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   280867篇
  免费   22522篇
  国内免费   44314篇
测绘学   9070篇
大气科学   39366篇
地球物理   64288篇
地质学   117792篇
海洋学   36485篇
天文学   56316篇
综合类   4848篇
自然地理   19538篇
  2021年   2709篇
  2020年   4340篇
  2019年   8021篇
  2018年   8798篇
  2017年   8321篇
  2016年   10794篇
  2015年   8693篇
  2014年   10844篇
  2013年   18622篇
  2012年   10581篇
  2011年   11375篇
  2010年   10759篇
  2009年   12666篇
  2008年   10852篇
  2007年   10081篇
  2006年   11553篇
  2005年   9720篇
  2004年   9966篇
  2003年   9214篇
  2002年   8551篇
  2001年   7618篇
  2000年   7182篇
  1999年   6537篇
  1998年   6682篇
  1997年   6576篇
  1996年   5843篇
  1995年   5492篇
  1994年   5121篇
  1993年   4779篇
  1992年   4400篇
  1991年   4163篇
  1990年   4287篇
  1989年   3966篇
  1988年   3685篇
  1987年   4167篇
  1986年   3666篇
  1985年   4413篇
  1984年   4913篇
  1983年   4548篇
  1982年   4470篇
  1981年   4033篇
  1980年   3716篇
  1979年   3584篇
  1978年   3500篇
  1977年   3296篇
  1976年   3045篇
  1975年   2956篇
  1974年   2917篇
  1973年   3076篇
  1972年   2026篇
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
141.
Histological study on the ontogeny of the lymphoid organs, kidney, thymus and spleen of Japanese flounder, Paralichthys olivaceus, from hatching to 40 d was carried out. The pronephric kidney duct appeared early in hatching although the primordial haemopoietic stem cells were observed within a week after hatching. The spleen was first seen after 8d of hatching. The thymus appeared after 15d, situated near the pronephric kidney. Small lymphoid cells appeared during the later phase of the post-larval stage in the sequence of thymus, kidney and spleen. During the 40d of observations, there were no distinct inner or outer zones in thymus and no red or white pulp in spleen. These results suggest that the nonspecific defense immune system plays a very important role in the early larval stage of Japanese flounder.  相似文献   
142.
143.
144.
145.
146.
147.
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them.  相似文献   
148.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
149.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems.  相似文献   
150.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during 1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles. The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed.  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