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   4523篇
  免费   32篇
  国内免费   6篇
安全科学   71篇
废物处理   164篇
环保管理   207篇
综合类   1627篇
基础理论   947篇
环境理论   9篇
污染及防治   1122篇
评价与监测   212篇
社会与环境   192篇
灾害及防治   10篇
  2022年   41篇
  2021年   35篇
  2019年   47篇
  2018年   91篇
  2017年   107篇
  2016年   113篇
  2015年   109篇
  2014年   116篇
  2013年   210篇
  2012年   121篇
  2011年   200篇
  2010年   130篇
  2009年   134篇
  2008年   222篇
  2007年   189篇
  2006年   163篇
  2005年   125篇
  2004年   119篇
  2003年   111篇
  2002年   139篇
  2001年   87篇
  2000年   76篇
  1999年   55篇
  1998年   40篇
  1997年   42篇
  1996年   45篇
  1995年   51篇
  1994年   49篇
  1993年   30篇
  1992年   46篇
  1991年   38篇
  1990年   38篇
  1989年   33篇
  1981年   32篇
  1976年   32篇
  1968年   29篇
  1967年   35篇
  1966年   30篇
  1965年   48篇
  1964年   38篇
  1963年   36篇
  1962年   33篇
  1961年   45篇
  1960年   40篇
  1959年   44篇
  1958年   45篇
  1957年   43篇
  1956年   42篇
  1955年   37篇
  1954年   40篇
排序方式: 共有4561条查询结果,搜索用时 31 毫秒
991.
992.
Methods of tapping the nipa palm (Nipa fruticans Wurmb.) were developed in the course of a 3-year study conducted in seven villages in the Baimuru district of the Gulf Province of Papua New Guinea. The experimental sites were located about 1.5–23 km from the sea. Their plant density varied from 4700 to 2400 palms per ha and the mean height of the plants was nearly 13 m.Five pre-treatment patterns were tested. Once-a-week treatment, maintained for 12 weeks, yielded an average of 155 ml of sap/24 h/palm for one month, whereas 3–5-times-a-week treatment, for 5–12 weeks, yielded an average of 1300 ml of sap. The duration of treatment depended on the state of development of the flower stalk. The pre-treatment pattern chosen as the standard was to bend the stalk 12 times in one direction, to pat with the hands the length of the stalk backwards and forwards 64 times, and to give the base four kicks; this was repeated four times a week.In the course of the 3-year study it was found that the average sugar content of fresh nipa sap is rather high, about 16.4% w/v, as compared with, for example, about 12% w/v of sugar cane. The results of the present study are discussed with reference to producing fuel alcohol from the Papua New Guinea nipa palm.  相似文献   
993.
994.
995.
Hexactinellid sponges are often considered to be the most ancient metazoans. Lipid biomarkers from 23 species were studied for information on their phylogenetic properties, particularly their disputed relation to the two other sponge classes (Demospongiae, Calcarea). The most prominent lipid compounds in the Hexactinellida comprise C28 to C32 polyenoic fatty acids. Their structures parallel the unique patterns found in demosponge membrane fatty acids ('demospongic acids') and strongly support a close phylogenetic association of the Demospongiae and the Hexactinellida. Both taxa also show unusual mid-chain methylated fatty acids (C15-C25) and irregular C25- and C40-isoprenoid hydrocarbons, tracers for specific eubacteria and Archaea, respectively. These biomarkers indicate a similar, highly conservative symbiont community, although some shift in the abundance of the associated microbiota was observed. The lack of these features in calcareous sponges further contradicts the still common view that Calcarea and Demospongiae are more closely related to each other than either is to the Hexactinellida.  相似文献   
996.
The study of the fetal platelet count and size can, according to the literature, be used for the prenatal diagnosis of the Wiskott-Aldrich syndrome (WAS). So far, no affected fetuses have been identified by this method. All pregnancies in which this method had been applied to resulted, as correctly predicted, in the birth of normal children. Here we report on a familial case of WAS where the haematological parameters failed to reveal the affected second child. Hence we assume that the platelet count and size of platelets remain normal in fetuses with WAS to the gestational age of 22 weeks and cannot be used for prenatal diagnosis.  相似文献   
997.
One hundred and ninety women who contracted toxoplasmosis after the seventh week of pregnancy underwent antenatal diagnosis, including ultrasound examination and biological tests. Tests included Toxoplasma isolation in fetal blood and amniotic fluid by mouse inoculation, specific IgM and IgA in fetal blood, and non-specific tests. Twenty fetuses had positive specific as well as non-specific tests for Toxoplasma infection. At birth, four of these presented with clinical congenital toxoplasmosis and 12 with subclinical forms. Antenatal diagnosis enabled the detection of 83 per cent of the infected fetuses. Under specific conditions, cordocentesis permits early diagnosis and considerably reduces the number of terminations of pregnancy.  相似文献   
998.
We describe a rare case of non-immune hydrops fetalis caused by mediastinal teratoma. The sonographic appearance was that of a mixed cystic and solid mass in the antero-superior mediastinum. The teratoma, on post mortem, extended cranially to the upper part of the thyroid, exerting pressure and causing deviation of the trachea, oesophagus, and aortic arch. The pathogenesis of non-immune hydrops fetalis suggests obstruction of venous return caused by this tumour.  相似文献   
999.
1000.
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