全文获取类型
收费全文 | 60459篇 |
免费 | 5115篇 |
国内免费 | 47篇 |
出版年
2023年 | 196篇 |
2022年 | 526篇 |
2021年 | 1090篇 |
2020年 | 611篇 |
2019年 | 824篇 |
2018年 | 1229篇 |
2017年 | 969篇 |
2016年 | 1718篇 |
2015年 | 2797篇 |
2014年 | 3150篇 |
2013年 | 3638篇 |
2012年 | 4733篇 |
2011年 | 4498篇 |
2010年 | 2866篇 |
2009年 | 2510篇 |
2008年 | 3578篇 |
2007年 | 3328篇 |
2006年 | 3060篇 |
2005年 | 2803篇 |
2004年 | 2730篇 |
2003年 | 2448篇 |
2002年 | 2088篇 |
2001年 | 1677篇 |
2000年 | 1566篇 |
1999年 | 1261篇 |
1998年 | 592篇 |
1997年 | 508篇 |
1996年 | 441篇 |
1995年 | 434篇 |
1994年 | 335篇 |
1993年 | 326篇 |
1992年 | 679篇 |
1991年 | 538篇 |
1990年 | 496篇 |
1989年 | 491篇 |
1988年 | 427篇 |
1987年 | 411篇 |
1986年 | 327篇 |
1985年 | 343篇 |
1984年 | 290篇 |
1983年 | 236篇 |
1982年 | 202篇 |
1981年 | 173篇 |
1980年 | 165篇 |
1979年 | 233篇 |
1978年 | 201篇 |
1977年 | 183篇 |
1976年 | 174篇 |
1974年 | 203篇 |
1972年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
791.
792.
793.
Seungdo Kim Taeyoun Hwang Kun M. Lee 《The International Journal of Life Cycle Assessment》1997,2(4):217-222
Allocation in LCA is defined as partitioning the responsibility for environmental burdens from the economic activities to a reference flow or a reference life cycle system in some proper shares. The result of LCA study involving a multi-input/output system or an open loop recycling system is affected significantly by the choice of the allocation method. For the case of allocation in a cascade recycling system, the quality of material as well as the material flow should be considered. Therefore, environmental burdens from the primary material production, the recycling process and the waste management process have to be allocated in proportion to the quality degradation of a material and to the quantity of a material used in each life cycle system. This paper proposes an allocation method for the cascade recycling system that considers both quality and quantity of a material used. 相似文献
794.
We define the species boundaries of white-toothed shrews (genus Crocidura ) in Taiwan using karyological and morphological characteristics. Ninety-nine animals were obtained from all over Taiwan at capture rates usually less than 10%. Three species are recognized by distinct cytotypes: Crocidura attenuata tanakae 2n = 40, FN = 56; Crocidura suaveolens hosletti 2n = 40, FN = 50; Crocidura kurodai 2n = 40, FN = 54. A suite of six morphological characters diagnose the three species: shape of skull, position of incisive foramina, shape of fourth upper premolar, shape of pinna, tail vibrissae, and foot pads. A species key and notes on the life history of each species are provided. Finally, we discuss chromosomal evolution and biolgeography of Crocidura in East and South East Asia. 相似文献
795.
796.
The effects of calystegines isolated from edible fruits and vegetables on mammalian liver glycosidases 总被引:1,自引:0,他引:1
Asano Naoki; Kato Atsushi; Matsui Katsuhiko; Watson Alison A.; Nash Robert J.; Molyneux Russell J.; Hackett Lucy; Topping Joanna; Winchester Bryan 《Glycobiology》1997,7(8):1085-1088
The polyhydroxylated nortropane alkaloids called calyste-ginesoccur in many plants of the Convolvulaceae, Solanaceae, andMoraceae families. Certain of these alkaloids exhibit potentinhibitory activities against glycosidases and the recentlydemonstrated occurrence of calystegines in the leaves, skins,and sprouts of potatoes (Solatium tuberosum), and in the leavesof the eggplant (S.melongena), has raised concerns regardingthe safety of these vegetables in the human diet. We have surveyedthe occurrence of calystegines in edible fruits and vegetablesof the families Convolvulaceae, Solanaceae, and Moraceae byGC-MS. Calystegines A3, B1, B2, and C1 were detected in allthe edible fruits and vegetables tested; sweet and chili peppers,potatoes, eggplants, tomatoes, Physalis fruits, sweet potatoes,and mulberries. Calystegines B1 and C1 were potent competitiveinhibitors of the bovine, human, and rat β-glucosidaseactivities, with K1 values of 150, 10, and 1.9 µM, respectivelyfor B1 and 15,1.5, and 1 µM, respectively, for C1. CalystegineB2 was a strong competitive inhibitor of the -galactosidaseactivity in all the livers. Human β-xylosidase was inhibitedby all four nortropanes, with calystegine C1 having a K1 of0.13 µM. Calystegines A3 and B2 selectively inhibitedthe rat liver β-glucosidase activity. The potent inhibitionof mammalian β-glucosidase and -galactosidase activitiesin vitro raises the possibility of toxicity in humans consuminglarge amounts of plants that contain these compounds. edible plants calystegines glycosidase inhibitors bovine, human, and rat liver 相似文献
797.
798.
Identification of three genetic loci controlling leaf senescence in Arabidopsis thaliana 总被引:23,自引:3,他引:20
Sung Aeong Oh Joon-Hyun Park Gyu In Lee Kyung Hee Paek Soon Ki Park Hong Gil Nam 《The Plant journal : for cell and molecular biology》1997,12(3):527-535
Four mutants that show the delayed leaf senescence phenotype were isolated from Arabidopsis thaliana . Genetic analyses revealed that they are all monogenic recessive mutations and fall into three complementation groups, identifying three genetic loci controlling leaf senescence in Arabidopsis . Mutations in these loci cause delay in all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II, relative amount of the large subunit of Rubisco, and RNase and peroxidase activity. Delay of the senescence symptoms was observed during both age-dependent in planta senescence and dark-induced artificial senescence in all of the mutant plants. The results indicate that the three genes defined by the mutations are key genetic elements controlling functional leaf senescence and provide decisive genetic evidence that leaf senescence is a genetically programmed phenomenon controlled by several monogenic loci in Arabidopsis . The results further suggest that the three genes function at a common step of age-dependent and dark-induced senescence processes. It is further shown that one of the mutations is allelic to ein2-1 , an ethylene-insensitive mutation, confirming the role of ethylene signal transduction pathway in leaf senescence of Arabidopsis . 相似文献
799.
800.
Qian Shi Pascal Verdier-Pinard Arnold Brossi Ernest Hamel Kuo-Hsiung Lee 《Bioorganic & medicinal chemistry》1997,5(12):2277-2282
(+)-Thiocolchicine (2b) was prepared from (±)-colchicine (1) in a five-step reaction sequence that included chromatographic separation of appropriate camphanylated diastereomers. Acid hydrolysis of the (+)-diastereomer, followed by acetylation, yielded the desired product 2b. (+)-Thiocolchicine has 15-fold lower inhibitory activity against tubulin polymerization than (−)-thiocolchicine, and is 29-fold less potent for inhibiting growth of human Burkitt lymphoma cells. The enantiomer 2a, prepared from the (−)-camphanylated diastereomer, had potent activity in all assays comparable to that of (−)-thiocolchicine prepared by other methods. These results support the hypothesis that the proper configuration of colchicine-related compounds is an important requirement for their anti-tubulin action. 相似文献