全文获取类型
收费全文 | 17014篇 |
免费 | 1846篇 |
国内免费 | 1928篇 |
出版年
2024年 | 45篇 |
2023年 | 261篇 |
2022年 | 323篇 |
2021年 | 427篇 |
2020年 | 672篇 |
2019年 | 823篇 |
2018年 | 706篇 |
2017年 | 689篇 |
2016年 | 672篇 |
2015年 | 765篇 |
2014年 | 911篇 |
2013年 | 1297篇 |
2012年 | 761篇 |
2011年 | 811篇 |
2010年 | 727篇 |
2009年 | 795篇 |
2008年 | 830篇 |
2007年 | 856篇 |
2006年 | 770篇 |
2005年 | 708篇 |
2004年 | 577篇 |
2003年 | 517篇 |
2002年 | 484篇 |
2001年 | 423篇 |
2000年 | 402篇 |
1999年 | 363篇 |
1998年 | 292篇 |
1997年 | 299篇 |
1996年 | 270篇 |
1995年 | 295篇 |
1994年 | 262篇 |
1993年 | 277篇 |
1992年 | 224篇 |
1991年 | 200篇 |
1990年 | 151篇 |
1989年 | 179篇 |
1988年 | 158篇 |
1987年 | 128篇 |
1986年 | 141篇 |
1985年 | 185篇 |
1984年 | 235篇 |
1983年 | 157篇 |
1982年 | 145篇 |
1981年 | 132篇 |
1980年 | 127篇 |
1979年 | 97篇 |
1978年 | 72篇 |
1977年 | 45篇 |
1976年 | 39篇 |
1975年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
T. Phanitchat C. Apiwathnasorn S. Sungvornyothin Y. Samung S. Dujardin J.‐P. Dujardin S. Sumruayphol 《Medical and veterinary entomology》2019,33(4):476-484
Wing geometry helps to identify mosquito species, even cryptic ones. On the other hand, temperature has a well‐known effect on insect metric properties. Can such effects blur the taxonomic signal embedded in the wing? Two strains of Aedes albopictus (laboratory and field strain) were examined under three different rearing temperatures (26, 30 and 33 °C) using landmark‐ and outline‐based morphometric approaches. The wings of each experimental line were compared with Aedes aegypti. Both approaches indicated similar associations between wing size and temperature. For the laboratory strain, the wing size significantly decreased as the temperature increased. For the field strain, the largest wings were observed at the intermediate temperature. The two morphometric approaches describing shape showed different sensibilities to temperature. For both strains and sexes, the landmark‐based approach disclosed significant wing shape changes with temperature changes. The outline‐based approach showed lesser effects, detecting significant changes only in laboratory females and in field males. Despite the size and shape changes induced by temperature, the two strains of Ae. albopictus were always distinguished from Ae. aegypti. The present study confirms the lability of size. However, it also suggests that, despite environmentally‐induced variation, the architecture of the wing still provides a strong taxonomic signal. 相似文献
22.
23.
Donald S. McLusky 《Aquatic Ecology》1993,27(2-4):489-493
The Proceedings of the ECSA 21 symposium on Marine and Estuarine Gradients are reviewed. It is emphasised that this is probably
the first time that a full set of papers on the tidal freshwater sections of estuaries has appeared. There is however some
ambiguity in the terminology applied to such waters, and a more consistent terminology is proposed. In particular the estuary
is defined as reaching upstream as far as the tidal limit, irrespective of salinity.
A wide variety of gradients within estuaries and coastal waters are considered, but it is apparent that the crucial spatial
gradients are based on salinity, oxygen and turbidity, and that many other gradients are co-variables with these parameters.
Temperature is also important for temporal gradients. 相似文献
24.
Growth of 2659 Atlantic cod Gadus morhua aged 4 to 9 years examined in Placentia Bay, Newfoundland, peaked in most cases in June and was at a minimum in October or November. Water temperature, partial fullness index ( I P ) and gonado‐somatic index ( I G ) explained between 31 and 52% of the monthly variability in growth. Temperature and I P of capelin Mallotus villosus had significant effects on growth of all age groups and explained most of the variance for ages 6–8 and 4–5 years, respectively. The I P of large invertebrates (ages 4 to 7 years), sandlance ( Ammodytes sp. age 6 years) and demersal fishes (age 9 years) had age‐specific effects in the model. Overall, amphipods, decapods and echinoderms dominated the Atlantic cod diet in most seasons, but fish consumption by Atlantic cod was high in June and July, particularly on capelin. The rapid increase in somatic mass during June and July occurred despite cold water temperatures ( < 3° C at 50 m) and moderate to high gonado‐somatic index. The findings of this study suggest that when food was not a limiting factor, growth tended to increase even when Atlantic cod occupied colder waters, but when food was limiting, the opposite may have occured. 相似文献
25.
Chloroflexus aurantiacus can be induced to shift from respiratory to photosynthetic energy production by introducing light and/or lowering the oxygen concentration of a culture. After induction, cells synthesize bacteriochlorophyll and proteins for the formation of a functional photosynthetic apparatus. Bacteriochlorophyll is detectable within 2 h after induction. Chlorosome polypeptides are detected after 8–12 h. Two proteins, Mr 60,000 and Mr 47,000, are present in both induced and noninduced cells and react specifically with antibodies against chlorosome polypeptides. Immunological data suggest that these proteins (Mr 60,000 and 47,000) are polyproteins which are transcribed and translated in the dark. When cells are exposed to light or low oxygen tension these proteins are processed into functional polypeptides required in the assembly of the chlorosome. The reaction center polypeptide (Mr 26,000) appears to be part of a separate genetic control system.Dedicated to Prof. G. Drews on occasion of his 60th birthday 相似文献
26.
The effect of caffeine,different fixation regimes and low temperature on microtubules in the cells of higher plants 总被引:2,自引:0,他引:2
Caffeine, (1:3:7-tri-methyl-xanthine), either as a prefixation treatment or included with glutaralde-hyde as the primary fixative, destroys or disorganises the microtubules associated with the formation of secondary walls in fibres from the flowering stem of the grass Lolium temulentum L. There is no observable effect of caffeine treatment on the microtubules associated with primary wall formation in collenchyma and young fibres from L. temulentum or in root cap cells of Zea mays L. and Phaseolus vulgaris L. The microtubules associated with primary wall formation are destroyed by cold treatment but not those associated with secondary wall formation. Tannic acid included in the fixative shows the microtubules associated with secondary wall formation in fibres of L. temulentum to be composed of 13 subunits. Treatment with lanthanum hydroxide does not stain the core or the halo of the microtubules.Abbreviation PIPES
Piperazine N-N- bis 2 ethanol sulphonic acid
The Grassland Research Institute is financed through the Agricultural Research Council 相似文献
27.
William P. Sullivan David F. Smith Thomas G. Beito Christopher J. Krco David O. Toft 《Journal of cellular biochemistry》1988,36(2):103-119
Avian progesterone receptor exists as two forms, A and B, with molecular weights of 79,000 and 110,000 daltons, respectively. The origin and significance of these two forms is an area of active investigation and debate. Monoclonal antibodies produced against these two forms were used to examine receptor stability in cytosol and changes in the receptor forms induced by hormone binding. The lability of hormone binding at elevated temperatures is well documented. Analysis by Western blotting showed the receptor was stable in freshly prepared oviduct cytosol for 2 hr at 37°C, while hormone binding was lost within 30 min. However, loss of receptor through degradation was seen when cytosol was prepared from frozen tissue or when homogenization was excessive. Progesterone was injected into diethylstilbestrol-stimulated chicks to examine, in vivo, effects of hormone treatment on receptor forms in the cytosol and nuclear fractions. Progesterone treatment caused a time- and dose-dependent conversion of the A receptor to a form (A′) with a slower electrophoretic mobility. The cytosolic progesterone receptor was divided equally between the B and A forms, while the nuclear receptor was predominantly A′. The amount of nuclear receptor was consistently less than cytosolic receptor. Receptor phosphorylation was analyzed by incubating tissue minces with [32P]orthophosphate with or without progesterone followed by immune isolation of receptor forms. Progesterone treatment caused a time-dependent increase in cytosol receptor phosphorylation which was evident after 5 min of treatment. This phosphorylation was observed with both the A and B receptor forms. The results indicate that receptor phosphorylation is a very early event during progesterone action. 相似文献
28.
Lucilia sericata has a facultative diapause in the third larval instar after cessation of feeding. Induction of the diapause is influenced by the photoperiod and temperature conditions experienced by insects in the parental generation as well as those experienced by the larvae themselves. The sensitive stage of the parental generation for induction of diapause was examined using diapause‐averting conditions of 16 h light : 8 h darkness (LD 16:8) at 25°C and diapause‐inducing conditions of LD 12:12 at 20°C. The incidence of diapause in the progeny was predominantly determined by the conditions experienced by the parents in the adult stage. Moreover, the results of reciprocal crosses showed that only the mother's experience is involved in the induction of diapause in the progeny. 相似文献
29.
Monte Carlo simulations and a modified Poisson–Boltzmann (MPB) theory are used to investigate the temperature dependence of the capacitance (around the potential of zero charge) of an electric double layer in the presence of surface polarization due to a dielectric boundary. Within the context of the restricted primitive model planar double layer, whose solvent dielectric constant is ε2, the cases when the electrode is an insulator (ε1 = 1), when the electrode and the electrolyte have the same permittivity (ε1 = ε2, no polarization), and when the electrode is a conductor (ε1 → ∞) are studied for the case where the electrolyte concentration is 0.1 M. The simulations reveal a capacitance anomaly, that is, a positive temperature dependence of the capacitance at low temperatures for the former two situations. The MPB theory also shows this effect for these two situations and is in qualitative or better agreement with the simulation data. In these two cases, both the simulations and theory show a dramatic increase of the diffuse layer potential in the temperature regime where capacitance anomaly occurs. However, in the latter situation, where the electrode is metallic, the capacitance always has a negative temperature derivative for the MPB theory and probably also for the simulation data. 相似文献
30.
A simple,band‐mounted device to measure behaviorally modified thermal microclimates experienced by birds
下载免费PDF全文
![点击此处可从《Journal of Field Ornithology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Birds encounter climate at the scale of microclimates that can vary rapidly in time and space and so understanding potential vulnerability and adaptations to those microclimates requires fine‐scale measurements that accurately track thermal exposures. However, few options exist for recording the microclimates actually experienced by birds (realized microclimates). We constructed and tested a simple, low‐cost, temperature logger for recording the realized microclimates of ground‐nesting birds. We developed attachment protocols for band‐mounting Thermochron iButtons on Ring‐billed Gull (Larus delawarensis) chicks. We tested these mounted, temperature‐logging devices on 20 chicks weighing > 200 g (device weight was 4 g), attaching devices for 48 h and observing behavior before and after attachment and removal. Devices recorded temperature immediately surrounding the leg at 2‐min intervals. Recorded temperatures were strong predictors of observed thermoregulatory behaviors (panting and sitting), outperforming predictions based on air temperatures measured by basic, static approaches. Through comparison with matched controls (chicks with just a band), we detected no adverse physiological effects of devices, no effects on social or feeding behavior, and only a short‐term decrease in inactivity immediately after device attachment (likely due to increased preening). By attaching iButtons to the legs of birds, we quantified realized thermal exposure, integrating air temperature, modes of environmental heat transfer, and bird behavior at microclimatic scales. Although not yet validated for broader use, our approach (including possible miniaturization) should be suitable to measure thermal exposure of adults, not just chicks, allowing collection of data concerning thermal exposures during flight under field conditions. At ~ $25 USD per device, our approach facilitates experimental protocols with robust sample sizes, even for relatively modest budgets. 相似文献