全文获取类型
收费全文 | 77732篇 |
免费 | 19679篇 |
国内免费 | 35篇 |
出版年
2022年 | 416篇 |
2021年 | 1095篇 |
2020年 | 2596篇 |
2019年 | 4204篇 |
2018年 | 4474篇 |
2017年 | 4645篇 |
2016年 | 5189篇 |
2015年 | 5961篇 |
2014年 | 5761篇 |
2013年 | 6905篇 |
2012年 | 5235篇 |
2011年 | 5004篇 |
2010年 | 5245篇 |
2009年 | 3724篇 |
2008年 | 3641篇 |
2007年 | 3270篇 |
2006年 | 3019篇 |
2005年 | 3162篇 |
2004年 | 3070篇 |
2003年 | 2992篇 |
2002年 | 2949篇 |
2001年 | 852篇 |
2000年 | 677篇 |
1999年 | 776篇 |
1998年 | 805篇 |
1997年 | 596篇 |
1996年 | 546篇 |
1995年 | 533篇 |
1994年 | 475篇 |
1993年 | 431篇 |
1992年 | 515篇 |
1991年 | 428篇 |
1990年 | 360篇 |
1989年 | 365篇 |
1988年 | 342篇 |
1987年 | 331篇 |
1986年 | 310篇 |
1985年 | 395篇 |
1984年 | 421篇 |
1983年 | 336篇 |
1982年 | 450篇 |
1981年 | 394篇 |
1980年 | 344篇 |
1979年 | 281篇 |
1978年 | 311篇 |
1977年 | 287篇 |
1976年 | 259篇 |
1975年 | 213篇 |
1974年 | 260篇 |
1973年 | 259篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
61.
Corine Vialas Geneviève Pratviel Jean Bernadou Bernard Meunier 《Nucleosides, nucleotides & nucleic acids》2013,32(4-5):1061-1063
Abstract Mn TMPyP in the presence of sulfite/O2 catalyses the oxidation of dG into dIz as selectively but slower and less efficiently than in the presence of KHSO5. 相似文献
62.
63.
64.
Ruth C. Paul B. Rainey Brian J. Sheehan Orla M. Keane Charles J. Dorman 《Current biology : CB》1999,9(24)
The relationship between environment and mutation is complex [1]. Claims of Lamarkian mutation [2] have proved unfounded [3], [4] and [5]; it is apparent, however, that the external environment can influence the generation of heritable variation, through either direct effects on DNA sequence [6] or DNA maintenance and copying mechanisms [7], [8], [9] and [10], or as a consequence of evolutionary processes [11], [12], [13], [14], [15] and [16]. The spectrum of mutational events subject to environmental influence is unknown [6] and precisely how environmental signals modulate mutation is unclear. Evidence from bacteria suggests that a transient recombination-dependent hypermutational state can be induced by starvation [5]. It is also apparent that chnages in the mutability of specific loci can be influenced by alterations in DNA topology [10] and [17]. Here we describe a remarkable instance of adaptive evolution in Salmonella which is caused by a mutation that occurs in intermediate-strength osmotic environments. We show that the mutation is not ‘directed’ and describe its genetic basis. We also present compelling evidence in support of the hypothesis that the mutational event is constrained by signals transmitted from the external environment via changes in the activity of DNA gyrase. 相似文献
65.
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
66.
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
67.
68.
69.
70.
This overview summarizes the data for how epithelial cells sort and deliver proteins and lipids to the apical and basolateral cell surface domains. The basolateral pathway uses a Rab-SNARE mechanism for docking and fusion, while the apical route employs a different machinery. This latter mechanism is based on lipid microdomains, composed of clusters of sphingolipids and cholesterol, which function as rafts for apical delivery. The sphingolipid-cholesterol raft mechanism seems to be employed generally by mammalian cells to transport raft-associated proteins to their post-Golgi destinations. 相似文献