全文获取类型
收费全文 | 45915篇 |
免费 | 17247篇 |
国内免费 | 103篇 |
出版年
2022年 | 284篇 |
2021年 | 732篇 |
2020年 | 2361篇 |
2019年 | 3914篇 |
2018年 | 4078篇 |
2017年 | 4299篇 |
2016年 | 4533篇 |
2015年 | 4780篇 |
2014年 | 4443篇 |
2013年 | 5095篇 |
2012年 | 3117篇 |
2011年 | 2754篇 |
2010年 | 3857篇 |
2009年 | 2512篇 |
2008年 | 1809篇 |
2007年 | 1326篇 |
2006年 | 1306篇 |
2005年 | 1349篇 |
2004年 | 1251篇 |
2003年 | 1206篇 |
2002年 | 1158篇 |
2001年 | 598篇 |
2000年 | 500篇 |
1999年 | 460篇 |
1998年 | 287篇 |
1997年 | 189篇 |
1996年 | 207篇 |
1995年 | 192篇 |
1994年 | 172篇 |
1993年 | 180篇 |
1992年 | 230篇 |
1991年 | 186篇 |
1990年 | 198篇 |
1989年 | 194篇 |
1988年 | 164篇 |
1987年 | 172篇 |
1986年 | 174篇 |
1985年 | 207篇 |
1984年 | 204篇 |
1983年 | 176篇 |
1982年 | 186篇 |
1981年 | 190篇 |
1980年 | 197篇 |
1979年 | 159篇 |
1978年 | 157篇 |
1977年 | 133篇 |
1976年 | 118篇 |
1975年 | 114篇 |
1974年 | 138篇 |
1973年 | 125篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
44.
QIAN‐WAN MENG YI‐BO LUO 《Botanical journal of the Linnean Society. Linnean Society of London》2008,158(4):689-695
A new species, Galearis huanglongensis Q.W.Meng & Y.B.Luo, is described and illustrated. It is similar to Galearis cyclochila (Franch. & Sav.) Soó and Galearis diantha (Schltr.) P.F.Hunt, but differs in having a short spur, two elliptical lateral stigma lobes and distinctly separated bursicles. This new species is known only from the type locality, the Huanglong Valley, Songpan County, western Sichuan, China, growing amongst mosses under alpine shrubs at an elevation of about 3000 m. Based on two years of observations of its population size, the species was categorized as critically endangered CR (B1a, B2a) according to the World Conservation Union (IUCN) Red List Categories and Criteria, Version 3.1. The micromorphology of pollinia and seeds was observed by scanning electron microscopy and compared with that of G. cyclochila and G. diantha. The results supported G. huanglongensis Q.W.Meng & Y.B.Luo as a new species. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 689–695. 相似文献
45.
The genes encoding the six polypeptide components of the alkene monooxygenase from Xanthobacter Py2 have been sequenced. The predicted amino acid sequence of the first ORF shows homology with the iron binding subunits of binuclear non-haem iron containing monooxygenases including benzene monooxygenase, toluene 4-monooxygenase (>60% sequence similarity) and methane monooxygenase (>40% sequence similarity) and that the necessary sequence motifs associated with iron co-ordination are also present. Secondary structure prediction based on the amino acid sequence showed that the predominantly α-helical structure that surrounds the binuclear iron binding site was conserved allowing the sequence to be modelled on the co-ordinates of the methane monooxygenase α-subunit. Significant differences in the residues forming the hydrophobic cavity which forms the substrate binding site are discussed with reference to the differences in reaction specificity and stereospecificity of binuclear non-haem iron monooxygenases. 相似文献
46.
Gray Flycatchers (Empidonax wrightii) breed in a variety of habitats in the arid and semi‐arid regions of the western United States, but little is known about their breeding biology, especially in the northern portion of their range where they nest in ponderosa pine (Pinus ponderosa) forests. From May to July 2014 and 2015, we conducted surveys for singing male Gray Flycatchers along the eastern slope of the Cascade Range in Washington, U.S.A, monitored flycatcher nests, and quantified nest‐site vegetation. We used a logistic‐exposure model fit within a Bayesian framework to model the daily survival probability of flycatcher nests. During the 2 yr of our study, we monitored 141 nests, with 93% in ponderosa pines. Mean clutch size was 3.6 eggs and the mean number of young fledged per nest was 3.2. Predation accounted for 90% of failed nests. We found a positive association between daily nest survival and both nest height and distance of nest substrates from the nearest tree. Flycatchers that locate their nests higher above the ground and further from adjacent trees may be choosing the safest alternative because higher nests may be less exposed to terrestrial predators and nests in trees that are farther from other trees may be less exposed to arboreal predators such as jays (Corvidae) that may forage in patches with connected canopies. Nests in trees farther from other trees may also allow earlier detection of approaching predators and thus aid in nest defense. 相似文献
47.
48.
49.
50.
Catherine E. Rogers Karl R. Mattox Kenneth D. Stewart 《American journal of botany》1980,67(5):774-783
Chlorokybus atmophyticus has a sarcinoid growth habit and produces scale-covered zoospores. Flagella are laterally inserted and attached internally to a multilayered structure characteristic of the Charophyceae. There are two kinds of pyrenoid in each cell, a feature previously observed in only one scaly green flagellate. C. atmophyticus demonstrates that the sarcinoid growth habit arose independently at least twice in the green algae and cannot be used to define taxonomic groups unless combined with other criteria. It is further concluded that C. atmophyticus should be classified in a separate family Chlorokybaceae and a separate order Chloroky bales. 相似文献