全文获取类型
收费全文 | 4347篇 |
免费 | 374篇 |
出版年
2023年 | 32篇 |
2022年 | 18篇 |
2021年 | 126篇 |
2020年 | 81篇 |
2019年 | 103篇 |
2018年 | 145篇 |
2017年 | 111篇 |
2016年 | 185篇 |
2015年 | 275篇 |
2014年 | 244篇 |
2013年 | 336篇 |
2012年 | 390篇 |
2011年 | 384篇 |
2010年 | 190篇 |
2009年 | 176篇 |
2008年 | 242篇 |
2007年 | 238篇 |
2006年 | 211篇 |
2005年 | 177篇 |
2004年 | 165篇 |
2003年 | 155篇 |
2002年 | 157篇 |
2001年 | 61篇 |
2000年 | 36篇 |
1999年 | 44篇 |
1998年 | 35篇 |
1997年 | 29篇 |
1996年 | 28篇 |
1995年 | 19篇 |
1994年 | 16篇 |
1993年 | 12篇 |
1992年 | 17篇 |
1991年 | 22篇 |
1990年 | 22篇 |
1989年 | 9篇 |
1988年 | 11篇 |
1987年 | 16篇 |
1986年 | 10篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 17篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1979年 | 12篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 11篇 |
1971年 | 7篇 |
1968年 | 9篇 |
1966年 | 7篇 |
排序方式: 共有4721条查询结果,搜索用时 31 毫秒
1.
2.
Luca Racca Andrea Villa Lukardis C. M. Wencker Marco Camaiti Hugues-Alexandre Blain Massimo Delfino 《Journal of morphology》2020,281(7):808-833
The skull osteology of Hierophis viridiflavus is here described and figured in detail on the basis of 18 specimens. The sample includes specimens from the ranges of both H. viridiflavus viridiflavus and H. viridiflavus carbonarius as well as specimens not identified at sub-specific level. The main characters that define H. viridiflavus in comparison to the parapatric congeneric species Hierophis gemonensis are wide maxillary diastema, basioccipital crest well distinct in three lobes and basioccipital process well marked. The foramina of the otoccipital and prootic, and the basioccipital process of the basioccipital are among the most ontogenetically variable characters, as indicated by two juvenile specimens included in the sample. A specimen-level phylogenetic analysis including H. gemonensis and other outgroups (overall 6 species, 26 specimens, 64 skull characters) recovered all H. viridiflavus specimens in one clade, indicating the presence of a clear phylogenetic signal in the applied characters. However, the resolution within the H. viridiflavus clade is poor the monophyly of H. viridiflavus carbonarius was retrieved, but not that of Hierophis v. viridiflavus. Probably due to the relatively high variability, the skull morphology does not support the recently proposed specific status of the two subspecies. 相似文献
3.
Luca Ceccato Simona Masiero Dola Sinha Roy Stefano Bencivenga Irma Roig-Villanova Franck Anicet Ditengou Klaus Palme Rüdiger Simon Lucia Colombo 《PloS one》2013,8(6)
Land plants are characterised by haplo-diploid life cycles, and developing ovules are the organs in which the haploid and diploid generations coexist. Recently it has been shown that hormones such as auxin and cytokinins play important roles in ovule development and patterning. The establishment and regulation of auxin levels in cells is predominantly determined by the activity of the auxin efflux carrier proteins PIN-FORMED (PIN). To study the roles of PIN1 and PIN3 during ovule development we have used mutant alleles of both genes and also perturbed PIN1 and PIN3 expression using micro-RNAs controlled by the ovule specific DEFH9 (DEFIFICENS Homologue 9) promoter. PIN1 down-regulation and pin1-5 mutation severely affect female gametophyte development since embryo sacs arrest at the mono- and/or bi-nuclear stages (FG1 and FG3 stage). PIN3 function is not required for ovule development in wild-type or PIN1-silenced plants. We show that sporophytically expressed PIN1 is required for megagametogenesis, suggesting that sporophytic auxin flux might control the early stages of female gametophyte development, although auxin response is not visible in developing embryo sacs. 相似文献
4.
Henri Berestycki Jean-Michel Roquejoffre Luca Rossi 《Journal of mathematical biology》2013,66(4-5):743-766
We propose here a new model to describe biological invasions in the plane when a strong diffusion takes place on a line. We establish the main properties of the system, and also derive the asymptotic speed of spreading in the direction of the line. For low diffusion, the line has no effect, whereas, past a threshold, the line enhances global diffusion in the plane and the propagation is directed by diffusion on the line. It is shown here that the global asymptotic speed of spreading in the plane, in the direction of the line, grows as the square root of the diffusion on the line. The model is much relevant to account for the effects of fast diffusion lines such as roads on spreading of invasive species. 相似文献
5.
Abstract A scanning electron microscopy (SEM) study of Cycas cuticle characteristics was undertaken in order to expand our knowledge of microscopic characters not observable under light microscopy and to clarify unresolved affinitites among some species within the genus. Whole leaf and isolated cuticle specimens from the middle region of leaflets of greenhouse-grown plants of Cycas revoluta, Cycas rumphii, Cycas circinalis, Cycas media, and Cycas normanbyana were examined using SEM for interior and exterior features. Characteristics in common include hypostomy, hair bases on abaxial and adaxial surfaces, adaxial cells randomly arranged, adaxial exterior cuticle smooth, and stomata sunken to various degrees but stomatal pit always formed by two layers of epidermal cells. Stomatal complex is of the polyperigenous type. Stomata randomly dispersed and oriented, and except C. revoluta, are not contiguous. Stomata deeply sunken in C. revoluta, intermediate in C. rumphii and C. normanbyana, and less sunken in C. circinalis and C. media. Aperture between guard cells extends the entire stomatal length in C. rumphii and C. normanbyana, ~80% in C. circinalis and C. media, and ~50% in C. revoluta. Cuticular features of C. revoluta show the greatest difference from the other species in complex relief of exterior cuticle and interior cuticular structure of subsidiary cells; C. media and C. circinalis show close similarity to each other and their stomatal complex dimensions fall within the same unique cluster using principal component analysis under normalized variables. C. normanbyana and C. rumphii show the most similarity to each other in cuticular micromorphology. Stomatal complex dimensions of these two species fall into a second cluster that also includes C. revoluta. These data contrast with current taxonomy placing C. normanbyana synonymous to C. media. 相似文献
6.
George F. Young Luca Scardovi Andrea Cavagna Irene Giardina Naomi E. Leonard 《PLoS computational biology》2013,9(1)
Flocks of starlings exhibit a remarkable ability to maintain cohesion as a group in highly uncertain environments and with limited, noisy information. Recent work demonstrated that individual starlings within large flocks respond to a fixed number of nearest neighbors, but until now it was not understood why this number is seven. We analyze robustness to uncertainty of consensus in empirical data from multiple starling flocks and show that the flock interaction networks with six or seven neighbors optimize the trade-off between group cohesion and individual effort. We can distinguish these numbers of neighbors from fewer or greater numbers using our systems-theoretic approach to measuring robustness of interaction networks as a function of the network structure, i.e., who is sensing whom. The metric quantifies the disagreement within the network due to disturbances and noise during consensus behavior and can be evaluated over a parameterized family of hypothesized sensing strategies (here the parameter is number of neighbors). We use this approach to further show that for the range of flocks studied the optimal number of neighbors does not depend on the number of birds within a flock; rather, it depends on the shape, notably the thickness, of the flock. The results suggest that robustness to uncertainty may have been a factor in the evolution of flocking for starlings. More generally, our results elucidate the role of the interaction network on uncertainty management in collective behavior, and motivate the application of our approach to other biological networks. 相似文献
7.
8.
9.