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   31034篇
  免费   234篇
  国内免费   124篇
安全科学   520篇
废物处理   1697篇
环保管理   4017篇
综合类   4334篇
基础理论   9075篇
环境理论   10篇
污染及防治   7016篇
评价与监测   2295篇
社会与环境   2294篇
灾害及防治   134篇
  2021年   148篇
  2019年   131篇
  2018年   1763篇
  2017年   1672篇
  2016年   1608篇
  2015年   427篇
  2014年   481篇
  2013年   1610篇
  2012年   1032篇
  2011年   2178篇
  2010年   1462篇
  2009年   1308篇
  2008年   1808篇
  2007年   2207篇
  2006年   773篇
  2005年   756篇
  2004年   693篇
  2003年   753篇
  2002年   778篇
  2001年   784篇
  2000年   587篇
  1999年   331篇
  1998年   286篇
  1997年   280篇
  1996年   280篇
  1995年   332篇
  1994年   302篇
  1993年   284篇
  1992年   277篇
  1991年   278篇
  1990年   294篇
  1989年   277篇
  1988年   249篇
  1987年   240篇
  1986年   222篇
  1985年   200篇
  1984年   272篇
  1983年   219篇
  1982年   271篇
  1981年   219篇
  1980年   184篇
  1979年   201篇
  1978年   184篇
  1977年   153篇
  1976年   138篇
  1975年   143篇
  1974年   159篇
  1973年   162篇
  1972年   148篇
  1971年   146篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
321.
322.
323.
324.
A very dry climate which prevailed during the last glacial maximum of the Pleistocene from India to the Sahara, was replaced by variable weather conditions with alternating more or less humid phases, starting from the Pleistocene-Holocene transition period and lasting up to about 3000 years ago, when present arid conditions set in. A particularly wet phase is recognized throughout the region some 5000 years ago. Phase shifts within the region and in particular changes in rain distribution patterns, are interpreted as due to shifting spheres of influence of the monsoonal circulation, of Atlantic depressions and moisture originating from the Mediterranean Sea. The condition of the Mediterranean Sea itself, which is influenced by runoff and melt waters from the European Continent and by the Nile floods, as well as by the eustatic sea-level changes, is not a good indicator of local climate conditions.  相似文献   
325.

Neither the “Green Revolution” in the developing countries nor “yield explosion” in the agricultural production of the industrialized nations have reached the genetic and physiologic limits of crop growth. Breeding of better varieties is still the most promising defense against hunger. The ambiguity of modern agroindustry throughout the world resides in its high dependence on the existence of functional technological, economical, and political systems.

  相似文献   
326.
Amniocentesis and subsequent tests are reported on a fetus conceived of a rare mating type: its mother has an intermediate level of β hexosaminidase A (HEX A), characteristic of carriers of Tay-Sachs disease (TSD), while the father suffers from an adult-onset GM2 ganglio-sidosis (AOG) with severe HEX A deficiency. Activity of HEX A in the cultured fetal cells was very low when measured by the heat-inactivation method, thus showing the typical biochemical phenotype of TSD fetuses. However, upon separation of HEX isozymes by ion exchange chromatography, residual HEX A (17 per cent of total HEX) was demonstrated. Also in contrast to TSD fetuses, this fetus' fibroblasts were able to synthesize the precursor of a chains of HEX, and ultrastructural examination of its brain revealed few atypical lamellar bodies, unlike those found in TSD fetuses of the same gestational age. It is therefore concluded that the fetus was not affected with TSD, but rather with AOG.  相似文献   
327.
328.
Peptide chemistry has reached a plateau on which more work is devoted to the improvement of known basic principles than to the development of really new methods. As a consequence, routine syntheses of peptides concentrate on few protecting groups and coupling procedures only. The recent progress in purification of peptides by liquid chromatography helped to promote this trend. Meanwhile, recombinant technology permits the synthesis of proteins and possibly of some lower peptides, too. It cannot compete, however, with chemical solid-phase synthesis of a newly discovered peptide with up to about 40 amino acids in speed but may take over the synthesis for large-scale production. Peptides containing modified or non-natural structures, which are often superior to the natural compounds as drugs, will remain subject of chemical synthesis, too. Present research focuses on the role of peptide and protein factors in the immune response and on structure and biological activities of neuropeptides.  相似文献   
329.
330.
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