全文获取类型
收费全文 | 2499篇 |
免费 | 253篇 |
出版年
2021年 | 25篇 |
2019年 | 18篇 |
2018年 | 31篇 |
2017年 | 22篇 |
2016年 | 41篇 |
2015年 | 75篇 |
2014年 | 92篇 |
2013年 | 88篇 |
2012年 | 94篇 |
2011年 | 89篇 |
2010年 | 75篇 |
2009年 | 57篇 |
2008年 | 87篇 |
2007年 | 87篇 |
2006年 | 93篇 |
2005年 | 73篇 |
2004年 | 82篇 |
2003年 | 71篇 |
2002年 | 70篇 |
2001年 | 54篇 |
2000年 | 58篇 |
1999年 | 46篇 |
1998年 | 29篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 26篇 |
1993年 | 20篇 |
1992年 | 29篇 |
1991年 | 36篇 |
1990年 | 41篇 |
1989年 | 43篇 |
1988年 | 43篇 |
1987年 | 40篇 |
1986年 | 45篇 |
1985年 | 45篇 |
1984年 | 35篇 |
1983年 | 33篇 |
1982年 | 18篇 |
1981年 | 32篇 |
1980年 | 24篇 |
1979年 | 36篇 |
1978年 | 21篇 |
1976年 | 18篇 |
1975年 | 21篇 |
1974年 | 30篇 |
1973年 | 29篇 |
1972年 | 21篇 |
1969年 | 19篇 |
1967年 | 25篇 |
1966年 | 19篇 |
排序方式: 共有2752条查询结果,搜索用时 15 毫秒
981.
Anther form and structure across the taxonomic groups inVerticordia were examined. The three anther types which were recognised — rectangular, oblong and saccate, accord well with the three subgenera into which the genus has been divided. The sporogenous part of the anther has a fairly typical angiosperm anatomy. However in many species there is a small or large gland in the upper filament/connective which contains lipidic contents. The anatomy of this structure is based on that of the oil glands which are ubiquitous in Myrtaceae primary tissues. However the gland is usually much larger than these and is schizolysigenous in origin. Evolutionary development of the anthers in the genus is related to pollination systems and the development of secondary pollen presentation from the upper style in some groups. Anther glands may have originally had a protective function for the sporogenous tissue. However in different groups the function has changed or the gland has disappeared. In some species in subgenusChrysoma (which does not have secondary pollen presentation) the gland contents seem to be an additional food source for pollinators. In other groups, with the development of secondary pollen presentation the protective function has become redundant and anther glands have either disappeared or produce contents which have become part of the process of pollen dispersal. 相似文献
982.
Fuentealba P Begum R Capogna M Jinno S Márton LF Csicsvari J Thomson A Somogyi P Klausberger T 《Neuron》2008,57(6):917-929
In the cerebral cortex, GABAergic interneurons are often regarded as fast-spiking cells. We have identified a type of slow-spiking interneuron that offers distinct contributions to network activity. "Ivy" cells, named after their dense and fine axons innervating mostly basal and oblique pyramidal cell dendrites, are more numerous than the parvalbumin-expressing basket, bistratified, or axo-axonic cells. Ivy cells express nitric oxide synthase, neuropeptide Y, and high levels of GABA(A) receptor alpha1 subunit; they discharge at a low frequency with wide spikes in vivo, yet are distinctively phase-locked to behaviorally relevant network rhythms including theta, gamma, and ripple oscillations. Paired recordings in vitro showed that Ivy cells receive depressing EPSPs from pyramidal cells, which in turn receive slowly rising and decaying inhibitory input from Ivy cells. In contrast to fast-spiking interneurons operating with millisecond precision, the highly abundant Ivy cells express presynaptically acting neuromodulators and regulate the excitability of pyramidal cell dendrites through slowly rising and decaying GABAergic inputs. 相似文献
983.
The rat is the preferred experimental animal in many biological studies. With the recent derivation of authentic rat embryonic stem (ES) cells it is now feasible to apply state-of-the art genetic engineering in this species using homologous recombination. To establish whether rat ES cells are amenable to in vivo recombination, we tested targeted disruption of the hypoxanthine phosphoribosyltransferase (hprt) locus in ES cells derived from both inbred and outbred strains of rats. Targeting vectors that replace exons 7 and 8 of the hprt gene with neomycinR/thymidine kinase selection cassettes were electroporated into male Fisher F344 and Sprague Dawley rat ES cells. Approximately 2% of the G418 resistant colonies also tolerated selection with 6-thioguanine, indicating inactivation of the hprt gene. PCR and Southern blot analysis confirmed correct site-specific targeting of the hprt locus in these clones. Embryoid body and monolayer differentiation of targeted cell lines established that they retained differentiation potential following targeting and selection. This report demonstrates that gene modification via homologous recombination in rat ES cells is efficient, and should facilitate implementation of targeted, genetic manipulation in the rat. 相似文献
984.
Bracken ferns (genus Pteridium) represent an ancient species complex with a natural worldwide distribution. Pteridium has historically been treated as comprising a single species, but recent treatments have recognized several related species. Phenotypic plasticity, geographically structured morphological variation, and geographically biased sampling have all contributed to taxonomic confusion in the genus. We sampled bracken specimens worldwide and used variable regions of the chloroplast genome to investigate phylogeography and reticulate evolution within the genus. Our results distinguish two major clades within Pteridium, a primarily northern hemisphere Laurasian/African clade, which includes all taxa currently assigned to P. aquilinum, and a primarily southern hemisphere Austral/South American clade, which includes P. esculentum and P. arachnoideum. All European accessions of P. aquilinum subsp. aquilinum appear in a monophyletic group and are nested within a clade containing the African P. aquilinum taxa (P. aquilinum subsp. capense and P. aquilinum subsp. centrali-africanum). Our results allow us to hypothesize the maternal progenitors of two allotetraploid bracken species, P. caudatum and P. semihastatum. We also discuss the biogeography of bracken in the context of the chloroplast phylogeny. Our study is one of the first to take a worldwide perspective in addressing variation in a broadly distributed species complex. 相似文献
985.
Sunderland NS Thomson SE Heffernan SJ Lim S Thompson J Ogle R McKenzie P Kirwan PJ Makris A Hennessy A 《Cytokine》2011,56(2):192-199
Preeclampsia is a common disease of pregnancy characterised by maternal hypertension and proteinuria. Abnormal placentation in early pregnancy and abnormal cytokine and anti-angiogenic factor expression are thought to contribute to the clinical syndrome of endothelial dysfunction evident in the second half of gestation. The mechanisms underlying both the placental pathology and its translation to the maternal clinical syndrome are not fully understood. A model of preeclampsia manifest by clinically evident endothelial dysfunction (increased blood pressure and proteinuria) was induced by administration of low-dose TNF-α for 2 weeks at mid-gestation in pregnant baboons (Papio hamadryas). Blood pressure was monitored continuously and remotely by intra-arterial radiotelemetry. Following TNF-α infusion, there was an increase in systolic and diastolic blood pressure and development of proteinuria in pregnant treated animals, but not in pregnant saline controls nor in non-pregnant TNF-α treated animals. The treated pregnant animals also developed elevated plasma soluble FMS-like tyrosine kinase-1 (sFLT-1) and increased placental mRNA expression of sFLT-1 and soluble endoglin (sEng). These results clearly demonstrate that the cytokine TNF-α can induce the clinical and biochemical features of human preeclampsia. The results identify a link between cytokines, placental dysfunction and endothelial dysfunction resulting in a loss of maternal blood pressure control. 相似文献
986.
D.H.S. Horn E.J. Middleton J.A. Thomson J.S. Wilkie 《Journal of insect physiology》1974,20(12):2433-2445
A study has been made, using Calliphora stygia at the time of puparium formation, of the incorporation of a number of labelled sterols into β-ecdysone. [1-3H]-Cholesterol and [4-14C]-cholesterol are incorporated to a similar extent (0·01-0·02%). [1-3H]-7-Dehydrocholesterol is better incorporated (0·025%) than cholesterol while [1-3H]-cholesterol sulphate, (22R)-22-hydroxy-[22-3H]-cholesterol, and 25-hydroxy-[26-14C]-cholesterol are not incorporated to a significant extent. 相似文献
987.
988.
Zhernakova A Stahl EA Trynka G Raychaudhuri S Festen EA Franke L Westra HJ Fehrmann RS Kurreeman FA Thomson B Gupta N Romanos J McManus R Ryan AW Turner G Brouwer E Posthumus MD Remmers EF Tucci F Toes R Grandone E Mazzilli MC Rybak A Cukrowska B Coenen MJ Radstake TR van Riel PL Li Y de Bakker PI Gregersen PK Worthington J Siminovitch KA Klareskog L Huizinga TW Wijmenga C Plenge RM 《PLoS genetics》2011,7(2):e1002004
989.
990.
Richard D. Pillans Douglas Bearham Andrew Boomer Ryan Downie Toby A. Patterson Damian P. Thomson Russel C. Babcock 《PloS one》2014,9(9)
Off the Ningaloo coast of North West Western Australia, Spangled Emperor Lethrinus nebulosus are among the most highly targeted recreational fish species. The Ningaloo Reef Marine Park comprises an area of 4,566 km2 of which 34% is protected from fishing by 18 no-take sanctuary zones ranging in size from 0.08–44.8 km2. To better understand Spangled Emperor movements and the adequacy of sanctuary zones within the Ningaloo Reef Marine Park for this species, 84 Spangled Emperor of a broad spectrum of maturity and sex were tagged using internal acoustic tags in a range of lagoon and reef slope habitats both inside and adjacent to the Mangrove Bay Sanctuary zone. Kernel Utilisation Distribution (KUD) was calculated for 39 resident individuals that were detected for more than 30 days. There was no relationship with fish size and movement or site fidelity. Average home range (95% KUD) for residents was 8.5±0.5 km2 compared to average sanctuary zone size of 30 km2. Calculated home range was stable over time resulting in resident animals tagged inside the sanctuary zone spending ∼80% of time within the sanctuary boundaries. The number of fish remaining within the array of receivers declined steadily over time and after one year more than 60% of tagged fish had moved outside the sanctuary zone and also beyond the 28 km2 array of receivers. Long term monitoring identified the importance of shifting home range and was essential for understanding overall residency within protected areas and also for identifying spawning related movements. This study indicates that despite exhibiting stable and small home ranges over periods of one to two years, more than half the population of spangled emperor move at scales greater than average sanctuary size within the Ningaloo Reef Marine Park. 相似文献