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   89914篇
  免费   5540篇
  国内免费   46篇
林业   3736篇
农学   3473篇
基础科学   632篇
  11274篇
综合类   13556篇
农作物   3362篇
水产渔业   4944篇
畜牧兽医   47717篇
园艺   1168篇
植物保护   5638篇
  2020年   733篇
  2019年   957篇
  2018年   1590篇
  2017年   1848篇
  2016年   1663篇
  2015年   1406篇
  2014年   1722篇
  2013年   3726篇
  2012年   2936篇
  2011年   3597篇
  2010年   2343篇
  2009年   2334篇
  2008年   3480篇
  2007年   3218篇
  2006年   3138篇
  2005年   2674篇
  2004年   2590篇
  2003年   2601篇
  2002年   2377篇
  2001年   3392篇
  2000年   3308篇
  1999年   2559篇
  1998年   985篇
  1997年   1053篇
  1996年   885篇
  1995年   1022篇
  1994年   899篇
  1993年   900篇
  1992年   1848篇
  1991年   1919篇
  1990年   1859篇
  1989年   1863篇
  1988年   1610篇
  1987年   1597篇
  1986年   1630篇
  1985年   1531篇
  1984年   1227篇
  1983年   1035篇
  1982年   714篇
  1979年   1060篇
  1978年   817篇
  1975年   815篇
  1974年   899篇
  1973年   865篇
  1972年   806篇
  1971年   795篇
  1970年   803篇
  1969年   780篇
  1968年   708篇
  1967年   735篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
221.
222.
The effects of canopy cover, seeding depth, and soil moisture on emergence of Centaurea maculosa Lam. and C. diffusa Lam. were studied. Canopy cover had no effect on ermergence rate (seedlings/day/100 seeds) of either species. Seeds of both species emerged faster when placed on the soil surface. Emergence rate decreased as seeding depth increased. Both species required more than 55% initial soil moisture to initiate emergence, with 65–70% being optimum. Percentage emergence followed exactly the same trend as emergence rate. C. maculosa had better emergence characteristics over a wider range of conditions than C. diffusa, possibly being some of the reasons for the former having a wider geographical distribution in the United States and Canada.  相似文献   
223.
Summary The study reported in the present paper discusses the clinical and histological picture of bovine demodecosis and the morphology of Demodex mites as seen in four cows suffering from generalized demodecosis. There were no clinical signs of other skin affections. Changes in both the number and the appearance of visible skin lesions were seen and related to the level of nutrition and the exposure to sunshine of the cattle. Histological sections of some skin nodules showed the presence of mite colonies in the hair follicles. Only adults were seen in the sebaceous glands. Microscopical study of the morphology of the mites revealed the presence of two types of demodicids in the skin lesions and three types from epilated eyelashes. Morphological criteria are presented to aid in identification of species and of life stages.  相似文献   
224.
225.
226.
227.
The development of fuels from biomass can lead naturally to dispersed facilities that incorporate food or materials production (or both) with fuel production, forming adaptive systems that can be modified to meet evolving needs and constraints. The technology that is appropriate to each system needs to be worked out, taking into account associated food and materials opportunities in order to decrease the ultimate cost of energy delivered to the consumer. I analyze possible systems based on sugarcane, corn, and guayule.  相似文献   
228.
Partially purified alkaline phosphatase (ALP) from canine intestine, liver, and bone were injected into rabbits to elicit anti-canine intestinal, hepatic, and osseous ALP antibodies, respectively. The antibody formed a soluble enzyme-antienzyme complex when directly interacted with the ALP antigen. In order to form an insoluble complex, it was then necessary to interact the initial soluble complex with the goat anti-rabbit gamma-globulin antibody in the 2nd step. Antiintestinal ALP antibody was highly specific and did not cross react with canine hepatic, osseous, splenic, and renal ALP. Antiliver and antibone ALP antibodies, on the other hand, did cross react with hepatic, osseous, splenic, and renal ALP, but not with the intestinal ALP.  相似文献   
229.
230.
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