全文获取类型
收费全文 | 5629篇 |
免费 | 623篇 |
国内免费 | 5篇 |
专业分类
系统科学 | 1080篇 |
丛书文集 | 7篇 |
理论与方法论 | 275篇 |
现状及发展 | 868篇 |
综合类 | 4027篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 2篇 |
2020年 | 9篇 |
2019年 | 1篇 |
2018年 | 728篇 |
2017年 | 734篇 |
2016年 | 434篇 |
2015年 | 36篇 |
2014年 | 9篇 |
2013年 | 12篇 |
2012年 | 260篇 |
2011年 | 954篇 |
2010年 | 820篇 |
2009年 | 459篇 |
2008年 | 509篇 |
2007年 | 767篇 |
2006年 | 9篇 |
2005年 | 49篇 |
2004年 | 123篇 |
2003年 | 153篇 |
2002年 | 58篇 |
2001年 | 4篇 |
2000年 | 7篇 |
1999年 | 14篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 4篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1967年 | 1篇 |
排序方式: 共有6257条查询结果,搜索用时 15 毫秒
181.
General mechanisms of drought response and their application in drought resistance improvement in plants 总被引:6,自引:0,他引:6
Plants often encounter unfavorable environmental conditions because of their sessile lifestyle. These adverse factors greatly affect the geographic distribution of plants, as well as their growth and productivity. Drought stress is one of the premier limitations to global agricultural production due to the complexity of the water-limiting environment and changing climate. Plants have evolved a series of mechanisms at the morphological, physiological, biochemical, cellular, and molecular levels to overcome water deficit or drought stress conditions. The drought resistance of plants can be divided into four basic types-drought avoidance, drought tolerance, drought escape, and drought recovery. Various drought-related traits, including root traits, leaf traits, osmotic adjustment capabilities, water potential, ABA content, and stability of the cell membrane, have been used as indicators to evaluate the drought resistance of plants. In the last decade, scientists have investigated the genetic and molecular mechanisms of drought resistance to enhance the drought resistance of various crops, and significant progress has been made with regard to drought avoidance and drought tolerance. With increasing knowledge to comprehensively decipher the complicated mechanisms of drought resistance in model plants, it still remains an enormous challenge to develop water-saving and drought-resistant crops to cope with the water shortage and increasing demand for food production in the future. 相似文献
182.
Danielle Kamato Muhamad Ashraf Rostam Rebekah Bernard Terrence J. Piva Nitin Mantri Daniel Guidone Wenhua Zheng Narin Osman Peter J. Little 《Cellular and molecular life sciences : CMLS》2015,72(4):799-808
G protein-coupled receptor (GPCR) signalling is mediated through transactivation-independent signalling pathways or the transactivation of protein tyrosine kinase receptors and the recently reported activation of the serine/threonine kinase receptors, most notably the transforming growth factor-β receptor family. Since the original observation of GPCR transactivation of protein tyrosine kinase receptors, there has been considerable work on the mechanism of transactivation and several pathways are prominent. These pathways include the “triple membrane bypass” pathway and the generation of reactive oxygen species. The recent recognition of GPCR transactivation of serine/threonine kinase receptors enormously broadens the GPCR signalling paradigm. It may be predicted that the transactivation of serine/threonine kinase receptors would have mechanistic similarities with transactivation of tyrosine kinase pathways; however, initial studies suggest that these two transactivation pathways are mechanistically distinct. Important questions are the relative importance of tyrosine and serine/threonine transactivation pathways, the contribution of transactivation to overall GPCR signalling, mechanisms of transactivation and the range of cell types in which this phenomenon occurs. The ultimate significance of transactivation-dependent signalling remains to be defined but it appears to be prominent and if so will represent a new cell signalling frontier. 相似文献
183.
184.
Lígia O. Martins Paulo Durão Vânia Brissos Peter F. Lindley 《Cellular and molecular life sciences : CMLS》2015,72(5):911-922
The ubiquitous members of the multicopper oxidase family of enzymes oxidize a range of aromatic substrates such as polyphenols, methoxy-substituted phenols, amines and inorganic compounds, concomitantly with the reduction of molecular dioxygen to water. This family of enzymes can be broadly divided into two functional classes: metalloxidases and laccases. Several prokaryotic metalloxidases have been described in the last decade showing a robust activity towards metals, such as Cu(I), Fe(II) or Mn(II) and have been implicated in the metal metabolism of the corresponding microorganisms. Many laccases, with a superior efficiency for oxidation of organic compounds when compared with metals, have also been identified and characterized from prokaryotes, playing roles that more closely conform to those of intermediary metabolism. This review aims to present an update of current knowledge on prokaryotic multicopper oxidases, with a special emphasis on laccases, anticipating their enormous potential for industrial and environmental applications. 相似文献
185.
Beta integrals for several non-integer values of the exponents were calculated by Leonhard Euler in 1730, when he was trying
to find the general term for the factorial function by means of an algebraic expression. Nevertheless, 70 years before, Pietro
Mengoli (1626–1686) had computed such integrals for natural and half-integer exponents in his Geometriae Speciosae Elementa (1659) and Circolo(1672) and displayed the results in triangular tables. In particular, his new arithmetic–algebraic method allowed him to compute
the quadrature of the circle. The aim of this article is to show how Mengoli calculated the values of these integrals as well
as how he analysed the relation between these values and the exponents inside the integrals. This analysis provides new insights
into Mengoli’s view of his algorithmic computation of quadratures. 相似文献
186.
Nissan Zeldes 《Archive for History of Exact Sciences》2009,63(3):289-323
The present article considers Giulio Racah’s contributions to general physical theory and his establishment of theoretical
physics as a discipline in Israel. Racah developed mathematical methods that are based on tensor operators and continuous
groups. These methods revolutionized spectroscopy. Currently, these are essential research tools in atomic, nuclear and elementary
particle physics. He himself applied them to modernizing theoretical atomic spectroscopy. Racah laid the foundations of theoretical
physics in Israel. He educated several generations of Israeli physicists, and put Israel on the world map of physics. 相似文献
187.
Dennis Duke 《Archive for History of Exact Sciences》2009,63(6):635-654
In the Planetary Hypotheses, Ptolemy summarizes the planetary models that he discusses in great detail in the Almagest, but he changes the mean motions to account for more prolonged comparison of observations. He gives the mean motions in two
different forms: first, in terms of ‘simple, unmixed’ periods and next, in terms of ‘particular, complex’ periods, which are
approximations to linear combinations of the simple periods. As a consequence, all of the epoch values for the Moon and the
planets are different at era Philip. This is in part a consequence of the changes in the mean motions and in part due to changes
in Ptolemy’s time in the anomaly, but not the longitude or latitude, of the Moon, the mean longitude of Saturn and Jupiter,
but not Mars, and the anomaly of Venus and Mercury, the former a large change, the latter a small one. The pattern of parameter
changes we see suggests that the analyses that yielded the Planetary Hypotheses parameters were not the elegant trio analyses of the Almagest but some sort of serial determinations of the parameters based on sequences of independent observations. 相似文献
188.
Two letters from William Burnside have recently been found in the Nachlass of Robert Fricke that contain instances of the Burnside Problem prior to its first publication. We present these letters
as a whole to the public for the first time. We draw a picture of these two mathematicians and describe their activities leading
to their correspondence. We thus gain an insight into their respective motivations, reactions, and attitudes, which may sharpen
the current understanding of professional and social interactions of the mathematical community at the turn of the twentieth
century.
Dedicated to Heiko Harborth on the occasion of his seventieth birthday. 相似文献
189.
Peng Xing Bao-zhong Ma Peng Zeng Cheng-yan Wang Ling Wang Yong-lu Zhang Yong-qiang Chen Shuo Wang Qiu-yin Wang 《矿物冶金与材料学报》2017,24(11):1217-1227
Huge quantities of zinc leaching residues (ZLRs) generated from zinc production are dumped continuously around the world and pose a potential environmental threat because of their considerable amounts of entrained heavy metals (mainly lead). Most ZLRs have not been properly treated and the valuable metals in them have not yet been effectively recovered. Herein, the deep cleaning of a ZLR and recovery of valuable metals via a hydrometallurgical route were investigated. The cleaning process consists of two essential stages:acid leaching followed by calcium chloride leaching. The optimum conditions for extracting zinc, copper, and indium by acid leaching were a sulfuric acid concentration of 200 g·L-1, a liquid/solid ratio of 4:1 (mL/g), a leaching time of 2 h, and a temperature of 90℃. For lead and silver extractions, the optimum conditions were a calcium chloride concentration of 400 g·L-1, a pH value of 1.0, a leaching time of 1 h, and a temperature of 30℃. After calcium chloride leaching, silver and lead were extracted out and the lead was finally recovered as electrolytic lead by electrowinning. The anglesite phase, which poses the greatest potential environmental hazard, was removed from the ZLR after deep cleaning, thus reducing the cost of environmental management of ZLRs. The treatment of chlorine and spent electrolyte generated in the process was discussed. 相似文献
190.
The paleoflow direction of the basal conglomerate of the Changzhougou formation is obtained by the paleocurrent analysis of certain sedimentary structures, and the ancient sedimentary environment of the Qian'an Region is determined by observing the psephicity and gradation of gravel in conglomerate. The results show that the paleoflow direction is from northwest to southwest, and the paleocurrent data manifest a single-peak style in a rose diagram. The average paleoflow direction in the Qian'an region is 265°. The basal conglomerate is of littoral facies, and the sediments of conglomerates are mainly sourced from Shanhaiguan. 相似文献