全文获取类型
收费全文 | 110481篇 |
免费 | 13261篇 |
国内免费 | 412篇 |
出版年
2018年 | 990篇 |
2017年 | 964篇 |
2016年 | 1259篇 |
2015年 | 1552篇 |
2014年 | 1942篇 |
2013年 | 2462篇 |
2012年 | 2859篇 |
2011年 | 2809篇 |
2010年 | 1884篇 |
2009年 | 1845篇 |
2008年 | 2358篇 |
2007年 | 2343篇 |
2006年 | 2332篇 |
2005年 | 2132篇 |
2004年 | 2069篇 |
2003年 | 2104篇 |
2002年 | 2073篇 |
2001年 | 9641篇 |
2000年 | 9509篇 |
1999年 | 7195篇 |
1998年 | 1614篇 |
1997年 | 1792篇 |
1996年 | 1580篇 |
1995年 | 1407篇 |
1994年 | 1308篇 |
1993年 | 1249篇 |
1992年 | 4792篇 |
1991年 | 4522篇 |
1990年 | 3992篇 |
1989年 | 4012篇 |
1988年 | 3608篇 |
1987年 | 3084篇 |
1986年 | 2776篇 |
1985年 | 2678篇 |
1984年 | 1973篇 |
1983年 | 1729篇 |
1982年 | 1230篇 |
1981年 | 983篇 |
1980年 | 913篇 |
1979年 | 1762篇 |
1978年 | 1361篇 |
1977年 | 1199篇 |
1976年 | 1028篇 |
1975年 | 1149篇 |
1974年 | 1166篇 |
1973年 | 1162篇 |
1972年 | 1033篇 |
1971年 | 955篇 |
1970年 | 822篇 |
1969年 | 743篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
211.
Hydration of pollen of Narcissus pseudonarcissus was retardedand germination blocked in media with supra-optimal concentrationsof osmoticum. Activation of the grains, expressed in circulatorymovement in the vegetative cell, was not blocked. Wall developmentwas disrupted, and pectic material and callose were depositedthroughout. In the absence of calcium many grains burst on hydration.The survivors showed evidence of activation, but few tubes wereformed. In medium with supra-optimal Ca2+, activation proceeded,but where tube tips were produced they became occluded withcallose, which eventually formed a general lining to the intine.Nifedipine, a Ca2+-blocker, did not prevent activation at 104M, but reduced callose deposition and inhibited polarized movementin the vegetative cell. Prominences formed at the germinationsites were mostly low and rounded. During recovery in normalmedium, tube tips with normal callose linings were formed. Colchicine,a microtubule inhibitor, had no effect on activation or germination.Cytochalasin D, an actin inhibitor, prevented activation ofthe vegetative cell, but did not arrest all wall deposition.Movement began soon after transfer to normal medium, and somegrains produced adventitious tube tips. While Ca2+ appears notto be essential for activation, these results may be interpretedas indicating links in the normal course of germination betweenthe initial Ca2+ influx at the potential germination sites and:(a) polarization of movement in the vegetative cell, probablyrelated to re-orientation of the actin cytoskeleton; and (b)patterned deposition of callose, which appears to have an importantmorphogenetic role. Narcissus pseudonarcissus, pollen activation, pollen germination, osmotic effects, actin cytoskeleton, nifedipine, cytochalasin D, colchicine, role of Ca2+ flux 相似文献
212.
Cationic amphiphile-mediated delivery of plasmid DNA is the non-viral gene transfer method most often used. In the present work, we considered a new cysteine-detergent, ornithinyl-cysteinyl-tetradecylamide (C(14)-CO), able to convert itself, via oxidative dimerization, into a cationic cystine-lipid. By using fluorescence techniques, we first characterized the structure of complexes of plasmid DNA with C(14)-CO molecules either kept as monomers, or oxidized into dimers. Both forms are able to condense DNA, with the formation of hydrophobic micelle-like domains along the DNA chain. Domains with a larger molecular order were obtained with dimeric C(14)-CO/DNA complexes. In a second step, the interactions of these complexes with lipid vesicles considered as membrane models were investigated. In the presence of vesicles, we observed a decondensation of the DNA involved in complexes obtained with C(14)-CO monomers. With anionic vesicles, the DNA is released into the bulk solution, while with neutral vesicles, it remains bound to the vesicles via electrostatic interactions with inserted C(14)-CO molecules. In sharp contrast, the complexes with C(14)-CO dimers are unaffected by the addition of either neutral or anionic vesicles and show no interaction with them. These results may partly explain the low transfection efficiency of these complexes at the +/-charge ratios used in this study. 相似文献
213.
The CD genome species in the genus Oryza are endemic to Latin America, including O. alta, O. grandiglumis and O. latifolia. Origins and phylogenetic relationship of these species have long been in dispute and are still ambiguous due to their homogeneous genome type, similar morphological characteristics and overlapping distribution. In the present study, we sequenced two chloroplast fragments (matK and trnL-trnF) and portions of three nuclear genes (Adh1, Adh2 and GPA1) from sixteen accessions representing seven species with the C, CD, and E genomes, as well as one G genome species as the outgroup. Phylogenetic analyses using parsimony and distance methods strongly supported that the CD genome originated from a single hybridization event, and that the C genome species (O. officinalis or O. rhizomatis instead of O. eichingeri) served as the maternal parent while the E genome species (O. australiensis) was the paternal donor during the formation of CD genome. In addition, the consistent phylogenetic relationships among the CCDD species indicated that significant divergence existed between O. latifolia and the other two (O. alta and O. grandiglumis), which corroborated the suggestion of treating the latter two as a single species or as taxa within species.We thank Tao Sang of Michigan State University (East Lansing, USA) and Bao-rong Lu of Fudan University (Shanghai, China) for their encouragement and assistance. We are also grateful to the International Rice Research Institute (Manila, Philippines) for providing plant material for this study. This research was supported by the Chinese Academy of Sciences (kscxz-sw-101A), the National Natural Science Foundation of China (30025005) and the Program for Key International S & T Cooperation Project of P. R. China (2001CB711103). 相似文献
214.
The reactions of aliphatic and aromatic amines with reducing sugars are important in both drug stability and synthesis. The
formation of glycosylamines in solution, the first step in the Maillard reaction, does not typically cause browning but results
in decreased potency and is hence significant from the aspect of drug instability. The purpose of this research was to present
(1) unreported ionic equilibria of model reactant (kynurenine), (2) the analytical methods used to characterize and measure
reaction products, (3) the kinetic scheme used to measure reaction rates and (4) relevant properties of various reducing sugars
that impact the reaction rate in solution. The methods used to identify the reversible formation of two products from the
reaction of kynurenine and monosaccharides included LC mass spectrometry, UV spectroscopy, and 1-D and 2-D 1H–1H COSY NMR spectroscopy. Kinetics was studied using a stability-indicating HPLC method. The results indicated the formation
of α and β glycosylamines by a pseudo first-order reversible reaction scheme in the pH range of 1–6. The forward reaction
was a function of initial glucose concentration but not the reverse reaction. It was concluded that the reaction kinetics
and equilibrium concentrations of the glycosylamines were pH-dependent and also a function of the acyclic content of the reacting
glucose isomer. 相似文献
215.
Monoclonal antibodies against chick embryonic beta-galactoside-binding lectin were obtained. One of the monoclonal antibodies was ineffective in Western blotting and seemed to be unable to bind the SDS-denatured lectin. When the native lectin was dotted on a nitrocellulose filter and subjected to denaturation by treatment with SDS, urea or heat, binding of this antibody no longer occurred, though other monoclonal antibodies bound normally. This antibody seems to have been raised against an epitope which is destroyed upon denaturation. 相似文献
216.
217.
218.
Y. Yamada 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1995,91(4):655-658
Conclusions The comparison of different selection indices is justified only if the indices are constrated to achieve the same profit function, even when each index is not optimized with respect to that profit function.When a profit function is known and is non-linear, the desired gains index may be more efficient than the economic index. The optimum desired gains index should be determined by iterative techniques over several generations to compare the genetic progress with the economic index, because gains by the economic index are not linear and the changes observed in the initial generations of selection are not the same rates in future generations, although those changes are linear in the case of the desired gains index. 相似文献
219.
Theory of oviposition strategy of parasitoids. I. Effect of mortality and limited egg number 总被引:1,自引:0,他引:1
Y Iwasa 《Theoretical population biology》1984,26(2):205-227
The optimal oviposition strategies of parasitoids, the host range, and the number of eggs laid per host which result in the maximum lifetime performance of reproduction, are investigated. To study the effects of parasitoid mortality and of limiting total number of eggs laid by a parasitoid, a standard criterion used in previous theories of optimal diet and optimal patch use, the maximization of the foraging rate, is no longer suitable. The model is solved analytically by using dynamic programming. The results are as follows: The host preference of solitary parasitoids depends on the mortality during handling times; i.e., the forager tends to avoid hosts with high risk of foraging mortality. If the total number of eggs produced by a parasitoid is limited, and if the mortality during handling is negligible, the host range is wider when a larger number of eggs remains in the parasitoid's body. In general, however, the mortality-cost of forager and the egg-cost interplay, because the loss of future reproduction by mortality increases with the number of available eggs. In an example with two host types, host range is widest with an intermediate number of eggs available in the body. The optimal number of eggs per host laid by a gregarious parasitoid is also affected by the differential mortality of the forager, and by the number of available eggs. 相似文献
220.
Storage of Hg in the ileum of Blatella germanica: biochemical characterization of metallothionein 总被引:1,自引:0,他引:1
J M Bouquegneau C Ballan-Dufrancais A Y Jeantet 《Comp. Biochem. Physiol. C, Comp. Pharmacol. Toxicol.》1985,80(1):95-98
Cockroach ileum has a high capability to concentrate mercury compared with other tissues. Part of the mercury contained in the soluble phase of this organ is bound to metallothionein. It is suggested that mercury of the insoluble phase is stored in lysosomes under a polymerized metallothionein form. 相似文献