全文获取类型
收费全文 | 67076篇 |
免费 | 5651篇 |
国内免费 | 29篇 |
出版年
2021年 | 934篇 |
2020年 | 562篇 |
2019年 | 696篇 |
2018年 | 916篇 |
2017年 | 834篇 |
2016年 | 1441篇 |
2015年 | 2583篇 |
2014年 | 2729篇 |
2013年 | 3629篇 |
2012年 | 4571篇 |
2011年 | 4533篇 |
2010年 | 2945篇 |
2009年 | 2627篇 |
2008年 | 3812篇 |
2007年 | 3855篇 |
2006年 | 3654篇 |
2005年 | 3674篇 |
2004年 | 3568篇 |
2003年 | 3388篇 |
2002年 | 3321篇 |
2001年 | 919篇 |
2000年 | 721篇 |
1999年 | 900篇 |
1998年 | 955篇 |
1997年 | 669篇 |
1996年 | 669篇 |
1995年 | 667篇 |
1994年 | 646篇 |
1993年 | 672篇 |
1992年 | 686篇 |
1991年 | 617篇 |
1990年 | 524篇 |
1989年 | 537篇 |
1988年 | 537篇 |
1987年 | 478篇 |
1986年 | 449篇 |
1985年 | 488篇 |
1984年 | 550篇 |
1983年 | 467篇 |
1982年 | 520篇 |
1981年 | 479篇 |
1980年 | 433篇 |
1979年 | 314篇 |
1978年 | 374篇 |
1977年 | 370篇 |
1976年 | 294篇 |
1975年 | 301篇 |
1974年 | 322篇 |
1973年 | 311篇 |
1971年 | 214篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
81.
Statistical mechanics provides the link between microscopic properties of many-particle systems and macroscopic properties such as pressure and temperature. Observations of similar “microscopic” quantities exist for the motion of zooplankton, as well as many species of other social animals. Herein, we propose to take average squared velocities as the definition of the “ecological temperature” of a population under different conditions on nutrients, light, oxygen and others. We test the usefulness of this definition on observations of the crustacean zooplankton Daphnia pulicaria. In one set of experiments, D. pulicaria is infested with the pathogen Vibrio cholerae, the causative agent of cholera. We find that infested D. pulicaria under light exposure have a significantly greater ecological temperature, which puts them at a greater risk of detection by visual predators. In a second set of experiments, we observe D. pulicaria in cold and warm water, and in darkness and under light exposure. Overall, our ecological temperature is a good discriminator of the crustacean’s swimming behavior. 相似文献
82.
Eric S. J. Harris Shugeng Cao Sean D. Schoville Chengming Dong Wenquan Wang Zaiyou Jian Zhongzhen Zhao David M. Eisenberg Jon Clardy 《PloS one》2012,7(11)
Oridonin is a diterpenoid with anti-cancer activity that occurs in the Chinese medicinal plant Isodon rubescens and some related species. While the bioactivity of oridonin has been well studied, the extent of natural variation in the production of this compound is poorly known. This study characterizes natural variation in oridonin production in order to guide selection of populations of Isodon with highest oridonin yield. Different populations of I. rubescens and related species were collected in China, and their offspring were grown in a greenhouse. Samples were examined for oridonin content, genotyped using 11 microsatellites, and representatives were sequenced for three phylogenetic markers (ITS, rps16, trnL-trnF). Oridonin production was mapped on a molecular phylogeny of the genus Isodon using samples from each population as well as previously published Genbank sequences. Oridonin has been reported in 12 out of 74 species of Isodon examined for diterpenoids, and the phylogeny indicates that oridonin production has arisen at least three times in the genus. Oridonin production was surprisingly consistent between wild-collected parents and greenhouse-grown offspring, despite evidence of gene flow between oridonin-producing and non-producing populations of Isodon. Additionally, microsatellite genetic distance between individuals was significantly correlated with chemical distance in both parents and offspring. Neither heritability nor correlation with genetic distance were significant when the comparison was restricted to only populations of I. rubescens, but this result should be corroborated using additional samples. Based on these results, future screening of Isodon populations for oridonin yield should initially prioritize a broad survey of all species known to produce oridonin, rather than focusing on multiple populations of one species, such as I. rubescens. Of the samples examined here, I. rubescens or I. japonicus from Henan province would provide the best source of oridonin. 相似文献
83.
84.
Hydrogen metabolism of the unicellular cyanobacterium Chroococcidiopsis thermalis ATCC29380 总被引:1,自引:0,他引:1
Abstract Hydrogenase was induced in the unicellular cyanobacterium Chroococcidiopsis thermalis ATCC29380 when grown aerobically in a medium lacking combined nitrogen. Nitrogenase, however, was only observed after incubation of cells in a microaerobic environment. Hydrogen evolution could not be detected under aerobic conditions, but upon transfer of cells to dark anaerobic conditions, large amounts of hydrogen were immediately produced. This hydrogen evolution was sensitive to light and oxygen but not to inhibitors of protein synthesis. The enzyme activity catalyzing the formation of hydrogen was not membrane-bound; some functional properties were characterized in cell-free extracts. 相似文献
85.
Peter Ehrhardt 《Entomologia Experimentalis et Applicata》1960,3(2):114-117
Zusammenfassung Der Sauerstoffverbrauch virusfreier Myzus persicae (Sulz.) wird mit demjenigen blattrollvirusinfizierter Läuse verglichen. Der O2-Verbrauch ist bei infizierten Tieren um rund 30% gegenüber virusfreien Kontrolltieren erniedrigt. Auf verschiedenen. Wirtspflanzen gehaltene Läuse zeigen keinen signifikanten Unterschied in ihrer Atmungsintensität. Die grö\te Erniedrigung des O2-Bedarfs ist etwa 30 Stunden nach Ende des Infektionssaugens (d.h 38 Stunden nach Beginn der Nahrungsaufnahme) erreicht und steigt dann im Verlauf der folgenden 50 Stunden nicht weiter an. Der Stoffwechsel der Läuse wird offensichtlich durch die Aufnahme des Blattrollvirus gestört.
Summary The intensity of respiration of virus-free aphids was compared with that of the same insects infected with potato leafroll virus. The former showed an average oxygen-consumption of 3.018 cm3/gm/hour, and the latter 1.992 cm3/gm/hour; which indicates a reduction of 33.9% in the oxygen-uptake in the infected animals. This reduction is associated with the infection and the multiplication of the virus in the insect. The oxygen-consumption was the same before the beginning and at the end of the infective suction period, falling afterwards over a period of 30 hours by about 30% of the first reading, and then remaining almost constant. This indicates a correlation between this variation in the oxygen-consumption and the multiplication of the virus in the aphid.相似文献
86.
87.
88.
An important problem in reproductive medicine is deciding when people who have failed to become pregnant without medical assistance should begin investigation and treatment. This study describes a computational approach to determining what can be deduced about a couple''s future chances of pregnancy from the number of menstrual cycles over which they have been trying to conceive. The starting point is that a couple''s fertility is inherently uncertain. This uncertainty is modelled as a probability distribution for the chance of conceiving in each menstrual cycle. We have developed a general numerical computational method, which uses Bayes'' theorem to generate a posterior distribution for a couple''s chance of conceiving in each cycle, conditional on the number of previous cycles of attempted conception. When various metrics of a couple''s expected chances of pregnancy were computed as a function of the number of cycles over which they had been trying to conceive, we found good fits to observed data on time to pregnancy for different populations. The commonly-used standard of 12 cycles of non-conception as an indicator of subfertility was found to be reasonably robust, though a larger or smaller number of cycles may be more appropriate depending on the population from which a couple is drawn and the precise subfertility metric which is most relevant, for example the probability of conception in the next cycle or the next 12 cycles. We have also applied our computational method to model the impact of female reproductive ageing. Results indicate that, for women over the age of 35, it may be appropriate to start investigation and treatment more quickly than for younger women. Ignoring reproductive decline during the period of attempted conception added up to two cycles to the computed number of cycles before reaching a metric of subfertility. 相似文献
89.
90.