祁连山区大气降水和冰川融水,更重要的是与上游森林植被密不可分。但是,由于森林害虫猖獗成灾,严重威胁着这一地域森林的生存。近几年我们在森林昆虫普查的基础上,对黑河流域森林昆虫 相似文献
10.
Monoclonal antibodies against human myelomonocytic cells: detection of certain lineage-specific antigens on CFU-GM progenitor cells 总被引:1,自引:0,他引:1
A series of monoclonal antibodies (mAb) were raised against nonlymphoid leukemic cell lines. Three of them have been characterized in detail. mAb H8 (IgG2), mAB U2 (IgG1), and mAb ML143 (IgM) were established with HEL, an erythroleukemia cell line, U937, a monocytoid (histiocytic) line, and ML-1, a myeloid cell line as immunogen, respectively. A 65 to 75 KD polypeptide was precipitated from monocytes by mAb H8, a 160 KD protein from monocytes by mAb U2, and two broad bands in the regions of 150 and 195 KD from granulocytes by mAb ML143. All three mAb stained peripheral blood monocytes and granulocytes, but not lymphocytes, platelets, and erythrocytes. The mAb reacted with immature myeloid cells in bone marrow, ranging from myeloblasts to mature myelomonocytic cells. They also were reactive with various nonlymphoid cell lines and leukemia of myelomonocytic origin. They did not react with B cell lines and B cell CLL cells. By complement-mediated cytolysis and/or an immune rosette method, antigens H8 and U2 were found to be expressed on the vast majority of CFU-GM (14 days) progenitors but not on BFU-E. Antigen ML143 was not expressed by either progenitor. Furthermore, ML143 antigen was found on T leukemia cell lines, a subpopulation of mitogen-activated T cells, and certain non-T/non-B ALL cells. This reactivity was not found with mAb H8 and U2. The relationship between these mAb and those reported are discussed. The possibility of using these mAb to obtain a markedly enriched CFU-GM progenitor population is also raised. 相似文献