全文获取类型
收费全文 | 3311篇 |
免费 | 279篇 |
国内免费 | 1篇 |
出版年
2023年 | 22篇 |
2022年 | 41篇 |
2021年 | 66篇 |
2020年 | 61篇 |
2019年 | 53篇 |
2018年 | 51篇 |
2017年 | 55篇 |
2016年 | 88篇 |
2015年 | 160篇 |
2014年 | 170篇 |
2013年 | 206篇 |
2012年 | 251篇 |
2011年 | 228篇 |
2010年 | 145篇 |
2009年 | 117篇 |
2008年 | 134篇 |
2007年 | 147篇 |
2006年 | 123篇 |
2005年 | 116篇 |
2004年 | 99篇 |
2003年 | 92篇 |
2002年 | 76篇 |
2001年 | 63篇 |
2000年 | 64篇 |
1999年 | 55篇 |
1998年 | 30篇 |
1997年 | 21篇 |
1996年 | 20篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 24篇 |
1992年 | 41篇 |
1991年 | 45篇 |
1990年 | 49篇 |
1989年 | 45篇 |
1988年 | 46篇 |
1987年 | 30篇 |
1986年 | 29篇 |
1985年 | 33篇 |
1984年 | 44篇 |
1983年 | 30篇 |
1982年 | 32篇 |
1981年 | 27篇 |
1980年 | 25篇 |
1979年 | 18篇 |
1978年 | 30篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1974年 | 21篇 |
1973年 | 25篇 |
排序方式: 共有3591条查询结果,搜索用时 171 毫秒
1.
Abstract Electrofusion between cells of yeast strains with different genetic markers in isotonic sorbitol solutions leads to high yields of hybrids when 0.1 mM Ca2+ and 0.5 mM Mg2+ salts are aded. On average, 1000–2000 hybrids are obtained when electrofusion is performed (in a helical chamber) compared to a yield of about 40–120 in the absence of these bivalent cations. A further increase in yield can be achieved by the addition of 1 mg/ml albumin, which results in up to 4000 hybrids per experimental run. The entire fusion process leads to very reproducible results in the presence of these substances. 相似文献
2.
3.
4.
Katharina Bork Johannes Karstensen Martin Visbeck Andreas Zimmermann 《Ocean Development & International Law》2013,44(3):298-328
This article analyzes the legal status of unmanned instruments (particularly, floats and gliders) for observation purposes in the ocean environment. These new kinds of instruments are being deployed by the thousands into the oceans, not the least as part of the Argo Project of the International Oceanographic Commission. Their uncontrolled drifting has raised legal questions, especially when such instruments enter waters subject to the jurisdiction of foreign states. The authors argue that the current international legal framework is insufficient to address the pertinent issues, and that a new legal regime is needed. 相似文献
5.
6.
Energetics and intermediates of the assembly of Protein OmpA into the outer membrane of Escherichia coli 总被引:27,自引:0,他引:27
OmpA is a major protein of the outer membrane of Escherichia coli. It is made as a larger precursor, pro-OmpA, which requires a membrane potential for processing. We now show that pro-OmpA accumulates in the cytoplasm of cells treated with carbonyl cyanide m-chlorophenylhydrazone, an uncouple which lowers the membrane potential. Upon restoration of the potential, this pro-OmpA is secreted, processed, and assembled into the outer membrane. Pro-OmpA made in vitro is also recovered with the postribosomal supernatant. It is efficiently processed to OmpA by liposomes which have bacterial leader peptidase that is exclusively internally oriented. These experiments show that: (i) the insertion of pro-OmpA into the plasma membrane is not coupled to its synthesis; (ii) insertion is promoted by the transmembrane electrochemical potential; (iii) pro-OmpA can cross a bilayer spontaneously; and (iv) pro-OmpA is processed by the same leader peptidase which converts M13 procoat to coat. 相似文献
7.
Pampatheres are extinct, large‐bodied cingulates, which share morphological characters with both armadillos and glyptodonts but are considered to be more closely related to the latter. The osteoderm histology of six pampathere taxa was examined and compared to the histology of other cingulate osteoderms. This study investigates the development and functional adaptation of pampathere osteoderms as well as the phylogenetic relationships of the Pampatheriidae within the Cingulata. We found that pampathere osteoderms share a uniform histological organization based on a basic diploe‐like structure. After initial stages of intramembranous growth, metaplastic ossification, that is, the direct incorporation and mineralization of pre‐existing protein fibers, plays an important role in osteoderm development and provides information on various kinds of soft tissue otherwise not preserved. The latest stages of osteoderm growth are dominated by periosteal bone formation especially in the superficial cortex. Movable band osteoderms show regular arrangements of incorporated fibers that may increase the resistance of particularly weak areas against strain. The histological composition of pampathere osteoderms is plesiomorphic in its basic structure but shows a number of derived features. A unique array of Sharpey's fibers that are incorporated into the bone matrix at sutured osteoderm margins is interpreted as a synapomorphy of pampatheres. The arrangement of dermal fibers in the deep and superficial cortexes supports the close relationship between pampatheres and glyptodonts. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc. 相似文献
8.
Eliane S. Meier Thomas C. Edwards Jr Felix Kienast Matthias Dobbertin Niklaus E. Zimmermann 《Journal of Biogeography》2011,38(2):371-382
Aim During recent and future climate change, shifts in large‐scale species ranges are expected due to the hypothesized major role of climatic factors in regulating species distributions. The stress‐gradient hypothesis suggests that biotic interactions may act as major constraints on species distributions under more favourable growing conditions, while climatic constraints may dominate under unfavourable conditions. We tested this hypothesis for one focal tree species having three major competitors using broad‐scale environmental data. We evaluated the variation of species co‐occurrence patterns in climate space and estimated the influence of these patterns on the distribution of the focal species for current and projected future climates. Location Europe. Methods We used ICP Forest Level 1 data as well as climatic, topographic and edaphic variables. First, correlations between the relative abundance of European beech (Fagus sylvatica) and three major competitor species (Picea abies, Pinus sylvestris and Quercus robur) were analysed in environmental space, and then projected to geographic space. Second, a sensitivity analysis was performed using generalized additive models (GAM) to evaluate where and how much the predicted F. sylvatica distribution varied under current and future climates if potential competitor species were included or excluded. We evaluated if these areas coincide with current species co‐occurrence patterns. Results Correlation analyses supported the stress‐gradient hypothesis: towards favourable growing conditions of F. sylvatica, its abundance was strongly linked to the abundance of its competitors, while this link weakened towards unfavourable growing conditions, with stronger correlations in the south and at low elevations than in the north and at high elevations. The sensitivity analysis showed a potential spatial segregation of species with changing climate and a pronounced shift of zones where co‐occurrence patterns may play a major role. Main conclusions Our results demonstrate the importance of species co‐occurrence patterns for calibrating improved species distribution models for use in projections of climate effects. The correlation approach is able to localize European areas where inclusion of biotic predictors is effective. The climate‐induced spatial segregation of the major tree species could have ecological and economic consequences. 相似文献
9.
H. Mussauer V.L. Sukhorukov A. Haase U. Zimmermann 《The Journal of membrane biology》1999,170(2):121-133
The interaction of human red blood cells (RBCs) with diethylenetriamine-pentaacetic acid (DTPA) or its Gd-complex (Magnevist,
a widely used clinical magnetic resonance contrast agent containing free DTPA ligands) led to the following, obviously interrelated
phenomena. (i) Both compounds protected erythrocytes against electrohemolysis in isotonic solutions caused by a high-intensity
DC electric field pulse. (ii) The inhibition of electrohemolysis was observed only when cells were electropulsed in low-conductivity
solutions. (iii) The uptake of Gd-DTPA by electropulsed RBCs was relatively low. (iv) (Gd-) DTPA reduced markedly deformability
of erythrocytes, as revealed by the electrodeformation experiments using high-frequency electric fields. Taken together, the
results indicate that (Gd-) DTPA produce stiffer erythrocytes that are more resistant to electric field exposure. The observed
effects of the chelating agents on the mechanical properties and the electropermeabilization of RBCs must have an origin in
molecular changes of the bilayer or membrane-coupled cytoskeleton, which, in turn, appear to result from an alteration of
the ionic equilibrium (e.g., Ca2+ sequestration) in the vicinity of the cell membrane.
Received: 19 January 1999/Revised: 1 April 1999 相似文献
10.
G. Zimmermann 《BioControl》1978,23(2):181-187
Zusammenfassung Von pilzbefallenen Larven der KohlschabePlutella maculipennis
Curt. wurdeEntomophthora blunckii
Lakon exZimmermann isoliert. Die Art wird mit lateinischer Diagnose gültig beschrieben, die morphologischen Merkmale werden erg?nzend angegeben
und erstmals durch Fotos belegt.E. blunckii w?chst gut auf koaguliertem Eidotter, weniger gut auf Sabouraud-Glucose-Agar. Mycelwachstum wurde zwischen 8° und 28°C beobachtet,
dagegen nicht bei 32°C. Die Symptome infizierter Larven werden geschildert und der Pilz wird mit ?hnlichen Arten verglichen.
Summary Entomophthora blunckii Lakon exZimmermann was isolated from larvae of the diamondback-mothPlutella maculipennis Curt. and redescribed. The fungus is characterized by its elliptical to pear-shaped conidia (from infected larvae 13–20×7–11 μm, mostly 15–18×7–9 μm) with the outer membrane often inflated and its branched conidiophores forming a dense gray to faint greenish-yellowish covering over the body of the insect. The symptoms of larvae infected withE. blunckii are described. The species grows on coagulated egg yolk, less well on Sabouraud-Dextrose-Agar. Growth was observed between 8° and 28°C but not at 32°C. The fungus is compared with similar species.相似文献