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   60260篇
  免费   3493篇
  国内免费   33篇
林业   2427篇
农学   1852篇
基础科学   302篇
  6632篇
综合类   10180篇
农作物   2059篇
水产渔业   2810篇
畜牧兽医   33047篇
园艺   722篇
植物保护   3755篇
  2018年   899篇
  2017年   913篇
  2016年   811篇
  2015年   777篇
  2014年   845篇
  2013年   2591篇
  2012年   1674篇
  2011年   2025篇
  2010年   1275篇
  2009年   1254篇
  2008年   1893篇
  2007年   1857篇
  2006年   1732篇
  2005年   1586篇
  2004年   1504篇
  2003年   1547篇
  2002年   1476篇
  2001年   1814篇
  2000年   1786篇
  1999年   1468篇
  1998年   621篇
  1997年   609篇
  1996年   546篇
  1995年   642篇
  1994年   601篇
  1993年   572篇
  1992年   1224篇
  1991年   1371篇
  1990年   1257篇
  1989年   1265篇
  1988年   1163篇
  1987年   1137篇
  1986年   1194篇
  1985年   1196篇
  1984年   961篇
  1983年   903篇
  1982年   584篇
  1979年   891篇
  1978年   702篇
  1977年   696篇
  1976年   712篇
  1975年   716篇
  1974年   870篇
  1973年   847篇
  1972年   813篇
  1971年   728篇
  1970年   769篇
  1969年   733篇
  1968年   651篇
  1967年   659篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Abstract – The reproductive biology of three species of rainbowfish (Melanotaeniidae) in northeastern Australian rainforest streams was investigated. Two species, Melanotaenia eachamensis and Cairnsichthys rhombosomoides are endemic to the area, whereas the third, M. splendida splendida, is more widespread. The species were all highly fecund, producing many hundreds of eggs between 1.10 and 1.24 mm in diameter. Melanotaenia eachamensis was the most fecund, produced the largest eggs of the three species, and consequently exhibited the greatest maternal investment (as measured by gonadosomatic index). The majority of reproductive effort occurred during the dry season, although reproductively active fish were present year-round for each of the species, but particularly so for M. s. splendida and C. rhombosomoides. No evidence for a role by temperature or photoperiod as environmental cues for reproduction was found, and it was suggested that gonad development was strongly tied to somatic growth. The concentration of reproduction to the dry season ensures that larvae are produced during a period of relatively stable and benign physical conditions. Comparison of temporal changes in gonadosomatic index values suggest that the spawning season of M. eachamensis , which occurs in high-elevation streams, is more restricted and commences about 1 month earlier than either other species. A similar phenology was observed for the M. s. splendida population found at high elevation and highlights the potential for spatial differences in stream productivity to influence life history. Note  相似文献   
32.
33.
Images with high resolution and exceptionally broad gray scale can be obtained by the application of video contrast enhancement to an optimized procedure for imaging transparent objects with oblique rays of illumination. This technique is simple to set up. A conventional microscope with a light source whose position can be adjusted and a video camera with controls for gain and black level are the only essential components. Features such as high resolution, optical sectioning, control of contrast, and operation under low light intensity make this technique preferable, in several instances, to currently used video microscopy techniques.  相似文献   
34.
Washed and unwashed red blood cells (RBC) from young calves, adult cattle, hamsters and humans were incubated with Leptospira interrogans serovars pomona and ballum. Washed cells suspended in saline were always haemolysed while unwashed cells and those which were washed and resuspended in plasma were never haemolysed, despite the presence of large numbers of organisms within the culture supernatant. Pomona produced greater haemolysis of cattle and human RBC than did ballum, but with hamster RBC ballum produced greater haemolysis than did pomona. A group of 6- to 9-month-old cattle infected with pomona showed no signs of clinical disease and RBC taken from them before infection and during the development of antibodies to pomona were haemolysed by pomona only after the cells were washed. Plasma therefore appears to have a protective function. This in vitro protective function of plasma even extended to plasma from young seronegative calves.  相似文献   
35.
36.
37.
38.
39.
40.
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