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   193529篇
  免费   7569篇
  国内免费   901篇
  201999篇
  2023年   561篇
  2022年   423篇
  2021年   994篇
  2020年   934篇
  2019年   954篇
  2018年   13567篇
  2017年   12237篇
  2016年   10204篇
  2015年   5375篇
  2014年   5137篇
  2013年   6832篇
  2012年   11970篇
  2011年   19498篇
  2010年   16010篇
  2009年   11226篇
  2008年   15254篇
  2007年   16683篇
  2006年   5795篇
  2005年   5676篇
  2004年   5793篇
  2003年   5455篇
  2002年   4855篇
  2001年   2416篇
  2000年   2295篇
  1999年   1867篇
  1998年   772篇
  1997年   613篇
  1996年   528篇
  1995年   547篇
  1994年   552篇
  1993年   444篇
  1992年   1257篇
  1991年   1190篇
  1990年   1039篇
  1989年   960篇
  1988年   914篇
  1987年   779篇
  1986年   692篇
  1985年   772篇
  1984年   672篇
  1983年   568篇
  1982年   430篇
  1981年   415篇
  1979年   571篇
  1978年   442篇
  1975年   432篇
  1974年   447篇
  1973年   455篇
  1972年   578篇
  1971年   617篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
P. Vargas  T. Cabello 《BioControl》1985,30(3):225-230
Trichogramma cordubensis n. sp. is a parasite ofHeliothis armigera Hb. eggs in cotton crops in western Andalusia. Reproduced onEphestia kuehniella Zeller eggs, it is described morphologically and compared withT. chilonis Ishii andT. evanescens Westwood as both present similarities with this species.T. cordubensis can be distinguished in the male genitalia by the shape of the dorsal expansion of the gonobase and the size, shape and relative position of the median ventral projection. Male antennal setae are longer inT. cordubensis than in the other 2 species. In addition, unlikeT. chilonis andT. evanescens, T. cordubensis is thelytokous at 20°C. in our laboratory, though males and gynandromorphs can be obtained by altering the rearing temperature, this being another difference of this species.  相似文献   
52.
53.
54.
55.
The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a) seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and (b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance, regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of the need to terminate ongoing stocking programs for introduced salmonines worldwide.  相似文献   
56.
57.
Following arteriolar occlusion, tissue oxygen concentration decreases and anoxic tissue eventually develops. Although anoxia first appears in the region most distal to the capillary at the venous end, it eventually spreads throughout the entire region of supply. In this paper the changing oxygen concentration, from the time of occlusion until the tissue is entirely anoxic, is examined mathematically. The equations governing oxygen transport to tissue are solved by iterating a nonlinear integral equation. This solution is valid until anoxia first appears. After anoxia develops it is necessary to solve a moving boundary problem. This is done using the method of matched asymptotic expansions, and accurate solutions are obtained for a wide range of physiological conditions.  相似文献   
58.
The discovery of epipsammitic Foraminifera in sediments of Moorea and Scilly (French Polynesia) and the study of close relationships between embedded specimens and host-grains indicate that some species participate in the weakening and subsequent breakdown of skeletal grains and consequently contribute to the production of silt-sized particles. The study of 36 stations around the islands shows the factors that control the distribution and abundance of this incrusted microfauna. Specimens are more abundant on sand particles larger than 1,000 microns and in the high energy areas; this abundance decreases with depth. The physiological mechanism of penetrations is presumably chemical, but each species apparently has its own process: complete dissolution with removal of ions for the cytoplasm of calcareous species; partial dissolution with transport of silt-size aggregates on the test of agglutinated species. The purpose of such penetration may be to protect themselves against water turbulence and to provide material for test construction.  相似文献   
59.
60.
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