全文获取类型
收费全文 | 5518篇 |
免费 | 467篇 |
国内免费 | 6篇 |
出版年
2023年 | 39篇 |
2022年 | 79篇 |
2021年 | 158篇 |
2020年 | 83篇 |
2019年 | 112篇 |
2018年 | 134篇 |
2017年 | 117篇 |
2016年 | 165篇 |
2015年 | 302篇 |
2014年 | 339篇 |
2013年 | 373篇 |
2012年 | 458篇 |
2011年 | 391篇 |
2010年 | 246篇 |
2009年 | 237篇 |
2008年 | 300篇 |
2007年 | 285篇 |
2006年 | 227篇 |
2005年 | 226篇 |
2004年 | 234篇 |
2003年 | 210篇 |
2002年 | 205篇 |
2001年 | 63篇 |
2000年 | 40篇 |
1999年 | 50篇 |
1998年 | 41篇 |
1997年 | 41篇 |
1996年 | 35篇 |
1995年 | 31篇 |
1994年 | 22篇 |
1993年 | 36篇 |
1992年 | 29篇 |
1991年 | 30篇 |
1990年 | 38篇 |
1989年 | 22篇 |
1988年 | 33篇 |
1987年 | 26篇 |
1986年 | 13篇 |
1984年 | 20篇 |
1983年 | 23篇 |
1982年 | 23篇 |
1981年 | 25篇 |
1980年 | 21篇 |
1979年 | 24篇 |
1978年 | 23篇 |
1977年 | 16篇 |
1976年 | 18篇 |
1975年 | 19篇 |
1974年 | 16篇 |
1973年 | 12篇 |
排序方式: 共有5991条查询结果,搜索用时 15 毫秒
951.
We examined frugivore visitation and seed dispersal of five large-seeded (≥ 5 mm) tree species in tropical montane forest based on their occurrence in frugivorous primate diets: Ekebergia capensis, Olea capensis, Parinari excelsa, Prunus africana , and Syzygium guineense. A total of 21 frugivores in five assemblages ( i.e. , chimpanzees, cercopithecines, large-bodied birds, small-bodied birds, and squirrels) were observed over the study period (August 2006 and October–April 2007). We observed seed dispersal in four of five tree species studied; no dispersal was observed for P. excelsa . Frugivore assemblages did not visit tree species equally. Primates spent the most time in trees and had the largest group size. Large-bodied birds (LB) and chimpanzees dispersed the highest number of seeds per minute. LB and cercopithecines potentially dispersed the greatest number of seeds for E. capensis , and chimpanzees for S. guineense . Our analyses indicated that the mean fruiting duration of the focal tree, time in the tree, and number of species present are important predictor variables for seed dispersal by small- and large-bodied birds, and cercopithecines. The number of fruiting trees in the immediate vicinity of the focal tree further predicted seed dispersal for small-bodied birds (SB). Large-bodied birdseed dispersal also was predicted by time in tree by SB, and the number of individuals for SB and cercopithecines. Cercopithecines (CS) were further explained by the time in tree and number of species (SB & LB), and number of individuals for CS. Our study highlights the complexity of describing the relative importance of a frugivore assemblage to the dispersal of a tree species seeds. 相似文献
952.
Julian Santiago-Moreno Rafael J. Astorga Inmaculada Luque Miguel A. Coloma Adolfo Toledano-Díaz Antonio Pulido-Pastor Felix Gómez-Guillamon Ricardo Salas-Vega Antonio López-Sebastián 《Cryobiology》2009,59(3):357-362
The method of sperm recovery may influence the initial quality of sperm samples and their response to freezing–thawing. The aim of the present work was to compare two methods for collecting epididymal spermatozoa in order to improve the quality of recovered sperm and reduce possible contamination. Testes were obtained from 23 legally hunted, adult ibex males. The sperm mass of the right epididymis was collected by small longitudinal and transverse cuts made in the cauda epididymidis. The sperm mass of the left epididymis was collected by retrograde flushing from the vas deferens to the cauda epididymidis (using a cannula), employing a Tris, citric acid, glucose, egg yolk-based medium. The flushing method recovered more spermatozoa (P < 0.001) than the cutting method. After freezing–thawing, greater acrosomes damage (P < 0.001) and more morphological abnormalities (P < 0.05) were seen among the sperm cells recovered by the cutting method than among those obtained by retrograde flushing. The method of sperm recovery did not, however, influence the microbial contamination rate. In frozen–thawed samples that were microbially contaminated, motility was significantly reduced (P < 0.05) and membrane integrity tended to be poorer (P = 0.06). In conclusion, retrograde flushing is recommended for ibex sperm collection since it would appear that microbial contamination is no more of a problem than that encountered with the cutting method, while a larger number of sperm cells more resistant to freezing–thawing can be obtained. 相似文献
953.
Monostroma latissimum (Kuetzing) Wittrock is a monostromatic green alga of commercial importance in Japan. Here we report the serendipitous discovery of asexually reproducing specimens collected from Usa, on the Pacific coast of Kochi Prefecture, south-western Japan. Zoids were found to be biflagellate and negatively phototactic. Germination of settled zoids was observed to follow erect-filamentous ontogeny similar to that of the previously reported sexual strain. Moreover, the newly discovered asexual strain had identical sequences of nuclear encoded ITS (Internal Transcribed Spacer) region to that of the sexual strain. On the basis of this finding, we postulate that the ITS sequences may have been maintained in these conspecific strains despite the evolution in sexuality. Relationships were investigated among M. latissimum and other monostromatic taxa within the class Ulvophyceae using ITS sequences in order to understand relative phylogenetic position of this species. 相似文献
954.
The attractive and spacing interaction between pairs of filaments via cross-linkers, e.g. myosin oligomers connecting actin
filaments, is modeled by global integral kernels for negative binding energies between two intersecting stiff and long rods
in a (projected) two-dimensional situation, for simplicity. Whereas maxima of the global energy functional represent intersection
angles of ‘minimal contact’ between the filaments, minima are approached for energy values tending to −∞, representing the
two degenerate states of parallel and anti-parallel filament alignment. Standard differential equations of negative gradient
flow for such energy functionals show convergence of solutions to one of these degenerate equilibria in finite time, thus
called ‘super-stable’ states. By considering energy variations under virtual rotation or translation of one filament with
respect to the other, integral kernels for the resulting local forces parallel and orthogonal to the filament are obtained.
For the special modeling situation that these variations only activate ‘spring forces’ in direction of the cross-links, explicit
formulas for total torque and translational forces are given and calculated for typical examples. Again, the two degenerate
alignment states are locally ‘super-stable’ equilibria of the assumed over-damped dynamics, but also other stable states of
orthogonal arrangement and different asymptotic behavior can occur. These phenomena become apparent if stochastic perturbations
of the local force kernels are implemented as additive Gaussian noise induced by the cross-link binding process with appropriate
scaling. Then global filament dynamics is described by a certain type of degenerate stochastic differential equations yielding
asymptotic stationary processes around the alignment states, which have generalized, namely bimodal Gaussian distributions.
Moreover, stochastic simulations reveal characteristic sliding behavior as it is observed for myosin-mediated interaction
between actin filaments. Finally, the forgoing explicit and asymptotic analysis as well as numerical simulations are extended
to the more realistic modeling situation with filaments of finite length.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
955.
A comprehensive evaluation of SNP genotype imputation 总被引:1,自引:0,他引:1
Michael Nothnagel David Ellinghaus Stefan Schreiber Michael Krawczak Andre Franke 《Human genetics》2009,125(2):163-171
956.
The aim of this study was to test the effectiveness of physically acting substances (oils and silicas) and plant preparations for the control of the poultry red mite Dermanyssus gallinae (De Geer 1778). Reproduction and survival of fed D. gallinae females were evaluated in vitro for a total of 168 h using the “area under the survival curve” (AUC) to compare survival of the mites between treatments. Four oils (two plant oils, one petroleum spray oil and diesel), one soap, three silicas (one synthetic amorphous silica, one diatomaceous earth (DE) and one DE with 2% pyrethrum extract) and seven plant preparations (derived from Chrysanthemum cineariaefolium, Allium sativum, Tanacetum vulgare, Yucca schidigera, Quillaja saponaria, Dryopteris filix-mas, and Thuja occidentalis) were tested at various concentrations. All the oils, diesel and soap significantly reduced D. gallinae survival. All silicas tested inhibited reproduction. DE significantly reduced mite survival, but amorphous silica was less effective in vitro. Except for pure A. sativum juice and the highest concentration of C. cineariaefolium extract, the plant preparations tested resulted in statistically insignificant control of D. gallinae. 相似文献
957.
Mateus G. Godoy Melissa L.E. Gutarra Fábio M. Maciel Shayany P. Felix Juliana V. Bevilaqua Olga L.T. Machado Denise M.G. Freire 《Enzyme and microbial technology》2009,44(5):317-322
The castor bean (Ricinus communis) represents a potential candidate for biodiesel production. The Petrobras Research Center is developing a biodiesel production process from castor bean seeds, in which an unwanted byproduct named castor bean waste is produced. This extremely alkaline waste is toxic and allergenic and, as such, poses a significant environmental problem. Solid-state fermentation (SSF) of castor bean waste was carried out to achieve ricin detoxification, reduce allergenic potential and stimulate lipase production. The fungus, Penicillium simplicissimum, an excellent lipase producer, was able to grow and produce lipase enzyme. After an optimization process, the maximum lipase activity achieved was 44.8 U/g. Moreover, the fungus P. simplicissimum was able to reduce the ricin content to non-detectable levels in addition to diminishing castor bean waste allergenic potential by approximately 16%. In this way, SSF of castor bean waste by P. simplicissimum may increase the utility of the waste by promoting enzyme production and eliminating the principal toxic element, ricin. 相似文献
958.
959.
Quantitative identification of teratoma tissues formed by human embryonic stem cells with TeratomEye
Steve Kah Weng Oh Pauline Chua Koh Lin Foon Emily Ng Angela Chin Andre Boon Hwa Choo Rajagopalan Srinivasan 《Biotechnology letters》2009,31(5):653-658
An automated vision system, TeratomEye, was developed for the identification of three representative tissue types: muscle,
gut and neural epithelia which are commonly found in teratomas formed from human embryonic stem cells. Muscle tissue, a common
structure was identified with an accuracy of 90.3% with high specificity and sensitivity greater than 90%. Gut epithelia were
identified with an accuracy of 87.5% with specificity and sensitivity greater than 80%. Neural epithelia which were the most
difficult structures to distinguish gave an accuracy of 47.6%. TeratomEye is therefore useful for the automated identification
of differentiated tissues in teratoma sections. 相似文献
960.
Arturo Andrade Alejandro Sandoval Ricardo Gonzlez-Ramírez Diane Lipscombe Kevin P. Campbell Ricardo Felix 《Cell calcium》2009,46(4):282-292
The auxiliary CaVα2δ-1 subunit is an important component of voltage-gated Ca2+ (CaV) channel complexes in many tissues and of great interest as a drug target. Nevertheless, its exact role in specific cell functions is still unknown. This is particularly important in the case of the neuronal L-type CaV channels where these proteins play a key role in the secretion of neurotransmitters and hormones, gene expression, and the activation of other ion channels. Therefore, using a combined approach of patch-clamp recordings and molecular biology, we studied the role of the CaVα2δ-1 subunit on the functional expression and the pharmacology of recombinant L-type CaV1.3 channels in HEK-293 cells. Co-expression of CaVα2δ-1 significantly increased macroscopic currents and conferred the CaV1.3α1/CaVβ3 channels sensitivity to the antiepileptic/analgesic drugs gabapentin and AdGABA. In contrast, CaVα2δ-1 subunits harboring point mutations in N-glycosylation consensus sequences or the proteolytic site as well as in conserved cysteines in the transmembrane δ domain of the protein, reduced functionality in terms of enhancement of CaV1.3α1/CaVβ3 currents. In addition, co-expression of the δ domain drastically inhibited macroscopic currents through recombinant CaV1.3 channels possibly by affecting channel synthesis. Together these results provide several lines of evidence that the CaVα2δ-1 auxiliary subunit may interact with CaV1.3 channels and regulate their functional expression. 相似文献