全文获取类型
收费全文 | 17210篇 |
免费 | 1426篇 |
国内免费 | 8篇 |
专业分类
18644篇 |
出版年
2024年 | 21篇 |
2023年 | 130篇 |
2022年 | 305篇 |
2021年 | 534篇 |
2020年 | 311篇 |
2019年 | 400篇 |
2018年 | 458篇 |
2017年 | 352篇 |
2016年 | 664篇 |
2015年 | 1074篇 |
2014年 | 1160篇 |
2013年 | 1346篇 |
2012年 | 1629篇 |
2011年 | 1460篇 |
2010年 | 930篇 |
2009年 | 774篇 |
2008年 | 1020篇 |
2007年 | 997篇 |
2006年 | 888篇 |
2005年 | 810篇 |
2004年 | 773篇 |
2003年 | 618篇 |
2002年 | 618篇 |
2001年 | 111篇 |
2000年 | 90篇 |
1999年 | 111篇 |
1998年 | 125篇 |
1997年 | 97篇 |
1996年 | 92篇 |
1995年 | 65篇 |
1994年 | 61篇 |
1993年 | 49篇 |
1992年 | 50篇 |
1991年 | 39篇 |
1990年 | 37篇 |
1989年 | 48篇 |
1988年 | 40篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 28篇 |
1984年 | 33篇 |
1983年 | 23篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 23篇 |
1979年 | 19篇 |
1978年 | 13篇 |
1976年 | 17篇 |
1974年 | 13篇 |
1968年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Miquel Barbera Laura Escriva Jorge Mariano Collantes-Alegre Giuseppe Meca Ezio Rosato David Martinez-Torres 《Insect Science》2020,27(2):224-238
Aphids display life cycles largely determined by the photoperiod.During the warm long-day seasons.most aphid species reproduce by viviparous parthenogenesis.The shortening of the photoperiod in autumn induces a switch to sexual reproduction.Males and sexual females mate to produce overwintering resistant eggs.In addition to this full life cycle(holocycle),there are anholocyelic lineages that do not respond to changes in photoperiod and reproduce continuously by parthenogenesis.The molecular or hormonal events that trigger the scasonal response(i.c,induction of the sexual phenotypes)are still unknown.Although circadian synthesis of melatonin is known to play a key role in vertebrate photoperiodism,the involvement of the circadian clock and/or of the hor-mone melatonin in insect seasonal responses is not so well established.Here we show that melatonin levels in the aphid Acyrthosiphon pisum are significantly higher in holocyclice aphids reared under short days than under long days,while no differences were found between anholoeyelic aphids under the same conditions.We also found that melatonin is localized in the aphid suboesophageal ganglion(SOG)and in the thoracic ganglionic mass(TGM).In analogy to vertcbrates,insect-type arylalkxylamine N-acetyltransferases(i-AANATs)are thought to play a key role in melatonin synthesis.We measured the expression of four I-AANAT genes identified in A.pisum and localized two of them in situ in the insect central nervous systems(CNS).Levels of expression of these genes were compatible with the quantities of melatonin observed.Moreover,like melatonin,expression of these genes was found in the SOG and the TGM. 相似文献
102.
Laura Stanbery Nisha J D'Silva Julia S Lee Carol R Bradford Thomas E Carey Mark E Prince Gregory T Wolf Francis P Worden Kitrina G Cordell Elizabeth M Petty 《Translational oncology》2010,3(4):239-245
The purpose of this work was to determine SEPT9_v1 expression levels in head and neck squamous cell carcinoma (HNSCC) and to analyze whether SEPT9_v1 expression is relevant to clinical outcomes. Recently, the SEPT9 isoform SEPT9_v1 has been implicated in oncogenesis, and methylation of the SEPT9 promoter region was reported in HNSCC. These findings led us to hypothesize that SEPT9_v1 could be differently expressed in HNSCC. To determine whether SEPT9_v1 is expressed in HNSCC, tissue microarray immunohistochemical analysis was performed using a SEPT9_v1-specific antibody. Tissue microarrays stained with a polyclonal SEPT9_v1-specific antibody was used to determine protein expression levels in HNSCC tissue samples, some with known clinical outcomes. This analysis showed that SEPT9_v1 is in fact highly expressed in HNSCC compared with normal epithelium, and high expression levels directly correlated with poor clinical outcomes. Specifically, a high SEPT9_v1 expression was associated with decreased disease-specific survival (P = .012), time to indication of surgery at primary site (P = .008), response to induction chemotherapy (P = .0002), and response to chemotherapy (P = .02), as well as advanced tumor stage (P = .012) and N stage (P = .0014). The expression of SEPT9_v1 was also strongly correlated with smoking status (P = .00094). SEPT9_v1 is highly expressed in HNSCC, and a high expression of SEPT9_v1 is associated with poor clinical outcomes. These data indicate that SEPT9_v1 warrants additional investigation as a potential biomarker for HNSCC. 相似文献
103.
104.
105.
106.
107.
108.
Sigrid van Grinsven Jaap S. Sinninghe Damsté Alejandro Abdala Asbun Julia C. Engelmann John Harrison Laura Villanueva 《Environmental microbiology》2020,22(2):766-782
Methanotrophic bacteria play a key role in limiting methane emissions from lakes. It is generally assumed that methanotrophic bacteria are mostly active at the oxic-anoxic transition zone in stratified lakes, where they use oxygen to oxidize methane. Here, we describe a methanotroph of the genera Methylobacter that is performing high-rate (up to 72 μM day−1) methane oxidation in the anoxic hypolimnion of the temperate Lacamas Lake (Washington, USA), stimulated by both nitrate and sulfate addition. Oxic and anoxic incubations both showed active methane oxidation by a Methylobacter species, with anoxic rates being threefold higher. In anoxic incubations, Methylobacter cell numbers increased almost two orders of magnitude within 3 days, suggesting that this specific Methylobacter species is a facultative anaerobe with a rapid response capability. Genomic analysis revealed adaptations to oxygen-limitation as well as pathways for mixed-acid fermentation and H2 production. The denitrification pathway was incomplete, lacking the genes narG/napA and nosZ, allowing only for methane oxidation coupled to nitrite-reduction. Our data suggest that Methylobacter can be an important driver of the conversion of methane in oxygen-limited lake systems and potentially use alternative electron acceptors or fermentation to remain active under oxygen-depleted conditions. 相似文献
109.
Michael J. Smith Laura Ruykys Bryony Palmer Nicola Palmer Georgia Volck Adele Thomasz Noel Riessen 《Restoration Ecology》2020,28(2):468-474
Introduced predators are a serious threat to Australian vertebrates. However, the consequences of predation for an area's avifauna have rarely been quantified. We took advantage of the establishment of a 7,832 ha fox‐ and cat‐free safe haven at Mt Gibson, in Western Australia, to assess the consequences of excluding introduced mammal predators on the bird fauna. Bird surveys were conducted over 6 years, before and after the establishment of the introduced predator‐free safe haven. After 3 years, half the sites were enclosed by the fence that excluded introduced predators, while the remainder of sites remained outside the fence and were exposed to fox and cat activity. The sites were stratified by four major vegetation types. A total of 91 bird species were variously detectable with the survey approach, but were typically more detectable during morning surveys. Site occupancy varied considerably among species, but overall, occupancy by all species was most likely to be either not impacted or positively impacted by the safe haven. The most notable change was that avifaunal richness appeared to increase in woodland and shrubland habitats within, as compared to outside, the safe haven. We conclude that: (1) the safe haven had an overall positive impact on bird occupancy; and (2) there were no consistent trends with respect to the kinds of species whose occupancy was positively impacted, beyond them all being small‐ to medium‐sized birds and mostly insectivorous. However, these conclusions must be tempered by the poor detection probability of many species. 相似文献
110.