全文获取类型
收费全文 | 8291篇 |
免费 | 778篇 |
国内免费 | 2篇 |
专业分类
9071篇 |
出版年
2022年 | 74篇 |
2021年 | 122篇 |
2020年 | 85篇 |
2019年 | 99篇 |
2018年 | 119篇 |
2017年 | 113篇 |
2016年 | 174篇 |
2015年 | 258篇 |
2014年 | 317篇 |
2013年 | 433篇 |
2012年 | 413篇 |
2011年 | 475篇 |
2010年 | 341篇 |
2009年 | 303篇 |
2008年 | 425篇 |
2007年 | 399篇 |
2006年 | 349篇 |
2005年 | 345篇 |
2004年 | 335篇 |
2003年 | 310篇 |
2002年 | 327篇 |
2001年 | 158篇 |
2000年 | 198篇 |
1999年 | 146篇 |
1998年 | 107篇 |
1997年 | 90篇 |
1996年 | 77篇 |
1995年 | 80篇 |
1994年 | 76篇 |
1992年 | 121篇 |
1991年 | 97篇 |
1990年 | 115篇 |
1989年 | 89篇 |
1988年 | 126篇 |
1987年 | 90篇 |
1986年 | 80篇 |
1985年 | 110篇 |
1984年 | 79篇 |
1983年 | 81篇 |
1982年 | 94篇 |
1981年 | 58篇 |
1980年 | 60篇 |
1979年 | 67篇 |
1978年 | 68篇 |
1977年 | 75篇 |
1976年 | 71篇 |
1975年 | 67篇 |
1974年 | 71篇 |
1973年 | 55篇 |
1972年 | 62篇 |
排序方式: 共有9071条查询结果,搜索用时 15 毫秒
991.
The leafminer Liriomyza trifolii (Burgess) (Diptera: Agromyzidae) is a worldwide pest of ornamental and vegetable crops. The most promising nonchemical approach for controlling Liriomyza leafminers in greenhouses is regular releases of the parasitoid Diglyphus isaea (Walker) (Hymenoptera: Eulophidae). In the current study, we examine the hypothesis that the use of D. isaea for biological control of leafminers in greenhouse crops may be more practical and efficient when supplemented with additional control strategies, such as the sterile insect technique (SIT). In small cages, our SIT experiments suggest that release of sterile L. trifolii males in three sterile-to-fertile male ratios (3:1, 5:1, and 10:1) can significantly reduce the numbers of the pest offspring. In large cage experiments, when both parasitoids and sterile males were released weekly, the combined methods significantly reduced mine production and the adult leafminer population size. Moreover, a synergistic interaction effect between these two methods was found, and a model based on our observed data predicts that because of this effect, only the use of both methods can eradicate the pest population. Our study indicates that an integrated pest management approach that combines the augmentative release of the parasitoid D. isaea together with sterile leafminer males is more efficient than the use of either method alone. In addition, our results validate previous theoretical models and demonstrate synergistic control with releases of parasitoids and sterile insects. 相似文献
992.
von Scheven G Alvares LE Mootoosamy RC Dietrich S 《Development (Cambridge, England)》2006,133(14):2731-2745
Recent knockout experiments in the mouse generated amazing craniofacial skeletal muscle phenotypes. Yet none of the genes could be placed into a molecular network, because the programme to control the development of muscles in the head is not known. Here we show that antagonistic signals from the neural tube and the branchial arches specify extraocular versus branchiomeric muscles. Moreover, we identified Fgf8 as the branchial arch derived signal. However, this molecule has an additional function in supporting the proliferative state of myoblasts, suppressing their differentiation, while a further branchial arch derived signal, namely Bmp7, is an overall negative regulator of head myogenesis. 相似文献
993.
Roy Watling 《Mycoscience》2006,47(1):18-24
Those fungi that have been linked to the gasteroid genus Scleroderma by molecular techniques are discussed in relation to secondary metabolites, development, spore morphology, and anatomy. The
group contains epigeous and hypogeous components and sequestrate and boletoid members. An intratribal classification is proposed
to accommodate these life forms. 相似文献
994.
Regulation of bacterial virulence by two-component systems 总被引:1,自引:0,他引:1
In bacteria, two-component systems (TCS) are widely used signal transduction devices which are engaged in a multitude of gene regulatory systems that respond to changing growth conditions. Many pathogenic bacteria encounter different microenvironments during their infectious cycle and their ability to efficiently adapt to different niches inside and outside of their host organisms is frequently mediated by TCSs, which can, therefore, be considered as an essential prerequisite for their pathogenicity. Although significant progress has been made in the elucidation of basic principles of the signal transduction process itself, in many pathogens the contribution of TCS to bacterial virulence is insufficiently recognized. 相似文献
995.
Since its invasion of North America in 2000, the soybean aphid, Aphis glycines Matsumura (Homoptera: Aphididae) has notably changed the arthropod community of the soybean agroecosystem. The objectives
of this study were to characterize the foliar predatory fauna associated with A. glycines in commercial soybean fields in Québec, Canada, and to evaluate the suitability of A. glycines as prey for three coccinellid species: Harmonia axyridis Pallas, Propylea quatuordecimpunctata L. and Coleomegilla maculata lengi Timberlake (Coleoptera: Coccinellidae). Field surveys showed that several predators responded rapidly to expanding and increasing
populations of the soybean aphid. Coccinellidae were the most important aphidophagous predators observed in 2002 (58.6%) and
2003 (44.8%), with mainly four native and naturalized species co-occurring with the soybean aphid throughout the growing season.
Measurement of fitness parameters under laboratory conditions (survival, development time, longevity, fecundity) indicated
that A. glycines is an excellent prey for the development and reproduction of all three of the coccinellid species studied. The intrinsic
rate of natural increase (rm) was highest for H. axyridis (0.238 d−1), intermediate for P. quatuordecimpunctata (0.215 d−1) and lowest for C. maculata (0.134 d−1). 相似文献
996.
The Influence of Previous Mountain Pine Beetle (Dendroctonus ponderosae) Activity on the 1988 Yellowstone Fires 总被引:1,自引:0,他引:1
We examined the historical record of mountain pine beetle (Dendroctonus ponderosae Hopkins) activity within Yellowstone National Park, Wyoming, for the 25-years period leading up to the 1988 Yellowstone fires
(1963–86) to determine how prior beetle activity and the resulting tree mortality affected the spatial pattern of the 1988
Yellowstone fires. To obtain accurate estimates of our model parameters, we used a Markov chain Monte Carlo method to account
for the high degree of spatial autocorrelation inherent to forest fires. Our final model included three statistically significant
variables: drought, aspect, and sustained mountain pine beetle activity in the period 1972–75. Of the two major mountain pine
beetle outbreaks that preceded the 1988 fires, the earlier outbreak (1972–75) was significantly correlated with the burn pattern,
whereas the more recent one (1980–83) was not. Although regional drought and high winds were responsible for the large scale
of this event, the analysis indicates that mountain pine beetle activity in the mid-1970s increased the odds of burning in
1988 by 11% over unaffected areas. Although relatively small in magnitude, this effect, combined with the effects of aspect
and spatial variation in drought, had a dramatic impact on the spatial pattern of burned and unburned areas in 1988. 相似文献
997.
998.
Membrane protein complexes can support both the generation and utilisation of a transmembrane electrochemical proton potential ('proton-motive force'), either by transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane or by transmembrane proton transfer. Here we provide the first evidence that both of these mechanisms are combined in the case of a specific respiratory membrane protein complex, the dihaem-containing quinol:fumarate reductase (QFR) of Wolinella succinogenes, so as to facilitate transmembrane electron transfer by transmembrane proton transfer. We also demonstrate the non-functionality of this novel transmembrane proton transfer pathway ('E-pathway') in a variant QFR where a key glutamate residue has been replaced. The 'E-pathway', discussed on the basis of the 1.78-Angstrom-resolution crystal structure of QFR, can be concluded to be essential also for the viability of pathogenic varepsilon-proteobacteria such as Helicobacter pylori and is possibly relevant to proton transfer in other dihaem-containing membrane proteins, performing very different physiological functions. 相似文献
999.
Neess D Kiilerich P Sandberg MB Helledie T Nielsen R Mandrup S 《Molecular and cellular biochemistry》2006,289(1-2):149-157
The differential alterations of the spliceosomal UsnRNAs (U1, U2, U4, U5, and U6) were reported to be associated with cellular proliferation and development. The attempt was made in this study to analyze the metabolic pattern of the spliceosomal UsnRNAs during the development of pre-malignant lung lesions induced in experimental mice model system by benzo(a)pyrene (BP) and also to see how tea polyphenols, epigallocatechin gallate (EGCG) and epicatechin gallate (ECG), modulate the metabolism of these UsnRNAs during the lung carcinogenesis. No significant changes in the level of the UsnRNAs were seen in the inflammatory lung lesions at 9th week due to treatment of BP. However, there was significant increase in the level of U1 (∼2.5 fold) and U5 (∼47%) in the hyperplastic lung lesions at 17th week. But in the mild dysplastic lung lesions at 26th week, the level of UsnRNAs did not change significantly. Whereas, in the dysplastic lung lesions at 36th week there was significant increase in the level of the U2 (∼2 fold), U4 (∼2.5 fold) and U5 (∼2 fold). Due to the EGCG and ECG treatment the lung lesions at 9th week appeared normal and in the 17th, 26th, and 36th week it appeared as hyperplasia. The level of the UsnRNAs was significantly low in the lung lesions at 9th week (only U2 and U4 by EGCG), at 17th week (only U1 by EGCG/ECG), at 26th week (U1 by ECG; U2, U4 and U5 by EGCG/ECG) and at 36th week (U1 by ECG, U2 and U4 by EGCG/ECG). Whereas, there was significant increase in the level of U5 (by EGCG/ECG) and U6 (by EGCG only) in the lung lesions at 36th and 26th week respectively. This indicates that the metabolism of the spliceosomal UsnRNAs differentially altered during the development of pre-malignant lung lesions by BP as well as during the modulation of the lung lesions by the tea polyphenols. 相似文献
1000.
The folding behavior of cytochrome C (Cyt-C) conjugated with CdS nanorods (CdSnr) is amenable to monitoring by bright field microscopy, the porosity and percolating behavior of such protein conjugated nanoclusters depending on the folding history prior to the conjugation. The method has been used to predict the thermal melting behavior as well as guanidine hydrochloride induced unfolding of Cyt-C. Dynamic light scattering studies indicate that the size distribution of the nanoforms widens in presence of the protein. Furthermore, there is emergence of clusters with higher conductivity and altered zeta potential. Increase of second virial coefficient of CdS nanoforms in the presence of Cyt-C (obtained from static light scattering experiments) implies presence of protein coat over the hydrophobic nanosurface. The results are supported by morphological changes observed through scanning electron microscopy (SEM). Accordingly, the X-ray diffraction pattern shows a change of crystallographic orientations of CdSnr in presence of Cyt-C. 相似文献