全文获取类型
收费全文 | 11392篇 |
免费 | 1007篇 |
国内免费 | 2篇 |
出版年
2024年 | 18篇 |
2023年 | 109篇 |
2022年 | 269篇 |
2021年 | 466篇 |
2020年 | 218篇 |
2019年 | 270篇 |
2018年 | 296篇 |
2017年 | 266篇 |
2016年 | 443篇 |
2015年 | 671篇 |
2014年 | 697篇 |
2013年 | 803篇 |
2012年 | 1072篇 |
2011年 | 1022篇 |
2010年 | 584篇 |
2009年 | 496篇 |
2008年 | 628篇 |
2007年 | 653篇 |
2006年 | 573篇 |
2005年 | 508篇 |
2004年 | 470篇 |
2003年 | 418篇 |
2002年 | 440篇 |
2001年 | 66篇 |
2000年 | 70篇 |
1999年 | 87篇 |
1998年 | 88篇 |
1997年 | 70篇 |
1996年 | 53篇 |
1995年 | 46篇 |
1994年 | 46篇 |
1993年 | 29篇 |
1992年 | 36篇 |
1991年 | 23篇 |
1990年 | 14篇 |
1989年 | 18篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 19篇 |
1984年 | 23篇 |
1983年 | 26篇 |
1982年 | 21篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1976年 | 16篇 |
1975年 | 9篇 |
1974年 | 14篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Intrathecally injected dynorphin A (1-13) in rats results in a reversible hindlimb paralysis and an irreversible loss of the tail-flick reflex. Histologic examination of the spinal cords of dynorphin treated rats demonstrated dead and/or dying neurons predominately localized in the central area which approximates Rexed lamina VII and X. In this area a maximum effect of the dynorphin-induced neurotoxicity is evident. Thus, the dynorphin-induced neuron death is suggestive of an anatomical selectivity. 相似文献
42.
We examine the performance of Hebbian-like attractor neural networks, recalling stored memory patterns from their distorted
versions. Searching for an activation (firing-rate) function that maximizes the performance in sparsely connected low-activity
networks, we show that the optimal activation function is a threshold-sigmoid of the neuron's input field. This function is shown to be in close correspondence with the dependence of the firing rate
of cortical neurons on their integrated input current, as described by neurophysiological recordings and conduction-based
models. It also accounts for the decreasing-density shape of firing rates that has been reported in the literature.
Received:9 December 1994 / Accepted in revised form: 9 January 1996 相似文献
43.
Continuous pollen monitoring of an urban network consisting of three stations has been undertaken for a period of 2 years
in Perugia, central Italy. The aim has been to establish whether the Perugia pollen trap, active since 1983, is still representative
of the area following recent urbanisation. Quantitative differences were found between the stations, reflecting different
vegetational areas, but only slight differences were detected in relation to the timing of the principal period of pollination.
Therefore, although individual pollen traps are necessary to characterize fully the different areas, one trap is sufficient
to determine the key allergenic thresholds in the studied area. 相似文献
44.
Continuous pollen monitoring of an urban network consisting of three stations has been undertaken for a period of 2 years in Perugia, central Italy. The aim has been to establish whether the Perugia pollen trap, active since 1983, is still representative of the area following recent urbanisation. Quantitative differences were found between the stations, reflecting different vegetational areas, but only slight differences were detected in relation to the timing of the principal period of pollination. Therefore, although individual pollen traps are necessary to characterize fully the different areas, one trap is sufficient to determine the key allergenic thresholds in the studied area. 相似文献
45.
46.
Yasmin H. Neggers Mary B. Dubard Robert L. Goldenberg Tsunenobu Tamura Kelley E. Johnston Rachel L. Copper John C. Hauth 《Biological trace element research》1996,55(1-2):127-135
Plasma zinc (Zn) concentrations were measured in 4376 indigent women (86% African-American), at a mean (±SD) gestational age
of 15 (±7.8) wk to determine the relationship between various maternal characteristics and plasma Zn levels during pregnancy.
Mean plasma Zn levels were lower in African-American women than in Caucasian women, in multiparous women than in primiparous
women, and in women with body weight >69.9 kg than in those with body weight ≤69.9 kg (p≤0.001 for each comparison). There were no significant differences related to maternal age, marital status, education, or
smoking habit. Multiple regression analysis, including maternal prepregnancy weight, race, age, parity, smoking habit, education,
and marital status indicated that race, parity, and pregnancy weight were significantly associated with maternal plasma Zn
levels, adjusted for gestational age. Maternal race was the best predictor of plasma Zn concentrations among the population
of pregnant women studied A significant proportion of variance in maternal plasma Zn levels remained unexplained after taking
into account various maternal characteristics. The reasons for lower plasma Zn levels in African-American women, compared
to Caucasian women, during pregnancy are unknown. 相似文献
47.
In comparison to wild type Arabidopsis thaliana, the auxin resistant mutants axr1 and axr2 exhibit reduced inhibition of root elongation in response to auxins. Several auxin-regulated physiological processes are also altered in the mutant plants. When wild-type, axr1 and axr2 seedlings were grown in darkness on media containing indoleacetic acid (IAA), promotion of root growth was observed at low concentrations of IAA (10?11 to 10?7M) in 5-day-old axr2 seedlings, but not in axr1 or wild-type seedlings. In axr1 there was little or no measurable root growth response over the same concentration range. In wild type, root growth was inhibited at concentrations greater than 10?10M and no detectable root growth response was observed at lower concentrations. In addition, production of lateral roots in response to IAA increased in axr2 seedlings and decreased in axr1 seedlings relative to wild type. Promotion of root elongation and initiation of lateral roots in axr2 seedlings in response to auxin indicate that axr2 seedlings are able to perceive and respond to IAA. 相似文献
48.
Kirsty Barr Henrik Moller Emma Christmas Philip Lyver Jacqueline Beggs 《Oecologia》1996,105(2):266-270
An introduced social wasp Vespula vulgaris may compete with native birds for honeydew and invertebrates in New Zealand forests. Experimentally hidden mealworms (Tenebrio molitor) persisted longer at two sites following wasp poisoning that at two sites where wasps were not poisoned. Mealworms persisted longer in the morning than in the afternoon within all study sites. An unusually low mealworm removal rate during a morning trial before wasp poisoning heavily influences the results of this experiment but we have no ecological reason to ignore it. Wasps may therefore be having a heavy impact on invertebrate abundance on very short time scales (within a day following dawn emergence). They may also remove cached food items that would otherwise be retrieved by the South Island robin (Petroica australis australis) during cold or dark feeding conditions. 相似文献
49.
Rachel Y. Reams H. Leon Thacker Daniel D. Harrington Meliton N. Novilla Billie Wilson 《Mycopathologia》1996,135(2):115-118
Aspergillus flavus Link ex Fries and A. parasiticus Speare can invade peanut kernels and under certain environmental conditions produce unacceptable levels of the mycotoxin aflatoxin. A concerted effort is underway to reduce aflatoxin contamination in peanut and peanut products. A potentially effective method of control in peanut is the discovery and use of genes for resistance to either fungal invasion or aflatoxin formation. The objective of the present experimental study was to develop an effective and efficient procedure for screening individual plants or pods of single plants for resistance to invasion by the aflatoxigenic fungi and subsequent aflatoxin production. Methods of obtaining adequate drought-stress and fungal infection were developed through this series of experiments. By completely isolating the pods from the root zone and imposing drought-stress only on pegs and pods, high levels of fungal infection were observed. High amounts of preharvest aflatoxin accumulation were also produced by completely isolating the pods from the root zone. Mid-bloom inoculation with A. parasiticus-contaminated cracked corn and drought-stress periods of 40 to 60 days were the most effective procedures. This technique was used to assess peanut genotypes previously identified as being partially resistant to A. parasiticus infection or aflatoxin contamination, and segregating populations from four crosses. Variability in aflatoxin contamination was found among the 11 genotypes evaluated, however, none were significantly lower than the standard cultivars. Broad-sense heritability of four crosses was estimated through evaluation of seed from individual plants in the F2 generation. The heritability estimates of crosses GFA-2 × NC-V11 and Tifton-8 × NC-V11 were 0.46 and 0.29, respectively, but mean aflatoxin contamination levels were high (73,295 and 27,305 ppb). This greenhouse screening method could be an effective tool when genes for superior aflatoxin resistance are identified.Cooperative investigation of the USDA-ARS and the University of Georgia, College of Agriculture. 相似文献
50.
Measures of geographic range size: the effects of sample size 总被引:2,自引:0,他引:2
A number of methods have been used for quantifying the sizes of the geographic ranges of species. The consequences of different levels of sampling (the proportion of actual spatial occurrences) are explored for eight of these, using data on the occurrences of butterfly species on a 10 × 10 km grid across Britain. For all methods, the percentage error of estimation (PEE) decreases with the number of 10 × 10 km squares which a species occupies, most rapidly for extent measures, and more rapidly for area measures than for measures of numbers of units occupied. The rate of decline in PEE itself falls as sampling effort increases. At a given sampling level, rank correlations between range sizes measured by different methods are generally high, but there is no consistent change in the magnitude of these correlations as the level of sampling increases. The composition of the set of species with the smallest range sizes changes with the level of sampling. 相似文献