收费全文 | 3445篇 |
免费 | 283篇 |
国内免费 | 3篇 |
2023年 | 13篇 |
2022年 | 11篇 |
2021年 | 36篇 |
2020年 | 31篇 |
2019年 | 51篇 |
2018年 | 52篇 |
2017年 | 40篇 |
2016年 | 67篇 |
2015年 | 105篇 |
2014年 | 167篇 |
2013年 | 167篇 |
2012年 | 219篇 |
2011年 | 207篇 |
2010年 | 161篇 |
2009年 | 148篇 |
2008年 | 215篇 |
2007年 | 231篇 |
2006年 | 209篇 |
2005年 | 218篇 |
2004年 | 191篇 |
2003年 | 167篇 |
2002年 | 190篇 |
2001年 | 58篇 |
2000年 | 42篇 |
1999年 | 42篇 |
1998年 | 50篇 |
1997年 | 34篇 |
1996年 | 34篇 |
1995年 | 40篇 |
1994年 | 30篇 |
1993年 | 35篇 |
1992年 | 27篇 |
1991年 | 29篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 23篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 30篇 |
1984年 | 24篇 |
1983年 | 17篇 |
1982年 | 23篇 |
1981年 | 16篇 |
1980年 | 26篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1974年 | 14篇 |
1973年 | 11篇 |
Structural features of parasitism ofOrobanche III. The differentiation of xylem connexion ofO. crenata
Summary Haustorial cells of the root parasiteOrobanche within the xylem of the host tissue were investigated by light- and electronmicroscopy. Coming into contact with the tracheary elements of the host these cells show an unusual differentiation before turning into a water conducting xylem element. From haustorial cells with thickened front walls they develop into typical transfer cells, bearing wall ingrowth in those parts of the wall orientated towards the tracheary elements of the host. During further differentiation the transfer cell changes into a water conducting element by developing the typical secondary wall thickenings of xylem elements within the cell. Partly these wall thickenings are formed inside the labyrinth structures of the transfer cell, always situated opposite those of the tracheary element of the host. Simultaneously the labyrinth disintegrates—some small remnants of wall ingrowths may persist. The protoplast degenerates, and finally a haustorial water conducting element results. Host- und parasitic tracheary elements are connected by pits.
Wir danken FrauChristl Glockmann für ihre stets verantwortungsvolle Mitarbeit. Den GÄrtnern des Botanischen Gartens in Kiel sei Dank für die oft schwierige Anzucht des Pflanzenmaterials. Die Untersuchungen wurden durch Mittel der Deutschen Forschungsgemeinschaft gefördert. 相似文献
A model for the spatial-temporal signal transmission properties of the human visual system based on the linear system theory of continuous media
In this paper, a three dimensional homogeneous neural web serves as a model for the transmission of spatial-temporal luminous pattern by the human visual system. The propagation of neuronal excitation in afferent and lateral direction is assumed to be anisotropic. It is described by a modified heat conduction equation a main feature of which is a regenerative signal component. The time-dependent solution describes f.e. the temporal transmission characteristics for flickering grids in the whole frequency range. The stationary solution contains the spatial transmission characteristics for grids as well as the structure of receptive fields with inhibitory surrounds. The structure of the solution makes it under-standable how the transition from spatial selectivity to spatial-frequency-selectivity develops with growing depth of the neuronal web.相似文献
Zusammenfassung Die Lymphocyten eines Patienten mit Ringchromosom 4 wurden in der Meta-, Ana-und Interphase beobachtet. 90,1% der Metaphasen wiesen einen Ring auf. Besondere Aufmerksamkeit wurde abnormen, aus dem Ring entstandenen Chromosomen zugewandt. Es fanden sich u. a.: partielle Doppel-und Dreifachringe, Ringe mit 3 Zentromeren, Ringe aus 3 miteinander verketteten Ringen, stabförmige Chromosomen und pulverisierte Ringe.Die Symptome des Patienten (Minderwuchs, Schwachsinn, Hypoplasie der Daumen, Ptosis palpebrae, Hypoplasie des äußeren genitale, abnorme Dermatoglyphen) waren zum größten Teil unspezifish.
This work was supported in part by grants from the Deutsche Forschungsgemeinschaft and is part of a thesis by R.N. for a medical doctorate. 相似文献
Despite its fundamental role in providing an extensive surface for gas exchange, the alveolar epithelium (AE) serves as an immunological barrier through, e.g., the release of proinflammatory cytokines and secretion of surfactant to prevent alveolar collapse. Thus, AE is important for sustaining lung homeostasis. Extracellular ATP secreted by alveolar epithelial cells (AECs) is involved in physiological and pathological conditions and acts mainly through the activation of purine receptors (P2Rs). When studying P2R-mediated processes, primary isolated type II AECs (piAECs) still represent the gold standard in in vitro research, although their preparation is time-consuming and requires the sacrifice of many animals. Hence, cultivated immortalized and tumor-derived AEC lines may constitute a valuable alternative. In this work, we examined P2R expression and functionality in piAECs, in immortalized and tumor-derived AEC lines with the purpose of gaining a better understanding of purinergic signaling in different cell systems and assisting researchers in the choice of a suitable cell line with a certain P2R in demand. We combined mRNA and protein analysis to evaluate the expression of P2R. For pharmacological testing, we conducted calcium ([Ca2+]) measurements and siRNA receptor knockdown. Interestingly, the mRNA and protein levels of P2Y2, P2Y6, and P2X4 were detected on all cell lines. Concerning functionality, P2XR could be narrowed to L2 and piAECs while P2YR were active in all cell lines.
相似文献