全文获取类型
收费全文 | 4907篇 |
免费 | 462篇 |
国内免费 | 70篇 |
专业分类
5439篇 |
出版年
2024年 | 70篇 |
2023年 | 77篇 |
2022年 | 92篇 |
2021年 | 110篇 |
2020年 | 160篇 |
2019年 | 247篇 |
2018年 | 186篇 |
2017年 | 163篇 |
2016年 | 185篇 |
2015年 | 172篇 |
2014年 | 241篇 |
2013年 | 289篇 |
2012年 | 181篇 |
2011年 | 256篇 |
2010年 | 138篇 |
2009年 | 256篇 |
2008年 | 204篇 |
2007年 | 214篇 |
2006年 | 203篇 |
2005年 | 194篇 |
2004年 | 159篇 |
2003年 | 130篇 |
2002年 | 132篇 |
2001年 | 93篇 |
2000年 | 81篇 |
1999年 | 91篇 |
1998年 | 90篇 |
1997年 | 56篇 |
1996年 | 64篇 |
1995年 | 57篇 |
1994年 | 63篇 |
1993年 | 50篇 |
1992年 | 49篇 |
1991年 | 63篇 |
1990年 | 30篇 |
1989年 | 51篇 |
1988年 | 30篇 |
1987年 | 19篇 |
1986年 | 21篇 |
1985年 | 39篇 |
1984年 | 59篇 |
1983年 | 54篇 |
1982年 | 39篇 |
1981年 | 40篇 |
1980年 | 52篇 |
1979年 | 31篇 |
1978年 | 31篇 |
1977年 | 28篇 |
1976年 | 26篇 |
1975年 | 24篇 |
排序方式: 共有5439条查询结果,搜索用时 15 毫秒
1.
Jonathan M. Adams Wei Fang Ragan M. Callaway Don Cipollini Elizabeth Newell 《Biological invasions》2009,11(4):1005-1016
Acer platanoides (Norway maple) is a widespread native tree species in Europe. It has been introduced to North America where it has often
established dense stands in both secondary woodlands and relatively undisturbed mature woodlands. In Europe A. platanoides is also extending its original range, but generally seems to exist at much lower densities. One explanation for the ‘aggressiveness’
of invasive plants such as A. platanoides is that they have left behind pests and diseases which limit their population densities in their native lands (the enemy
release hypothesis or ERH). To assess the ERH for Norway maple, a large network of collaborators assessed leaf herbivory rates
in populations throughout Europe and North America. We found significantly lower total leaf herbivory (1.6% ± 0.19, n = 21 vs. 7.4% ± 1.94, n = 34) and lower fungal damage (1.0% ± 0.35, n = 13 vs. 3.7% ± 0.85, n = 34) in North America than in Europe over a 2 year period, which is consistent with the predictions of the Enemy Release
Hypothesis. Across years, the average total leaf herbivory was significantly correlated with average annual temperature of
the site (P < 0.05), although this was mostly due to sites in Europe (P < 0.001), and not sites in North America (P > 0.05). Furthermore, only populations in Europe showed very high levels of herbivory (e.g., nine sites had total leaf herbivory
ranging from 10.0 to 51.2% in at least 1 year) or leaf fungal damage (only one site in North America showed high levels of
fungal damage in 1 year), suggesting the possibility of more frequent episodic outbreaks in the native range. Leaf herbivory
and fungal damage are only two aspects of consumer pressure and we do not know whether the differences reported here are enough
to actually elicit release from top-down population control, but such large scale biogeographic differences in herbivory contribute
towards understanding exotic invasions.
Jonathan M. Adams and Wei Fang—equally contributed as first authors.
A list of the participating members of the Transatlantic Acer platanoides Invasion Network is given in the Appendix 3. 相似文献
2.
Mills (1982) formulated the satiationhypothesis in order to explain why ladybirdsare generally more effective in the biologicalcontrol of coccids than of aphids:aphidophagous ladybirds are supposedly lessefficient because they become more rapidlysatiated than coccidophagous ladybirds. If thisis true, the former should spend less timeeating than the latter. This hypothesis istested in this study using the coccidophagousCryptolaemus montrouzieri Mulsant(Coleoptera: Coccinellidae) and theaphidophagous Adalia bipunctata (L.) (Coleoptera: Coccinellidae). Contrary to Mills'(1982) prediction the coccidophagous ladybirddid not feed continuously and even spent lesstime feeding than the aphidophagous ladybird.Furthermore, the gut capacity of C. montrouzieri is smaller than that of A. bipunctata. 相似文献
3.
Percottus glehni is a new colonizer in the ichthyofauna of Lake Glubokoe. Recently, this species, established in the European part of the
USSR nearly 30 years ago, has become common in the lakes and ponds located 3–5 km from Lake Glubokoe. In Lake Glubokoe it
keeps mainly to water thyme (Elodea) in shallow water with individuals up to 7 cm in size being predominant. Apparently, these fishes are not abundant due to
strong predation pressure from larger predators. 相似文献
4.
Catherine M. Matassa Geoffrey C. Trussell 《Proceedings. Biological sciences / The Royal Society》2014,281(1796)
The ecological impacts of predation risk are influenced by how prey allocate foraging effort across periods of safety and danger. Foraging decisions depend on current danger, but also on the larger temporal, spatial or energetic context in which prey manage their risks of predation and starvation. Using a rocky intertidal food chain, we examined the responses of starved and fed prey (Nucella lapillus dogwhelks) to different temporal patterns of risk from predatory crabs (Carcinus maenas). Prey foraging activity declined during periods of danger, but as dangerous periods became longer, prey state altered the magnitude of risk effects on prey foraging and growth, with likely consequences for community structure (trait-mediated indirect effects on basal resources, Mytilus edulis mussels), prey fitness and trophic energy transfer. Because risk is inherently variable over time and space, our results suggest that non-consumptive predator effects may be most pronounced in productive systems where prey can build energy reserves during periods of safety and then burn these reserves as ‘trophic heat’ during extended periods of danger. Understanding the interaction between behavioural (energy gain) and physiological (energy use) responses to risk may illuminate the context dependency of trait-mediated trophic cascades and help explain variation in food chain length. 相似文献
5.
At the end of the exponential growth phase, the enzyme UDP-glueose pyrophosphorylase is present in the vegetative cells of Dictyostelium discoideum NC4 (haploid) at a low level (about 0.05% of total protein). During the initial stages of fruiting body construction, while the cells are entering into multicellular aggregates, the enzyme level remains constant, but increases dramatically thereafter reaching a peak (about 0.5% of total protein) at the end of fruiting body construction, and then partially decreasing. Previous studies have shown that both the accumulation and disappearance are keyed to the flow of morphogenetic events.In this study, cells were labeled with amino acids for different periods throughout the sequence. The enzyme was quantitatively immune-precipitated from crude cell extracts, the precipitate was washed and redissolved, and the enzyme protein separated by acrylamide gel electrophoresis in order to estimate the differential incorporation ratio, i.e. for each labeling period. During the initial stages, when the enzyme level remained relatively constant, this ratio was about 0.03 to 0.04%. As the enzyme began to accumulate it rose progressively, attaining levels of 0.6 to 0.8% toward the end of fruiting body construction before declining. The data are not consistent with the theory of Gustafson and Wright (1973) that differential turnover controls the level of this enzyme during the development of D. discoideum. They are consistent with the conclusion that directed changes in the differential rate of synthesis of UDP-glucose pyrophosphorylase is the controlling element.The estimates of enzyme content are based on a value for the specific enzyme activity of 100,000 units/mg enzyme, which had been determined previously using samples of the enzyme purified to apparent physical homogeneity. This figure has been confirmed in the present study by quantitative immuneprecipitation of the enzyme from crude extracts of homogeneously labeled cells. The method can be generally used to determine if a specific biological activity estimate obtained with a purified protein is consistent with its activity when measured before or during purification. 相似文献
6.
7.
Kater L Houtepen R De Vries R Widdershoven G 《Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences》2003,34(4):669-684
Over the past three or four decades, the concept of medical ethics has changed from a limited set of standards to a broad field of debate and research. We define medical ethics as an arena of moral issues in medicine, rather than a specific discipline. This paper examines how the disciplines of health care ethics and health care law have developed and operated within this arena. Our framework highlights the aspects of jurisdiction (Abbott) and the assignment of responsibilities (Gusfield). This theoretical framework prompted us to study definitions and changing responsibilities in order to describe the development and interaction of health care ethics and health law. We have opted for the context of the Dutch debate about end-of-life decisions as a relevant case study. We argue that the specific Dutch definition of euthanasia as 'intentionally taking the life of another person by a physician, upon that person's request' can be seen as the result of the complex jurisdictional process. This illustrates the more general conclusion that the Dutch debate on end-of-life decisions and the development of the two disciplines must be understood in terms of mutual interaction. 相似文献
8.
Francis P Martinez DM Taghipour F Bowen BD Haynes CA 《Biotechnology and bioengineering》2006,95(6):1207-1217
Controlled shear affinity filtration (CSAF) is a novel integrated processing technology that positions a rotor directly above an affinity membrane chromatography column to permit protein capture and purification directly from cell culture. The conical rotor is intended to provide a uniform and tunable shear stress at the membrane surface that inhibits membrane fouling and cell cake formation by providing a hydrodynamic force away from and a drag force parallel to the membrane surface. Computational fluid dynamics (CFD) simulations are used to show that the rotor in the original CSAF device (Vogel et al., 2002) does not provide uniform shear stress at the membrane surface. This results in the need to operate the system at unnecessarily high rotor speeds to reach a required shear stress of at least 0.17 Pa at every radial position of the membrane surface, compromising the scale-up of the technology. Results from CFD simulations are compared with particle image velocimetry (PIV) experiments and a numerical solution for low Reynolds number conditions to confirm that our CFD model accurately describes the hydrodynamics in the rotor chamber of the CSAF device over a range of rotor velocities, filtrate fluxes, and (both laminar and turbulent) retentate flows. CFD simulations were then carried out in combination with a root-finding method to optimize the shape of the CSAF rotor. The optimized rotor geometry produces a nearly constant shear stress of 0.17 Pa at a rotational velocity of 250 rpm, 60% lower than the original CSAF design. This permits the optimized CSAF device to be scaled up to a maximum rotor diameter 2.5 times larger than is permissible in the original device, thereby providing more than a sixfold increase in volumetric throughput. 相似文献
9.
10.
A two-state hopping model was proposed to study the permeation of ion channel. The Nernst equation in equilibrium and the Michaelis-Menten relation in steady state were derived from the two-state kinetic model. The current-voltage relationship obtained in the symmetrical solutions case was linear when the applied potential was less than 100 mV, which met Ohm’s law. The conductance-concentration relationship exhibited the saturation property. Moreover, the characteristic time reaching the steady state of the KcsA channel was also discussed. Translated from Acta Biophysica Sinica, 2005, 21(4): 289–294 [译自: 生物物理学报] 相似文献