全文获取类型
收费全文 | 2734篇 |
免费 | 136篇 |
国内免费 | 205篇 |
出版年
2023年 | 23篇 |
2022年 | 36篇 |
2021年 | 43篇 |
2020年 | 37篇 |
2019年 | 87篇 |
2018年 | 120篇 |
2017年 | 68篇 |
2016年 | 52篇 |
2015年 | 37篇 |
2014年 | 102篇 |
2013年 | 153篇 |
2012年 | 71篇 |
2011年 | 101篇 |
2010年 | 71篇 |
2009年 | 86篇 |
2008年 | 101篇 |
2007年 | 102篇 |
2006年 | 101篇 |
2005年 | 90篇 |
2004年 | 104篇 |
2003年 | 137篇 |
2002年 | 141篇 |
2001年 | 110篇 |
2000年 | 79篇 |
1999年 | 69篇 |
1998年 | 62篇 |
1997年 | 51篇 |
1996年 | 47篇 |
1995年 | 74篇 |
1994年 | 54篇 |
1993年 | 53篇 |
1992年 | 42篇 |
1991年 | 55篇 |
1990年 | 35篇 |
1989年 | 27篇 |
1988年 | 21篇 |
1987年 | 30篇 |
1986年 | 20篇 |
1985年 | 26篇 |
1984年 | 58篇 |
1983年 | 45篇 |
1982年 | 49篇 |
1981年 | 32篇 |
1980年 | 32篇 |
1979年 | 16篇 |
1978年 | 34篇 |
1977年 | 29篇 |
1976年 | 24篇 |
1975年 | 9篇 |
1973年 | 14篇 |
排序方式: 共有3075条查询结果,搜索用时 31 毫秒
1.
2.
Antonia Eastwood Quentin C. B. Cronk Johannes C. Vogel Andreas Hemp Mary Gibby 《Plant Systematics and Evolution》2004,245(1-2):93-106
The endemic elaphoglossoid ferns, Elaphoglossum dimorphum, E. nervosum and Microstaphyla furcata of St Helena, form a closely related group within section Lepidoglossa when analysed phylogenetically using sequences from the chloroplast trnL intron (partial) and trnL-F intergenic spacer. Microstaphyla furcata, traditionally placed in its own genus, is clearly shown to belong to Elaphoglossum confirming the previous transfer of this species to Elaphoglossum as E. bifurcatum. There is hardly any trnL-F sequence divergence between the species, in fact sequences of E. nervosum and E.
dimorphum are identical. These results are consistent with the possible origin of E. dimorphum as a hybrid between E. bifurcatum and E. nervosum or with the view that the three species are the result of a recent radiation. The potential conflict between phylogenetic and morphological distinctness in determining species conservation priorities is discussed. 相似文献
3.
Michael R. Heaven Anthony W. Herren Daniel L. Flint Natasha L. Pacheco Jiangtao Li Alice Tang Fatima Khan James E. Goldman Brett S. Phinney Michelle L. Olsen 《Molecular & cellular proteomics : MCP》2022,21(1):100180
Alexander disease (AxD) is a rare and fatal neurodegenerative disorder caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). In this report, a mouse model of AxD (GFAPTg;Gfap+/R236H) was analyzed that contains a heterozygous R236H point mutation in murine Gfap as well as a transgene with a GFAP promoter to overexpress human GFAP. Using label-free quantitative proteomic comparisons of brain tissue from GFAPTg;Gfap+/R236H versus wild-type mice confirmed upregulation of the glutathione metabolism pathway and indicated proteins were elevated in the peroxisome proliferator-activated receptor (PPAR) signaling pathway, which had not been reported previously in AxD. Relative protein-level differences were confirmed by a targeted proteomics assay, including proteins related to astrocytes and oligodendrocytes. Of particular interest was the decreased level of the oligodendrocyte protein, 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (Ugt8), since Ugt8-deficient mice exhibit a phenotype similar to GFAPTg;Gfap+/R236H mice (e.g., tremors, ataxia, hind-limb paralysis). In addition, decreased levels of myelin-associated proteins were found in the GFAPTg;Gfap+/R236H mice, consistent with the role of Ugt8 in myelin synthesis. Fabp7 upregulation in GFAPTg;Gfap+/R236H mice was also selected for further investigation due to its uncharacterized association to AxD, critical function in astrocyte proliferation, and functional ability to inhibit the anti-inflammatory PPAR signaling pathway in models of amyotrophic lateral sclerosis (ALS). Within Gfap+ astrocytes, Fabp7 was markedly increased in the hippocampus, a brain region subjected to extensive pathology and chronic reactive gliosis in GFAPTg;Gfap+/R236H mice. Last, to determine whether the findings in GFAPTg;Gfap+/R236H mice are present in the human condition, AxD patient and control samples were analyzed by Western blot, which indicated that Type I AxD patients have a significant fourfold upregulation of FABP7. However, immunohistochemistry analysis showed that UGT8 accumulates in AxD patient subpial brain regions where abundant amounts of Rosenthal fibers are located, which was not observed in the GFAPTg;Gfap+/R236H mice. 相似文献
4.
G. Karrer 《Plant Ecology》1985,59(1-3):199-209
Five plant communities contrasting in successional status and human impact from the southern part of the ‘Wienerwald’ (Austria) are analyzed using vegetation relevés, spectra of area types and a newly proposed disjunction quotient. A climax community (Asperulo-Fagetum), a subclimax community (Querco-Carpinetum s.l.), an anthropogenous substitute community (Mesobromion) and two natural, non-climax permanent communities (Euphorbio saxatilis-Pinetum nigrae and Fumano-Stipetum eriocaulis) are recognized. The disjunction quotient is defined as the number of partial (discontinous) areas divided by the size of the total area of distribution of a species. In particular, the average disjunction quotients of the species in the first two communities reflect relatively table environments only slightly influenced by man, with many ancient, stable taxa. These communities are characterized by species with well-delimited, stable distribution areas. The species in the Mesobromion community have very low average disjunction quotients as its component species are widely and continuously distributed and are often promoted by man. In contrast to these communities, the species linked to the natural permanent, non-climax communities of extreme habitats, have high distribution quotients i.e. small, disconinuous areas; this illustrates the relic character of these plant communities and of the eastern edge of the Alps is a whole. Using the highly variable disjunction quotient of all species and communities examined, the concepts of climax and permanent communities (of different origin) are discussed with regard to European conditions. 相似文献
5.
Summary To explain the decline of Hippopha? scrub in the vegetation succession in the dunes of The Netherlands, the growth and nodulation
of Hippopha? plants grown in pots, using soil from an early stage (site AH) and a post-optimum stage (site HP), were investigated.
In HP-soil nodulation, yield, and the nitrogen and phosphorus content of test plants were always lower and the number of necrotic
nodules and the dry matter content were always higher than in AH-soil, even after inoculation with crushed nodules and the
addition of a nutrient solution. Plants in HP-soil also had darker roots, less root hairs, a higher number of short lateral
roots and a higher percentage of dead roots than those in AH-soil. These characteristics of adverse growth conditions disappeared
upon ignition or gamma-irradiation of HP-soil.
Possible explanations of these results are discussed. The degeneration of Hippopha? scrub cannot be ascribed to the age of
the plants, the absence of sufficient infective endophyte particles or to abiotic factors such as unfavourable physical (particle
size) or chemical soil conditions but is caused by biotic factors. No indications were obtained that plant-pathogenic fungi
and bacteria are involved. HP-soil in contrast to AH-soil, however, contained large numbers of the nematodeLongidorus sp., a species known to cause root deformations. The conclusion was that this nematode is one of the biotic factors involved
in the degeneration of the Hippopha? scrub. This degeneration is due to a restriction of the root system resulting in a low
phosphate uptake, a low nodulation capacity and, as a consequence, a low nitrogen content. The results demonstrate that biotic
soil factors are important in influencing succession in higher plant communities. 相似文献
6.
A R Jones 《Chemico-biological interactions》1973,6(1):47-57
The meso- and (±)-isomers of dimethylmyleran (DMM) have been synthesised and their chemical reactivity towards selected nucleophiles has been investigated. Both isomers react in vitro by hydrolysis to give 2,5-dimethyltetrahydrofuran and with the thiol groups of cysteine, cysteine ethyl ester and glutathione forming 2,5-dimethyltetrahydrothiophene. As each of these cyclic compounds exists in the cis- and trans-configurations, their production in stereochemically quantitative yields from meso- and (±)-dimethylmyleran respectively, indicates that both isomers react by a bimolecular process and not a unimolecular one as has been generally accepted. A comparison of the activity of the isomers with that of Myleran on anti-fertility action, neutrophildepressant activity and effect on the weight response suggests that these esters could exert their biological actions in vivo by the same mechanism. 相似文献
7.
8.
9.
Emanuel Lebenthal Gregory W. Morrissey 《Biochimica et Biophysica Acta (BBA)/General Subjects》1977,497(2):558-566
The subcellular localization of enterokinase is controversial. In this study, enterokinase was extracted from a soluble fraction and a brush border fraction of rat small intestine by differential centrifugation. The soluble fraction contained 41% of the initial enterokinase activity while the brush border fraction contained only 4.6% of the initial activity. In contrast, alkaline phosphatase monitored as a brush border marker, yielded 26.3 in the brush border fraction and only 6% in the soluble fraction. Further separation of the soluble fraction on a Sepharose 4B column revealed three peaks of enterokinase activity. One small peak (3%) of a bound enzyme (Mr, 2·10?6) and two larger peaks of free enzyme (Mr, 3·105 and 9·105). In contrast, alkaline phosphatase major fraction was in a high molecular weight peak of bound enzyme. When the brush border fraction was chromatographed only a single peak of bound enterokinase and alkaline phosphatase were found. In the lower part of the small intestine, no brush border-bound enterokinase was found, while the peak of alkaline phosphatase was the same as in the upper intestine. These data suggest that enterokinase activity in the rat intestine is mainly in a free form localized in the mucin and soluble fraction and to a negligible extent in the brush border. 相似文献
10.
Wilhelm Sauer 《Plant Systematics and Evolution》1987,155(1-4):257-276
Comparative morphological, karyological and chorological studies prove the distinctness of the SE European and Asiatic populations ofPulmonaria mollis s.l., to be treated asP. dacica (Figs. 4, 5).P. mollis s.str. is interpreted as a relatively young taxon, which probably has evolved from an older stock ofP. dacica ancestors during the Pleistocene (Figs. 6, 8) and might have migrated, perhaps with oak woodland communities, from SE to C. Europe. A similar evolution can be postulated for S—SE European dysploidP. rubra-carnica-stiriaca-vallarsae group; this may have involved still unknown tetraploids (P. carnica?, Fig. 7). This group is linked via the hypertriploid speciesP. vallarsae with theP. saccharata group and at least the eastern species of theP. australis group. The monotypic genusParaskevia apparently marks the earliest divergence from the common Tertiary ancestral stock (Fig. 8). It exhibits a tetraploid chromosome number (2n = 28) but has preserved the most primitive characters. — Some comments on the systematics and nomenclature ofP. australis andParaskevia are added. 相似文献