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   2787篇
  免费   253篇
  国内免费   1篇
  2023年   19篇
  2022年   12篇
  2021年   83篇
  2020年   61篇
  2019年   51篇
  2018年   93篇
  2017年   65篇
  2016年   140篇
  2015年   162篇
  2014年   168篇
  2013年   226篇
  2012年   249篇
  2011年   231篇
  2010年   143篇
  2009年   113篇
  2008年   171篇
  2007年   171篇
  2006年   130篇
  2005年   134篇
  2004年   111篇
  2003年   109篇
  2002年   81篇
  2001年   19篇
  2000年   18篇
  1999年   28篇
  1998年   20篇
  1997年   19篇
  1996年   13篇
  1995年   15篇
  1994年   7篇
  1993年   12篇
  1992年   17篇
  1991年   8篇
  1990年   13篇
  1989年   11篇
  1988年   9篇
  1987年   8篇
  1986年   9篇
  1985年   6篇
  1984年   5篇
  1982年   6篇
  1981年   6篇
  1979年   5篇
  1976年   3篇
  1974年   5篇
  1973年   8篇
  1972年   3篇
  1970年   5篇
  1958年   3篇
  1944年   3篇
排序方式: 共有3041条查询结果,搜索用时 15 毫秒
1.
First appearing in 2000, and with 20,000 copies sold as it entered its sixth printing in 2004, Richard Trudgen's Why Warriors Lie down and Die is a minor publishing phenomenon. Although scholarly forums have generally ignored the book, Christian media outlets, the popular press and neo‐conservative political activists have enthusiastically received and promoted it. An examination of the merits of Why Warriors Lie down and Die suggests that its popularity is only partly explained by original observations or insights on the part of the author. The most important explanation, and implication, of the book's remarkable take‐up lies in the opportune way in which it corresponds with now ascendant neo‐conservative political perspectives on indigenous policy.  相似文献   
2.
3.
Controlled generation of reactive oxygen species orchestrates numerous physiological signaling events (Finkel, T. (2011) Signal transduction by reactive oxygen species. J. Cell Biol. 194, 7–15). A major cellular target of reactive oxygen species is the thiol side chain (RSH) of Cys, which may assume a wide range of oxidation states (i.e. −2 to +4). Within this context, Cys sulfenic (Cys-SOH) and sulfinic (Cys-SO2H) acids have emerged as important mechanisms for regulation of protein function. Although this area has been under investigation for over a decade, the scope and biological role of sulfenic/sulfinic acid modifications have been recently expanded with the introduction of new tools for monitoring cysteine oxidation in vitro and directly in cells. This minireview discusses selected recent examples of protein sulfenylation and sulfinylation from the literature, highlighting the role of these post-translational modifications in cell signaling.  相似文献   
4.
We evaluated VER effect on RNA synthesis of quiescent and angiotensin II (AII)- stimulated cultured rat aortic vascular smooth muscle cells (VSMC). In a dose-dependent manner, VER decreased [3H]uridine uptake by quiescent VSMCs (ED50 7 x 10(-6)M), an effect that was shared by other calcium antagonists, but to a variable degree. VER caused a significant effect within 3 hours and attained a maximal effect at 7 hours. In addition VER caused a 22 +/- 2% decrease in [3H]uridine uptake by VSMCs stimulated with 10% fetal bovine serum, while it completely abolished [3H]uridine uptake by VSMCs induced by AII. We conclude that VER decreases basal and inhibits AII-induced increase in mRNA synthesis of VSMCs. These data may explain in part how VER causes a decrease in vascular resistance and alters the vasoconstrictor effect of AII.  相似文献   
5.
ABSTRACT.   Latitudinal variation in population structure during the winter has been reported in many migratory birds, but has been documented in few species of waterfowl. Variation in environmental and social conditions at wintering sites can potentially influence the population dynamics of differential migrants. We examined latitudinal variation in sex and age classes of wintering Pacific Black Brant ( Branta bernicla nigricans ). Brant are distributed along a wide latitudinal gradient from Alaska to Mexico during the winter. Accordingly, migration distances for brant using different wintering locations are highly variable and winter settlement patterns are likely associated with a spatially variable food resource. We used resightings of brant banded in southwestern Alaska to examine sex and age ratios of birds wintering at Boundary Bay in British Columbia, and at San Quintin Bay, Ojo de Liebre Lagoon, and San Ignacio Lagoon in Baja California from 1998 to 2000. Sex ratios were similar among wintering locations for adults and were consistent with the mating strategy of geese. The distribution of juveniles varied among wintering areas, with greater proportions of juveniles observed at northern (San Quintin Bay and Ojo de Liebre Lagoon) than at southern (San Ignacio Lagoon) locations in Baja California. We suggest that age-related variation in the winter distribution of Pacific Black Brant is mediated by variation in productivity among individuals at different wintering locations and by social interactions among wintering family groups.  相似文献   
6.
Summary In the Chihuahuan Desert of southern New Mexico, both water and nitrogen limit the primary productivity of Larrea tridentata, a xerophytic evergreen shrub. Net photosynthesis was positively correlated to leaf N, but only in plants that received supplemental water. Nutrient-use efficiency, defined as photosynthetic carbon gain per unit N invested in leaf tissue, declined with increasing leaf N. However, water-use efficiency, defined as the ratio of photosynthesis to transpiration, increased with increasing leaf N, and thus these two measures of resource-use efficiency were inversely correlated. Resorption efficiency was not significantly altered over the nutrient gradient, nor was it affected by irrigation treatments. Leaf longevity decreased significantly with fertilization although the absolute magnitude of this decrease was fairly small, in part due to a large background of insect-induced mortality. Age-specific gas exchange measurements support the hypothesis that leaf aging represents a redistribution of resources, rather than actual deterioration or declining resource-use efficiency.  相似文献   
7.
Summary The purpose of this study was to determine whether the organizer regions of early avian and amphibian embryos could induce supernumerary (SN) wing structures to develop when they were grafted to a slit in the anterior side of stage 19–23 chick wing buds. Supernumerary digits developed in 43% of the wings that received anterior grafts of Hensen's node from stage 4–6 quail or chick embryos; in addition, 16% of the wings had rods of SN cartilage, but not recognizable SN digits. The grafted quail tissue did not contribute to the SN structures. When tissue anterior or lateral to Hensen's node or lateral pieces of the area pellucida caudal to Hensen's node were grafted to anterior slits, the wings usually developed normally. No SN structures developed when Hensen's nodes were grafted to posterior slits in chick wing buds. Wings developed normally when pieces of the dorsal lip of the blastopore from stage 10–11.5 frog (Xenopus laevis and Rana pipiens) embryos were grafted to anterior slits. No SN digits developed when other tissues that have limb-inducing activity in adult urodele amphibians [chick otic vesicle, frog (Rana pipiens) lung and kidney] or that can act as heteroinductors in neural induction (rat kidney, lung, submaxillary gland and urinary bladder; mouse liver and submaxillary gland) were grafted to anterior slits in chick wing buds. SN digits also failed to develop following preaxial grafts of chick optic vesicles. These results suggest that although the anteroposterior polarity of the chick wing bud can be influenced by factors other than the ZPA (e.g., Hensen's node, retinoids), the wing is not so labile that it can respond to a wide variety of inductively-active tissues.  相似文献   
8.
In 1987 experimental schemes for distributing injecting equipment to intravenous drug users to help prevent the spread of the human immunodeficiency virus were started by the government. After six months the schemes were found to have been reasonably successful in attracting clients but were less successful in keeping them. It has been shown that equipment can be distributed to drug users on an exchange basis. Many of the clients who were attracted to these schemes had had no treatment or other help for their drug problems.  相似文献   
9.
Changes in intracellular drug localization accompany doxorubicin resistance in multidrug resistant tumor cells. The purpose of this study was to develop a method to quantify these changes and so detect different levels of resistance. Tumor cells were incubated with the fluorescent anthracycline doxorubicin (excitation at 480 nm; emission maximum at 560-590 nm) and were quantified using laser scanning microscopy. The fluorescent mode was used to record the intracellular drug distribution, whereas the absorption mode was used to define the nuclear and cytoplasmic boundaries. The cell compartments were delineated interactively on an image processing system and the ratio nuclear fluorescence/cytoplasmic fluorescence (N/C ratio) was determined. N/C ratios were: 1.8 in the Chinese hamster ovarian cell line AUXB1 and 0.1 in its MDR subline CHRC5; 3.8 in the human squamous lung cancer cell line SW-1573 and 1.8 and 0.4 in its MDR sublines SW-1573/2R120 and SW-1573/2R160, respectively; and 3.6 in the human myeloma cell line 8226/S and 2.1 and 1.0 in its MDR sublines 8226/Dox4 and 8226/Dox40, respectively. The doxorubicin distribution was independent of the doxorubicin concentration within a range from 1-32 microM. Furthermore, the progressive mean of the nuclear/cytoplasmic doxorubicin fluorescence ratio showed that a minimal sample size of 30 cells is necessary for reliable results. The results of two independent assessments showed a high reproducibility (r = 0.97). Thus, with the method described in this paper, it is possible to detect relatively low levels of doxorubicin resistance (factor 8).  相似文献   
10.
This paper describes a combined technique for gross skeletal staining and Feulgen staining of avian embryonic limbs. The gross skeletal stain uses Victoria blue B, and the Feulgen stain is done en bloc before the skeletal stain is applied. The method has been useful in determining the cellular origins of supernumerary structures arising from experiments in which quail wing mesoderm is grafted into chick wing buds.  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