全文获取类型
收费全文 | 113094篇 |
免费 | 1759篇 |
国内免费 | 821篇 |
出版年
2023年 | 54篇 |
2022年 | 182篇 |
2021年 | 412篇 |
2020年 | 247篇 |
2019年 | 311篇 |
2018年 | 12217篇 |
2017年 | 10919篇 |
2016年 | 8017篇 |
2015年 | 1690篇 |
2014年 | 1493篇 |
2013年 | 1673篇 |
2012年 | 5966篇 |
2011年 | 14320篇 |
2010年 | 12935篇 |
2009年 | 9007篇 |
2008年 | 10999篇 |
2007年 | 12417篇 |
2006年 | 1298篇 |
2005年 | 1465篇 |
2004年 | 1908篇 |
2003年 | 1780篇 |
2002年 | 1549篇 |
2001年 | 880篇 |
2000年 | 789篇 |
1999年 | 445篇 |
1998年 | 173篇 |
1997年 | 150篇 |
1996年 | 128篇 |
1995年 | 87篇 |
1994年 | 88篇 |
1993年 | 98篇 |
1992年 | 177篇 |
1991年 | 162篇 |
1990年 | 95篇 |
1989年 | 110篇 |
1988年 | 86篇 |
1987年 | 78篇 |
1986年 | 69篇 |
1985年 | 53篇 |
1984年 | 54篇 |
1983年 | 54篇 |
1982年 | 29篇 |
1978年 | 28篇 |
1976年 | 32篇 |
1975年 | 34篇 |
1973年 | 33篇 |
1972年 | 265篇 |
1971年 | 297篇 |
1970年 | 27篇 |
1962年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
931.
932.
933.
934.
935.
936.
937.
938.
Xitlali Aguirre-Dugua Edgar Pérez-Negrón Alejandro Casas 《Journal of ethnobiology and ethnomedicine》2013,9(1):76
Background
Selection criteria are important for analyzing domestication of perennial plant species, which experience a selection pressure throughout several human generations. We analyze the preferred morphological characteristics of Crescentia cujete fruits, which are used as bowls by the Maya of Yucatan, according to the uses they are given and the phenotypic consequences of artificial selection between one wild and three domesticated varieties.Methods
We performed 40 semi-structured interviews in seven communities. We calculated Sutrop’s salience index (S) of five classes of ceremonial and daily life uses, and of each item from the two most salient classes. We sampled 238 bowls at homes of people interviewed and compared their shape, volume and thickness with 139 fruits collected in homegardens and 179 from the wild. Morphology of varieties was assessed in fruit (n?=?114 trees) and vegetative characters (n?=?136 trees). Differences between varieties were evaluated through linear discriminant analysis (LDA).Results
Use of bowls as containers for the Day of the Dead offerings was the most salient class (S?=?0.489) with chocolate as its most salient beverage (S?=?0.491), followed by consumption of daily beverages (S?=?0.423), especially maize-based pozol (S?=?0.412). The sacred saka’ and balche' are offered in different sized bowls during agricultural and domestic rituals. Roundness was the most relevant character for these uses, as bowls from households showed a strong selection towards round shapes compared with wild and homegarden fruits. Larger fruits from domesticated varieties were also preferred over small wild fruits, although in the household different sizes of the domesticated varieties are useful. LDA separated wild from domesticated trees (p?<?0.001) according to both fruit and vegetative variables, but domesticated varieties were not different among themselves.Conclusions
The association between C. cujete bowls and traditional beverages in ritual and daily life situations has driven for centuries the selection of preferred fruit morphology in this tree. Selection of fruit roundness and volume has allowed for the differentiation between the wild variety and the three domesticated ones, counteracting gene flow among them. By choosing the best fruits from domesticated varieties propagated in homegardens, the Maya people model the domestication process of this important tree in their culture.939.
Immobilization of enzymes onto nanoparticles for enhanced biocatalytic activity via enzyme clustering is a growing field. In this paper, the effect of nanoparticle size on the hydrolytic activity of artificial cellulosomes was investigated. A simple method based on metal affinity coordination was employed to directly conjugate two enzymes, an endoglucanase CelA and an exoglucanase CelE, onto CdSe–ZnS core–shell quantum dots (QDs) without the use of any chemical modification or linker molecules such as streptavidin. Artificial cellulosomes were created by clustering the enzymes onto two different QDs (5 and 10 nm) to systematically study the influence of particle size and QD to enzyme ratio on the enhancement in cellulose hydrolysis. Our results indicate that enzyme proximity is the most important factor for activity enhancement while the influence of particle size is relatively modest. This detailed understanding will provide insights for the design of other artificial cellulosomes based on nanoclustering of multiple catalytic domains with significantly enhanced activities, and may be applicable for designing improved nanobiocatalysts for biofuel production, bioremediation, and drug design. 相似文献
940.
Jae Kwang Kim Soo-Yun Park Si Myung Lee Sun-Hyung Lim Hyo Jin Kim Sung-Dug Oh Yunsoo Yeo Hyun Suk Cho Sun-Hwa Ha 《Plant biotechnology reports》2013,7(1):121-128
Substantial equivalence is a critical concept for biosafety assessment of genetically modified (GM) crops. To investigate substantial equivalence among carotenoid-biofortified GM rice and five conventional rice cultivars having common white (three) and red (two) grain colors, profiles of 52 polar metabolites were analyzed using gas chromatography time-of-flight mass spectrometry. The results were compared to evaluate the differences among GM and non-GM rice cultivars using principal components analysis. The GM rice is more comparable to its non-transgenic counterpart rice variety according to the closer co-separation than for other cultivars tested. This suggests that profiling of unintended polar metabolites could be a useful tool to reveal substantial equivalence of GM rice. 相似文献