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   95880篇
  免费   2578篇
  国内免费   54篇
  2018年   1022篇
  2017年   1202篇
  2016年   3015篇
  2015年   6484篇
  2014年   6120篇
  2013年   6560篇
  2012年   5354篇
  2011年   2664篇
  2010年   2463篇
  2009年   2415篇
  2008年   1235篇
  2007年   1273篇
  2006年   1306篇
  2005年   7420篇
  2004年   6181篇
  2003年   4353篇
  2002年   1895篇
  2001年   1207篇
  2000年   400篇
  1999年   1599篇
  1998年   537篇
  1997年   367篇
  1992年   2042篇
  1991年   2098篇
  1990年   2161篇
  1989年   2085篇
  1988年   2036篇
  1987年   1874篇
  1986年   1696篇
  1985年   1739篇
  1984年   1187篇
  1983年   920篇
  1982年   564篇
  1981年   515篇
  1980年   451篇
  1979年   1114篇
  1978年   806篇
  1977年   649篇
  1976年   680篇
  1975年   898篇
  1974年   1030篇
  1973年   1038篇
  1972年   976篇
  1971年   940篇
  1970年   830篇
  1969年   847篇
  1968年   744篇
  1967年   758篇
  1966年   591篇
  1965年   437篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Root hydrotropism of an agravitropic pea mutant, ageotropum   总被引:3,自引:0,他引:3  
We have partially characterized root hydrotropism of an agravitropic pea mutant, ageotropum (from Pisum sativum L. cv. Weibull's Weitor), without interference of gravitropism. Lowering the atmospheric air humidity inhibited root elongation and caused root curvature toward the moisture-saturated substrate in ageotropum pea. Removal of root tips approximately 1.5 mm in length blocked the hydrotropic response. A computer-assisted image analysis showed that the hydrotropic curvature in the roots of ageotropum pea was chiefly due to a greater inhibition of elongation on the humid side than the dry side of the roots. Similarly, gravitropic curvature of Alaska pea roots resulted from inhibition of elongation on the lower side of the horizontally placed roots, while the upper side of the roots maintained a normal growth rate. Gravitropic bending of Alaska pea roots was apparent 30 min after stimulation, whereas differential growth as well as curvature in positive root hydrotropism of ageotropum pea became visible 4–5 h after the continuous hydrostimulation. Application of 2,3,5-triiodobenzoic acid or ethyleneglycol-bis-( β -aminoethylether)-N,N,N',N'-tetraacetic acid was inhibitory to both root hydrotropism of ageotropum pea and root gravitropism of Alaska pea. Some mutual response mechanism for both hydrotropism and gravitropism may exist in roots, although the stimulusperception mechanisms differ from one another.  相似文献   
962.
The frxC gene, found in liverwort chloroplast DNA, encodes aprotein of unknown function. The deduced amino acid sequenceof the protein shows significant homology to that of ni-trogenaseFe-protein encoded by the nifH gene. We have cloned the frxCand nifH genes from the nitrogen-fixing cyanobacterium Plectonemaboryanum, using frxC- and nifH-specific probes, and have determinedtheir nucleotide sequences. The amino acid sequence deducedfrom the frxC gene of P. boryanum exhibits 83% homology to thatof the protein encoded by the/rxCgene from liverwort, whereasit exhibits only 34% homology to that encoded by the nifH genefrom the same organism, namely, P. boryanum. Northern blot analysisshowed that the frxC gene was transcribed more actively undernitrogenase-repressed conditions than under nitrogenase-inducedconditions, suggesting that the FrxC protein has a functiondistinct from nitrogen fixation. These results, together withthe phylogenetic relationship between the nifH and frxC genes,indicate that the frxC and nifH genes are derived from a commonancestral gene but have evolved independently to encode proteinswith different functions. (Received April 27, 1991; Accepted August 12, 1991)  相似文献   
963.
Steroidal saponins from the rhizomes of Polygonatum orientale.   总被引:6,自引:0,他引:6  
E Ye?ilada  P J Houghton 《Phytochemistry》1991,30(10):3405-3409
  相似文献   
964.
965.
Phagocytic index and rosette test E have been determined in 50 patients, including 15 with diffuse peritonitis and the noncomplicated diffuse peritonitis, 20 patients with septic complications, and 15 patients who underwent abdominal surgery. It was found, that phagocytic index is decreased in all patients after abdominal surgery. The most marked decrease of this index was found in patients with complicated peritonitis. It still decreases in the course of peritonitis and increases in the reference group. The marked decrease in the number of T-cells was observed in complicated peritonitis during the whole period of follow-up while it normalizes in case of noncomplicated peritonitis as in the reference group. Using the tests under study it was possible to assess cell-mediated immunity which is depressed in peritonitis. The tests enable also the prediction of septic complications of peritonitis.  相似文献   
966.
An efficiency of the acute myeloblastic leukemia therapy has been assessed in 79 patients aged over 60 years. Twenty six patients out of this group have been treated with usual or reduced doses of doxorubicin and cytarabine (ADR-Ara-C) 35--low doses of cytarabine (LD Ara-C), 11-6-mercaptopurine (6 MP), and 7 patients died before chemotherapy. Complete remission in group treated with ADR-Ara-C was achieved in 23% of patients while partial remission in 42%. Median survival in this group was 5.8 months (range from 0.5 to 16 months). Percentage of the complete remissions in the group treated with LD-Ara-C was 6%, and partial remissions 40%. Median survival was 4.7 months (range from 0.5 to 14.2 months). Partial remission in 5 out of 11 patients treated with 6 MP (36%) and no complete remissions were noted. Median survival was 3.9 months. Therapy with ADR-Ara-C produced marked leucopenia and thrombocytopenia in the majority of treated patients. Vomiting, hemorrhagic complications, and bacterial infections have also been noted. These adverse reactions have been less frequent in patients treated with LD-Ara-C, and 6 MP. Ten patients (38%) treated with ADR-Ara-C and 7 patients treated with LD-Ara-C died during remission inducing therapy.  相似文献   
967.
An average follow-up period of 16 patients was 28 months following an implantation of the artificial aortic valve for its insufficiency. In 10 operated patients who were able to continue their occupation exercise tolerance increased by two classes, according to NYHA. Blood pressure gradient decreased significantly from 61.8 to 37.5 mmHg, cardiac volume index decreased from 639 to 602 ml/m2. Echocardiographically measured muscle mass of the left ventricle, end-diastolic and end-systolic volumes, and the left atrial dimensions decreased significantly following surgery. A significance of the relation of the left ventricle volume to its mass <4 as a prognostic factor in aortic valve replacement has also been confirmed.  相似文献   
968.
969.
Recurrence of duodenal ulcer was diagnosed in 15 patients who underwent highly selective vagotomy before 6-13 years, i.e. in 12.2%. Factors possibly contributing to such a recurrence were analysed. Patients with ulcer recurrence were: 1) non-qualified workers, 2) tobacco smokers, 3) heavy drinkers, 4) users of ulcerogenic medicines, and 5) were involved into conflict situations.  相似文献   
970.
Use of 36Cl, 82Br, and [3H]sucrose to estimate extracellular water volume was evaluated in 14 piglets (7-14 days old). 36Cl and 82Br were distributed in approximately the same volume, but a period of 5-6 hr after injection was required to reach equilibrium in the neonatal pig. Dilution volumes calculated before equilibration (2-5 hr) for 36Cl (326 +/- 11 ml/kg) and 82Br (328 +/- 13 ml/kg) were different from equilibration (6-8 hr) phase volumes (356 +/- 13 ml/kg and 355 +/- 13 ml/kg, respectively; P less than 0.001). A 3-hr sample estimated the same volume distribution calculated by extrapolation of the 6- to 8-hr period because of the relationship between the two slopes of the plasma clearance curves. After the 82Br and 36Cl had achieved equilibration, each was distributed in a volume equivalent to total body chloride space (362 +/- 29 ml/kg) measured by neutron activation; no statistical differences were found (P = 0.6). The early equilibration phase measured a 10% smaller, faster exchangeable fraction of total body Cl. Sucrose dilution volume (332 +/- 19 ml/kg) required multiple plasma samples for extrapolation and measured a dilution volume 7% smaller (P less than 0.05) than total body chloride space.  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