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   214572篇
  免费   11493篇
  国内免费   7909篇
林业   14317篇
农学   16098篇
基础科学   7789篇
  28740篇
综合类   44833篇
农作物   8876篇
水产渔业   9734篇
畜牧兽医   84960篇
园艺   3891篇
植物保护   14736篇
  2022年   2024篇
  2021年   3338篇
  2020年   3298篇
  2019年   3463篇
  2018年   3324篇
  2017年   4214篇
  2016年   4162篇
  2015年   4674篇
  2014年   4846篇
  2013年   9003篇
  2012年   8010篇
  2011年   9439篇
  2010年   7402篇
  2009年   7205篇
  2008年   8766篇
  2007年   8591篇
  2006年   8341篇
  2005年   7583篇
  2004年   5631篇
  2003年   5445篇
  2002年   4993篇
  2001年   5962篇
  2000年   6070篇
  1999年   5537篇
  1998年   3335篇
  1997年   3052篇
  1996年   2815篇
  1995年   3086篇
  1994年   2752篇
  1993年   2621篇
  1992年   4086篇
  1991年   4061篇
  1990年   3992篇
  1989年   3893篇
  1988年   3487篇
  1987年   3393篇
  1986年   3383篇
  1985年   3160篇
  1984年   2678篇
  1983年   2372篇
  1982年   1624篇
  1979年   2330篇
  1978年   1916篇
  1977年   1630篇
  1974年   2074篇
  1973年   2089篇
  1972年   2054篇
  1971年   1817篇
  1970年   1712篇
  1969年   1573篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Up to 60 percent of the total marine primary production (or about one-fourth of the total global carbon dioxide fixation) passes through the free-living bacterioplankton. Grazing by bacteriovores is probably the predominant fate of the bacteria, although data are scarce. Evidence is presented that previously uncharacterized, small eukaryotes that are able to pass even 0.6-micrometer filters may be responsible for a large fraction (more than 50 percent) of the total grazing in coastal waters. These organisms have not yet been observed microscopically.  相似文献   
993.
994.
995.
996.
1. It has been shown that both the variety of wheat and the environment under which growth took place influenced the quantities of starch and cell wall carbohydrate contents of wheat.

2. The different varieties of wheat investigated contained different proportions of water‐soluble to water‐insoluble hemicelluloses.

3. By comparison of the arabinose to xylose ratios it was concluded that water‐soluble and insoluble arabinoxylans of wheat were branched to the same degree.

4. Starch from six wheat varieties grown in the U.K. was very well digested by adult cockerels and true metabolisable energy values of the wheats were high.

5. Small amounts of cell wall carbohydrates were digested by adult cockerels.  相似文献   

997.
1. Conditions affecting the keeping quality of traditional farm‐fresh turkeys were investigated.

2. Storage of uneviscerated Wrolstad turkeys at 4 °C for 10 days caused no statistically significant changes in meat flavour or texture.

3. During further storage at —2 °C, however, there was a slight but significant change in flavour, which became more marked with time in birds which had been eviscerated after the initial period at 4 °C.

4. Both eviscerated and uneviscerated birds became slightly tougher during storage.

5. Initial holding at 4 °C increased the numbers of psychrotrophic bacteria on the skin by about 103 but subsequent changes at — 2 °C were slight for uneviscerated birds.

6. Eviscerated carcases had higher counts than uneviscerated birds after storage at — 2 °C and, although ‘off’ odours were not detected, spoilage appeared to be imminent at the end of the 20‐d period.  相似文献   

998.
The Colorado beetle is protected against predators by the secretions of defensive glands located on the pronotum and the elytra. The single major compound of the secretion was identified as gamma-L-glutamyl-L-2-amino-3(Z),5-hexadienoic acid by spectroscopic and chemical methods. This compound, which contains a nonprotein beta,gamma- unsaturated amino acid, is toxic to ants (Myrmica rubra) at a concentration 10(-2) molar, which is less than its estimated concentration in the secretion (1.8 x 10(-1) molar).  相似文献   
999.
The earliest fossil record of African anthropoid primates (monkeys and apes) comes from the Jebel Qatrani Formation in the Fayum depression of Egypt. Reevaluation of both geologic and faunal evidence indicates that this formation was deposited in the early part of the Oligocene Epoch, more than 31 million years ago, earlier than previous estimates. The great antiquity of the fossil higher primates from Egypt accords well with their primitive morphology compared with later Old World higher primates. Thus, the anthropoid primates and hystricomorph rodents from Fayum are also considerably older than the earliest higher primates and rodents from South America.  相似文献   
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