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   91805篇
  免费   5621篇
  国内免费   45篇
林业   3119篇
农学   2812篇
基础科学   557篇
  10482篇
综合类   17426篇
农作物   3801篇
水产渔业   4499篇
畜牧兽医   47842篇
园艺   1138篇
植物保护   5795篇
  2019年   853篇
  2018年   1256篇
  2017年   1429篇
  2016年   1357篇
  2015年   1188篇
  2014年   1434篇
  2013年   3636篇
  2012年   2698篇
  2011年   3197篇
  2010年   2074篇
  2009年   2038篇
  2008年   3113篇
  2007年   2990篇
  2006年   2847篇
  2005年   2696篇
  2004年   2521篇
  2003年   2568篇
  2002年   2420篇
  2001年   3008篇
  2000年   2899篇
  1999年   2368篇
  1998年   1015篇
  1997年   926篇
  1996年   821篇
  1995年   992篇
  1994年   894篇
  1993年   885篇
  1992年   1890篇
  1991年   1898篇
  1990年   1848篇
  1989年   1908篇
  1988年   1643篇
  1987年   1759篇
  1986年   1848篇
  1985年   1790篇
  1984年   1421篇
  1983年   1266篇
  1982年   908篇
  1981年   799篇
  1979年   1199篇
  1978年   986篇
  1977年   828篇
  1976年   776篇
  1975年   847篇
  1974年   1042篇
  1973年   1095篇
  1972年   1047篇
  1971年   998篇
  1970年   926篇
  1969年   858篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Archeological deposits have been discovered in three pimple mounds on the upper Texas coast. Such incorporation indicates that (i) these particular mounds are aggradational rather than erosional in origin; (ii) mound formation continued until at least A.D. 1100 to 1200; (iii) each mound took 300 to 500 years to form; (iv) mounds within the same mound field did not form simultaneously; (v) certain Gulf Coast pimple mounds have archeological research potential; and (vi) archeological data also offer a means for investigating mound formation.  相似文献   
962.
963.
For the scientist who wishes to synthesize complex organic compounds, the most difficult problem is often establishing the correct configuration at the various chiral centers as the synthesis is being carried out. In the past decade, there has been an increasing effort to find direct solutions to this problem, which is particularly acute in the synthesis of acyclic and other conformationally flexible molecules. One of the oldest organic reactions, the aldol condensation, is emerging as a powerful tool for use in achieving such stereocontrol.  相似文献   
964.
965.
Abstract Sir, - Andrews'((2)) checklist of helminth parasites of wild ruminants in New Zealand recorded the presence of 10 species of gastrointestinal nematodes in chamois (Rupicapra rupicapra rupicapra L.) from "Central South Island". In October 1978 we examined 28 freshly-shot chamois cadavers from the Harper-Avoca watershed in the headwaters of the Rakaia River. Six of the nematode species reported by Andrews were found again; viz. Ostertagia ostertagi, O. circumcincta, O. trifurcata, Nematodirus filicollis, Oesophagostomum venulosum and Trichuris ovis. The 4 species found by Andrews that were not recovered by us from the animals we examined were: Trichostrongylus axei, T. vitrinus, Spiculotragia spiculoptera and S. asymmetrica. On the other hand we found 4 helminths not encountered by Andrews: 3 nematodes: Nematodirus spathiger (Railliet, 1896), N. abnormalis (May, 1920), and Chabertia ovina (Fabricius, 1794), and 7 specimens of the cestode Moniezia expansa (Rud., 1805). Nematodirus spathiger was the most commonly encountered Nematodirus species, but generally the numbers of all 3 species encountered were low as might be expected in a sample of adult and almost year-old animals. It is probable that most of the adults had acquired some measure of immunity((3)), and the most susceptible group, the kids, would not be born until late November. Because peptic digestion was not used to recover juveniles from the intestinal submucosa, only small numbers of adults were obtained.  相似文献   
966.
Strips of rumen wall from bovine fetuses were incubated in an organ bath with prostaglandin F2 alpha (0.13 to 33.76 g/ml). The highest reactivity with a submaximal dose (17.03 g/ml) was observed in the period between 3.0 and 7.9 months of fetal age. A smaller response, but higher than in 1.0 to 2.9 months old fetuses, was observed in the 8.0 to 8.9 months fetuses. The period of the highest reactivity to prostaglandin F2 alpha coincides with the age of onset of papillary morphogenesis and the period of highest reactivity to autonomic and putative transmitter drugs.  相似文献   
967.
968.
969.
970.
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