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   575篇
  免费   39篇
  2021年   6篇
  2020年   7篇
  2019年   6篇
  2018年   13篇
  2017年   15篇
  2016年   13篇
  2015年   15篇
  2014年   18篇
  2013年   32篇
  2012年   27篇
  2011年   24篇
  2010年   16篇
  2009年   21篇
  2008年   20篇
  2007年   27篇
  2006年   21篇
  2005年   20篇
  2004年   19篇
  2003年   13篇
  2002年   14篇
  2001年   22篇
  2000年   18篇
  1999年   19篇
  1998年   7篇
  1997年   9篇
  1996年   5篇
  1994年   5篇
  1992年   13篇
  1991年   8篇
  1990年   8篇
  1989年   11篇
  1988年   12篇
  1987年   7篇
  1986年   10篇
  1985年   9篇
  1984年   5篇
  1983年   5篇
  1982年   3篇
  1981年   5篇
  1980年   5篇
  1979年   9篇
  1978年   6篇
  1977年   3篇
  1976年   5篇
  1975年   6篇
  1973年   6篇
  1972年   3篇
  1971年   6篇
  1970年   4篇
  1967年   6篇
排序方式: 共有614条查询结果,搜索用时 390 毫秒
51.
52.
Franz Boas's study of the changes in bodily form of descendants of immigrants has stood for over ninety years as proof of environmental influences on cranial form. Recent reanalysis of his data have shown differing interpretations of the importance of his findings. Here, we explore the historical, political, and social setting of the study that could have led Boas to overstate the importance of his findings. We also include a discussion of the issue at large with respect to the study of modern and prehistoric human variation. Given the current state of population research using craniometric data we conclude, as many of Boas's early criticism have, that while some of the changes observed by Boas have statistical credibility, they generally lack biological meaning when considered in the scope of the degree of modern human variation. [Keywords: plasticity, immigration, craniometries, cranial index, human variation]  相似文献   
53.
Fusarium verticillioides is an important fungus occupying dual roles in the maize plant. The fungus functions as an endophyte, a fungal/host interaction beneficial to the growth of some plants. At other times, the fungus may function as a mycotoxin producing pathogen. The advantages and/or disadvantages of the endophytic relationship must be established in order to target appropriate sites for controlling diseases and mycotoxins in maize. One possibility could be to ensure seed maize is fungal free prior to planting. Reciprocal inoculations were made with two fungal isolates on seed of two maize genotypes. Yield was measured at harvest by ear and seed characters and vegetative growth at one-month intervals for plant survival, height, weight and stem diameter. Yield and vegetative growth differed among mature plants only once based on seed inoculation status. In 1998, plant weight was reduced and seed weight per ear was increased for the dent maize, GT-MAS: gk, grown from F. verticillioides RRC 374- inoculated seed compared to other seed treatments. Most vegetative characters were reduced at the first collection for Silver Queen plants grown from F. verticillioides-inoculated seed in 1997 and 1999, but not in 1998. However, no significant differences occurred among mature Silver Queen plants during any of the three growing seasons. In conclusion, yield and vegetative growth of mature maize plants grown from F. verticillioides-inoculated seed were equal to or greater than plants grown from non-inoculated seed under south Georgia field conditions during 1997, 1998, and 1999.  相似文献   
54.
Prostaglandin H synthase (PGHS) is an autocatalytic enzyme which plays a key role in the arachidonic acid metabolic pathway. PGHS mediates the formation of prostaglandin H2, the precursor for a number of prostaglandins which are important in a wide variety of biological processes, including inflammation, blood clotting, renal function, and tumorigenesis. Here we present a Michaelis-Menten-style model for PGHS. A stability analysis determines when the reaction becomes self-sustaining, and can help explain the regulation of PGHS activity in vivo. We also consider a quasi-steady-state approximation (QSSA) for the model, and present conditions under which the QSSA is expected to be a good approximation. Applying the QSSA for this model can be useful in computationally intensive modeling endeavors involving PGHS.  相似文献   
55.
The last five decades have witnessed an intensification of agriculture with a marked increase in the use of agrochemicals. This study investigated whether arable edges affect ancient woods and the extent of any modifications to the ground flora. Plant species were recorded from a total of 84 transects into the centre of the woods, of which 59 were taken from the arable–woodland interface and 25 were taken from non-arable land or woodland rides. Ellenberg indicator and CSR values were used to infer fertility and features of the environment. Results suggested that, after adjusting for light and moisture conditions, elevated nitrogen levels might occur in ancient woodlands to a distance of at least 100 m from arable farmland. Furthermore, the transects from the arable fields had a substantially altered herb-layer with respect to species composition, notably a greater percentage cover of ruderal and nitrophilous species, and a lower cover of stress tolerator species. Smaller woods contained a greater cover of competitor and nitrophilous species and a lower cover of stress tolerators and ancient woodland indicator species. We conclude that farming may be unwittingly modifying the ground flora of ancient woodlands adjacent to arable land and that small and irregularly shaped woods are most at risk. It is recommended that farmers apply agrochemicals in such a way that drift is minimised, and that large, valuable ancient woods be protected by a substantial buffer zone.  相似文献   
56.
This paper discusses diseases of commercially important non-insect invertebrates from both a historical and a personal perspective. Early studies of curious scientists such as Metchnikoff using invertebrate systems led to important discoveries with much wider impact. As commercial culture of oysters, shrimp, and other invertebrates grew, so did studies of their diseases. Here, the impact of pioneering scientists and important new techniques on our understanding of these diseases is described.  相似文献   
57.
Here we demonstrate a novel p53-independent interaction between the nucleolar tumor suppressors, p14 Arf and Werners helicase (WRN). Binding of p14 Arf to WRN is multivalent and resembles the binding of p14 Arf to Mdm2. Residues 2-14 and 82-101 of p14 Arf and residues in the central region and C terminus of WRN have particular importance for binding. p14 Arf promotes small ubiquitin-like modifier (SUMO) modification of WRN in a synergistic manner with the SUMO-conjugating enzyme, UBCH9. p14 Arf causes redistribution of WRN within the nucleus, and this effect is reversed by expression of a SUMO-specific protease, thus implicating the SUMO conjugation pathway in WRN re-localization. We establish that the ability to promote SUMO conjugation is a general property of the p14 Arf tumor suppressor.  相似文献   
58.
Objective: Previous research has addressed the relationship between BMI and chronic disease in primary care; however, little has been done with regard to the association between obesity and depression in primary care. The purpose of this paper is to assess the relationship between obesity and chronic conditions including depression. Research Methods and Procedures : Data from primary care patients seen at a university‐based family medicine clinic in the southeastern United States were extracted for the time between January 1, 1999 and January 1, 2002. Data extracted included most recent height and weight, age, ethnicity, pregnancy status, number of office visits, blood pressure, cholesterol, hemoglobin A1C, current diagnoses, and medications. Results : A total of 8197 patients were included in the analysis. Sixty‐nine percent of patients seen in a 3‐year period were either overweight or obese. Comparing blood pressure, cholesterol, diagnoses, and medications between BMI groups found differences in virtually all categories. Diagnoses of high cholesterol, hypertension, diabetes, and depression significantly increased for obese patients. Discussion : Obese patients are over‐represented in primary care, and this over‐representation of obesity correlates with several diagnoses, including depression. Depression is a chronic disease that may interfere with health‐related behaviors and must be addressed within the health care system.  相似文献   
59.
Structural analyses of the protein-tyrosine phosphatase 1B (PTP1B) active site and inhibitor complexes have aided in optimization of a peptide inhibitor containing the novel (S)-isothiazolidinone (IZD) phosphonate mimetic. Potency and permeability were simultaneously improved by replacing the polar peptidic backbone of the inhibitor with nonpeptidic moieties. The C-terminal primary amide was replaced with a benzimidazole ring, which hydrogen bonds to the carboxylate of Asp(48), and the N terminus of the peptide was replaced with an aryl sulfonamide, which hydrogen bonds to Asp(48) and the backbone NH of Arg(47) via a water molecule. Although both substituents retain the favorable hydrogen bonding network of the peptide scaffold, their aryl rings interact weakly with the protein. The aryl ring of benzimidazole is partially solvent exposed and only participates in van der Waals interactions with Phe(182) of the flap. The aryl ring of aryl sulfonamide adopts an unexpected conformation and only participates in intramolecular pi-stacking interactions with the benzimidazole ring. These results explain the flat SAR for substitutions on both rings and the reason why unsubstituted moieties were selected as candidates. Finally, substituents ortho to the IZD heterocycle on the aryl ring of the IZD-phenyl moiety bind in a small narrow site adjacent to the primary phosphate binding pocket. The crystal structure of an o-chloro derivative reveals that chlorine interacts extensively with residues in the small site. The structural insights that have led to the discovery of potent benzimidazole aryl sulfonamide o-substituted derivatives are discussed in detail.  相似文献   
60.
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