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   145864篇
  免费   10061篇
  国内免费   136篇
林业   6384篇
农学   5013篇
基础科学   923篇
  20598篇
综合类   21784篇
农作物   5926篇
水产渔业   9369篇
畜牧兽医   73472篇
园艺   1988篇
植物保护   10604篇
  2021年   1301篇
  2020年   1784篇
  2019年   2375篇
  2018年   2983篇
  2017年   3282篇
  2016年   3099篇
  2015年   2550篇
  2014年   3111篇
  2013年   6487篇
  2012年   5104篇
  2011年   6089篇
  2010年   4043篇
  2009年   3896篇
  2008年   5670篇
  2007年   5264篇
  2006年   4854篇
  2005年   4543篇
  2004年   4291篇
  2003年   4373篇
  2002年   3868篇
  2001年   4372篇
  2000年   4475篇
  1999年   3518篇
  1998年   1517篇
  1997年   1439篇
  1996年   1332篇
  1995年   1571篇
  1994年   1382篇
  1993年   1258篇
  1992年   2629篇
  1991年   2746篇
  1990年   2635篇
  1989年   2654篇
  1988年   2405篇
  1987年   2386篇
  1986年   2472篇
  1985年   2392篇
  1984年   1913篇
  1983年   1711篇
  1979年   1786篇
  1978年   1434篇
  1977年   1179篇
  1976年   1225篇
  1975年   1299篇
  1974年   1531篇
  1973年   1562篇
  1972年   1433篇
  1971年   1334篇
  1970年   1259篇
  1969年   1296篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
A few of the scientific challenges of polar research are reviewed, with stress on the work being done by U.S. investigators. The accomplishments that have been made are impressive, but much remains to be done. An evaluation of future research goals and priorities in science per se and in the national interest is not attempted, but a research strategy for the future, presently planned by the National Research Council's Polar Research Board, should contribute to this end.  相似文献   
994.
In a consecutive and non-selected study of 54 kidney transplanted dogs, 12 cases of intussusception of the small intestine developed in 8 dogs. The patogenesis is still unknown. It has been postulated that the time of the operation (4--7 hours) and the segmental termination of the post-operative atony in the intestine are essential factors in this fatal complication. The last 16 dogs in this material were peroperatively treated with antikolinergics (Mestinon NFN), which was continued until normal function of the intestine. In these dogs there were no evidence of intussusception. The value of this profylaxis is open to discussion and it is essential to solve the problem of intussusception through a "blind" and controlled trial.  相似文献   
995.
The hemolysis of unsensitized human erythrocytes by fresh bovine serums was investigated. Lysis occurred in ethylene glycol bis-amino tetraacetate buffers and with serums depleted of Clq. Serums extensively absorbed with packed human erythrocytes at 0 C effectively lysed human erythrocytes, but optimal lytic capacity required target cells "sensitized" with a heat-stable serum factor. Lysis did not occur with serums absorbed with zymosan at 17 C or heat inactivated at 50 C. These results indicate that human erythrocytes can activate the alternative pathway of complement in bovine serums. Lysis can proceed in the apparent absence of antibodies, although their presence may enhance the reaction.  相似文献   
996.
997.
A mouse model system was used to study various aspects of host and parasite relationships in Trypanosoma vivax infections. These included the phenomenon of antigenic variation, the variable parasite antigens responsible for this phenomenon, parasite-host adaption, host immune responses and the role of genes in the major histocompatibility complex in the control of infection. While the mouse model system has allowed investigation of these aspects of host parasite relationships, it is clear that the system is much more limited than those generally used in T. brucei spp and T. congolense infections. This is indicated by the discovery that not all VATs of T. vivax were equally infective for mice, though in some cases infectivity could be improved by bovine serum supplementation and/or immunosuppression of the mouse host. In the case of rats, infection was even restricted to a smaller number of the VATs studied. It was, however, possible to biochemically characterize the variable surface antigen carried by T. vivax and show its similarity to those carried by T. brucei and T. congolense. The H-2 complex was found not to influence acquired resistance of inbred strains. Cyclic transmissions of T. vivax infections to goats combined with chemotherapy were carried out in an attempt to induce protection to subsequent infection as has been shown in T. brucei and T. congolense infections. Such protection could, however, not be obtained, The failure of the metacyclic VATs to induce immunity, was perhaps due to rapid decrease in antibody titres to bloodstream VATs found after treatment and prior to rechallenge. The usefulness of the mouse model system in elucidating the mechanisms responsible for the non-H-2 linked differences in susceptibility to T. vivax infections should be further explored and its relevance to mechanisms of trypanotolerance in domestic ruminants defined.  相似文献   
998.
Following concomitant intravenous administration of Tomanol and sodium penicillin G to six Dutch Friesian dairy cows a significant decrease in total body clearance of penicillin (34.7%) and a prolongation of the elimination half-life of penicillin (17.2%) was observed. Tomanol did not affect other pharmacokinetic parameters such as rate constants of drug transfer (k12/k21, alpha en beta), distribution volume of the central compartment (V1), and extrapolated serum drug levels. Intravenous or intramuscular administration of Tomanol had no effect on the tissue distribution of penicillin G, because neither a change in the ratios of muscle to serum and of kidney cortex to serum nor a change in an induced steady state level of low penicilline G serum concentrations was observed. From the data obtained it is concluded that concomitant Tomanol administration with penicillin induces an elevation of the serum penicillin concentration and prolongs the persistence of penicillin residues in carcass meat and organs.  相似文献   
999.
1000.

Epizootiological aspects of porcine respiratory diseases which were encountered in Quebec from September 1980 until February 1981  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