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   3007篇
  免费   134篇
  国内免费   172篇
  2023年   49篇
  2022年   51篇
  2021年   71篇
  2020年   60篇
  2019年   79篇
  2018年   80篇
  2017年   55篇
  2016年   70篇
  2015年   73篇
  2014年   144篇
  2013年   200篇
  2012年   107篇
  2011年   151篇
  2010年   95篇
  2009年   111篇
  2008年   111篇
  2007年   210篇
  2006年   151篇
  2005年   151篇
  2004年   130篇
  2003年   112篇
  2002年   84篇
  2001年   64篇
  2000年   56篇
  1999年   72篇
  1998年   61篇
  1997年   49篇
  1996年   57篇
  1995年   66篇
  1994年   40篇
  1993年   31篇
  1992年   32篇
  1991年   25篇
  1990年   34篇
  1989年   29篇
  1988年   20篇
  1987年   24篇
  1986年   22篇
  1985年   40篇
  1984年   41篇
  1983年   32篇
  1982年   35篇
  1981年   35篇
  1980年   24篇
  1979年   22篇
  1978年   10篇
  1977年   15篇
  1976年   5篇
  1975年   8篇
  1973年   7篇
排序方式: 共有3313条查询结果,搜索用时 15 毫秒
51.
Pharmacological treatment of obesity has been neglected as a viable therapeutic option for many years. Recent long term studies with combinations of obesity drugs gives promise that drugs may play a role in weight maintenance, which classically has been the most difficult aspect of treating obesity. Currently available obesity drugs include centrally acting adrenergic agents and serotonin agonists. Drugs still in development include a lipase inhibitor that produces fat malabsorption, a combined adrenergic-serotonergic reuptake inhibitor, various gut-central nervous system peptides, and a number of beta-3 agonists. Any of these obesity drugs given alone produces modest weight loss, and for most, weight loss continues for as long as medication is given. The most successful drug regimens to date are combinations of phentermine and fenfluramine or of ephedrine, caffeine, and/or aspirin. The former combination produces reduction in body weight and complications of obesity for 2 to almost 4 years in clinical trials to date. More research is needed to document long term efficacy and particularly the long term safety of these and other combinations.  相似文献   
52.
Population-based data have not been readily available on relatively short-term changes in weight. Therefore, we sought to determine the nature of self-reported substantial (> 10%) weight change over one year in a representative sample of the US population which participated in the 1989 National Health Interview Survey (NHIS). Across all ages, a larger proportion of women than men reported both weight loss as well as weight gain of any amount (18.9% vs. 16.1% for weight loss and 20.0% vs. 16.1% for weight gain). In sex-specific logistic regression analyses, significant risk factors common to both sexes for substantial weight loss included divorced/separated marital status, smoking, increased number of blood pressure checks, increased BMI (body mass index) and increased number of bed days. Black race reduced the risk of weight loss for both men and women. Sex-specific risk factors for weight loss in men only were widowhood or never married marital status, while increasing age was a protective factor in women only. Concerning weight gain > 10% over the past year, increased number of blood pressure checks and having one or more diabetic parents were significant risk factors among both men and women; while never being married, increased age, BMI, and education exerted a protective effect in both sexes. For women only, risk factors for weight gain included black race, increased number of contacts with a health professional, and being unemployed. Intention to lose weight was associated with both weight gain and weight loss in both sexes, although it did not serve as a confounder in any of these relationships. A greater likelihood of substantial weight loss among women relative to men was diminished for persons with higher BMI, higher number of blood pressure checks, being widowed, divorced or separated, and intention to lose weight. A greater likelihood of substantial weight gain among women relative to men was diminished for persons with low BMI. The results of this cross-sectional study of weight change, involving a one-year follow-up period, generally correspond with the results obtained by longitudinal studies involving a longer follow-up.  相似文献   
53.
Four methods for classifying cultivated rices (Oryza sativa L.) (including IR varieties) into indica and japonica types — waxy gene product in endosperm starch, glutelin 3 molecular weight in milled rice, RFLP polymorphism at the Wx locus and Glaszmann's isozyme method — were compared. On the basis of the two endosperm traits and the RFLP method Glaszmann's group 1 (indica) was classified as mainly indica and intermediate groups 2, 3 and 4 as exclusively indica. However, the endosperm traits classified Glaszmann's group 5 as mainly indica, while the RFLP method classified it as japonica. The RFLP waxy gene probe was closest to the isozyme method in classifying group 6 as japonicas; the waxy gene product gave mainly indica reaction even in group 6, and the glutelin 3 method was intermediate. All IR rices were classified as being indica on the basis of Wx gene product and by Glaszmann's method, but a few were classified as japonica by the glutelin 3 method and by the RFLP waxy gene probe.  相似文献   
54.
We isolated and sequenced Ha hsp 17.9, a DNA complementary (cDNA) of dry-seed stored mRNA that encodes a low-molecular-weight heat-shock protein (LMW HSP). Sequence analysis identified Ha hsp17.9, and the previously reported Ha hsp17.6, as cDNAs encoding proteins (HSP17.6 and HSP17.9) which belong to different families of cytoplasmic LMW HSPs. Using specific antibodies we observed differential expression of both proteins during zygotic embryogenesis under controlled environment, and a remarkable persistence of these LMW HSPs during germination. Immuno-blot analysis of HSP17.9 proteins in two-dimensional gels revealed that the polypeptides expressed in embryos were indistinguishable from LMW HSPs expressed in vegetative tissues in response to water deficit; but they appeared different from homologeous proteins expressed in response to thermal-stress. Tissue-print immunolocalization experiments showed that HSP17.9 and HSP17.6 were homogeneously distributed in every tissue of desiccation-tolerant dry seeds and young seedlings under non-stress conditions. These results demonstrate developmental regulation of specific, cytoplasmic, plant LMW HSPs, suggesting also their involvement in water-stress tolerance.  相似文献   
55.
Yang  X.  Römheld  V.  Marschner  H. 《Plant and Soil》1993,155(1):441-444
Pot experiments were conducted with a calcareous soil (Inceptisol) to elucidate the effects of bicarbonate (0 and 20 mM) and root zone temperature (15° and 25°C) on the uptake of Zn, Fe, Mn and Cu by "Zn-efficient" and "Zn-inefficient" rice cultivars. Bicarbonate decreased concentrations and total uptake of Zn in shoots of "Zn-inefficient" cultivars, especially of IR 26 at 25°C, but not in Zn-efficient cultivars. Bicarbonate decreased concentrations and uptake of Fe in shoots of Zn inefficient cultivars, particularly in IR 26. Concentrations and total uptake of Mn were lower in bicarbonate treatment in the Zn-inefficient cultivars at 15°C, and in all cultivars at 25°C. However, concentration and uptake of Cu were not affected by bicarbonate in all cultivars. Compared to the 25°C root zone temperature, the concentrations and total uptake of both Zn and Cu in shoots at 15°C were lower in Zn-inefficient than in the Zn-efficient cultivars. The results indicate that Zn-efficiency in rice is causally related to high tolerance of plant to elavated bicarbonate concentrations in soil solution.  相似文献   
56.
The leaf macroelement profile of fruiting shoots of persimmon was characterized by a modified diagnostic and recommendation integrated system (DRIS), using SLW as a primary determinant of leaf mineral content. Leaf N, P, Ca, and Mg content was positively and linearly correlated with SLW when expressed on leaf area basis (g mm–2). Potassium had a negative and higher correlation to SLW when expressed on %DW basis. Mineral ratios relevant for the DRIS analysis were calculated using all four possible combinations of Area (A) and Weight (W) expressions (A/A, A/W, W/A and W/W) and correlated with leaf SLW. The particular expressions chosen for the DRIS analysis were based on their highest correlation to SLW and included N/K, P/K and Ca/Mg, based on the A/W expression of the respective nutrients and the reciprocal expression (W/A) for all other ratios. Derivation of DRIS norms were based on the mineral profile of highly exposed shoots (SLW of 15.0±0.3 mg cm–2). Calculated indices of gradually less exposed shoots (SLW of 3.8–18.9 mg cm–2) revealed a strong exponential imbalance of N, K and P (increasingly positive) vs Ca and Mg (increasingly negative). The calculated Nutritional Imbalance Index (NII) value of leaves decreased exponentially as shoot leaf SLW decreased. The modified DRIS analysis detected successfully a distinct mineral profile of highly vigorous fruiting water shoots, as compared to regular fruiting shoots of comparable SLW.  相似文献   
57.
Dissolved organic carbon (DOC) is the dominant form of carbon in transport in blackwater rivers, and bacteria are the major biological agents of its utilization. This study describes longitudinal patterns in DOC concentration and relates them to suspended bacterial populations in the channel. Concentrations of total DOC, three molecular weight fractions, and bacterial numbers were determined at 12 sites along the Ogeechee River in 1985–1986 and 1989 during periods of low and high discharge. Suspended bacterial populations were compared with DOC concentrations to determine if differences in bacterial abundance were related to longitudinal patterns of DOC concentration. Three distinct longitudinal patterns were observed: (1) The longitudinal pattern followed by both total and intermediate molecular weight DOC concentrations was a linear function of the geographic distance along the river. (2) During low flow conditions, there was a high degree of correspondence between patterns of bacterial numbers and low MW DOC (< 1000 apparent MW). (3) During periods of high discharge, the proportion of high (> 10,000) and intermediate (1000–10,000) MW fractions increased, and there was no longer a clear relationship between bacterial cells and low MW DOC.  相似文献   
58.
We describe some of the sources of variation in gestation length among rhesus macaques. the data were obtained from the timed-mating breeding program at the California Regional Primate Research Center (CRPRC). Information about approximately 700 pregnancies that resulted in spontaneous vaginal deliveries of liveborn young is presented. The average length of these pregnancies was 166.5 days. In this population, older females with higher parities had significantly longer pregnancies and significantly heavier infants than other females did. Other factors, including infant sex, month of conception, maternal reproductive history, and paternal identity, had no consistent effect upon gestation length.  相似文献   
59.
从1985年开始,陆续从国外实验动物著名机构引进常用近交系小鼠共23个品系(亚系和亚群)。在多年来保种繁殖过程中,根据修饰平行线系统选择留种动物,同时对动物进行全兄妹近亲交配繁殖,保持完整的谱系记录,控制饲养环境条件,成功地保持了绝大多数品系。同时对多数品系的繁殖性能和体重增长进行了观察,为这些品系的实验应用提供基本参数。  相似文献   
60.
This study was carried out to clarify the reason for elevation of serum α-fetoprotein (AFP) level of nude mice bearing hepatoma cells after treatment with monoclonal antibodies (MoAbs) to AFP. MoAbs to AFP showed no effect on the cumulative amounts of AFP secreted from human hepatoma cell line, HuH-7, in vitro. However, the treatment of nude mice bearing HuH-7N cells (HuH-7 xenograft) with MoAbs to AFP led to elevation of the serum AFP level in spite of the fact that the growth curve of HuH-7N cells was similar to that for PBS treatment. This apparent elevation of the serum AFP level is thought to be due to the slow elimination of AFP-MoAb immune complexes with little lattice structure from circulation, but not the enhancement of AFP secretion of HuH-7N cells. Thus, when using a MoAb alone or MoAb-drug conjugate, the serum AFP level should only be cautiously used as a tumor marker for evaluating the targeting immunotherapy.  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