全文获取类型
收费全文 | 3149篇 |
免费 | 222篇 |
国内免费 | 1篇 |
专业分类
3372篇 |
出版年
2023年 | 12篇 |
2022年 | 30篇 |
2021年 | 39篇 |
2020年 | 41篇 |
2019年 | 32篇 |
2018年 | 39篇 |
2017年 | 35篇 |
2016年 | 83篇 |
2015年 | 131篇 |
2014年 | 145篇 |
2013年 | 204篇 |
2012年 | 252篇 |
2011年 | 232篇 |
2010年 | 132篇 |
2009年 | 116篇 |
2008年 | 187篇 |
2007年 | 187篇 |
2006年 | 178篇 |
2005年 | 177篇 |
2004年 | 165篇 |
2003年 | 151篇 |
2002年 | 150篇 |
2001年 | 31篇 |
2000年 | 22篇 |
1999年 | 26篇 |
1998年 | 32篇 |
1997年 | 36篇 |
1996年 | 18篇 |
1995年 | 21篇 |
1994年 | 28篇 |
1993年 | 27篇 |
1992年 | 20篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 21篇 |
1988年 | 22篇 |
1987年 | 16篇 |
1986年 | 18篇 |
1985年 | 15篇 |
1984年 | 22篇 |
1983年 | 17篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 22篇 |
1979年 | 15篇 |
1978年 | 12篇 |
1977年 | 15篇 |
1976年 | 20篇 |
1975年 | 15篇 |
1974年 | 21篇 |
排序方式: 共有3372条查询结果,搜索用时 15 毫秒
11.
Among the different subtypes of Ehlers-Danlos syndrome (EDS), the dominant types I–III have, so far, been uninformative biochemically and molecular genetically, and diagnostic problems with subgroup boundaries often arise. We have investigated the ultrastructural pattern of connective tissue macromolecules in skin biopsy specimens of some 85 patients aged 4 months-54 years who exhibit clinical symptoms or the suspicion of EDS I–IV. Based on the differential features of collagen fibrils and ground substance material, four distinct groups could be established. Group I (clinically EDS type I) showed disorganized collagen bundles and dense aggregations of collagen fibrils with bizarre shapes. Group II (clinically varying from EDS types I–III) revealed collagen bundles that regularly contained numerous “composite collagen fibrils” with enlarged “flower-like” cross-sections and rope-like longitudinal sections, often associated with increased amounts of matrix substances in the form of electron-dense irregular strands and filaments in a branched network. Group III (clinically EDS types II–III) presented smaller isolated collagen flowers and ropes associated with excessive filamentous ground substance material and flocculent material. Group IV (with clinical symptoms of EDS type IV) had a dermis thinned to one third of the normal and a reduced number of collagen bundles with small diameter fibrils. In 13 patients, the abnormal ultrastructural dermal architecture did not coincide with any of these four groups or with the pattern of any other inherited connective tissue disorder. In 16 additional patients with mostly mild clinical symptoms, such as muscle weakness and small joint hyperlaxity, no ultrastructural aberrations could be found. Even though the primary defects underlying the respective aberration of the collagen fibrils are still unknown, the differential ultrastructural changes of the collagen fibrils together with clinical symptoms should, as in other heterogeneous genetic disorders, facilitate the (provisional?) classification of EDS and permit the diagnosis of individual cases. 相似文献
12.
Frans A. Prins Ronnie van Diemen-Steenvoorde Jan Bonnet Ingrid Cornelese-ten Velde 《Histochemistry and cell biology》1993,99(6):417-425
Reflection contrast microscopy (RCM) of ultrathin sections was recently introduced as a sensitive technique for visualization with enhanced definition in immunogold histochemistry. Experience of using RCM as a major tool in immunocytochemical research in different fields is summarized, e.g. oncology, nephrology and embryology. The sensitive visualization of immunocytochemical labels, gold particles or peroxidase-diaminobenzidine deposits in or on ultrathin sections, by RCM instead of electron microscopy is demonstrated. RCM of ultrathin sections is an adequate light microscopical alternative for immunoelectron microscopy, since an overview of both label and tissue is obtained with a high image definition and high contrast of label. In the studies presented, RCM is shown to provide a better gradation in staining intensity and staining pattern than other light microscopical methods. Moreover, a precise localization of multiple labels is obtained with this method. Besides the applications shown, ultrathin section visualization by RCM is very useful for correlative light- and electron microscopical studies of fine structures. Commercially available fluorescence microscopes can be adapted for proper RCM functioning; an adaptation scheme and list of microscopes tested is provided. 相似文献
13.
The relative uptake rates of N, P, K, S, Ca, Mg, Fe, Mn, Zn, Cu, and Al were estimated in beech seedlings pot cultured in the field in six acid soils (treatments). The relative uptake rates were compared with the relative growth rates. The relative uptake rates of N, K and Ca agreed well with the growth rates of the seedlings irrespective of widely differing soil conditions (acid sand-clayey till, pH 4–6). The relative uptake rates of P, Fe, and Al differed from that predicted by the growth rate. The uptake rates of Fe and Al were highest at the lowest growth rates, and the P uptake rate was lower than the growth rate in these treatments. Thus the P availability probably limited growth in an eluvial (E) horizon of a podzol, and possibly in the illuvial (B) horizon of a podzol and in an acid clayey till (Dystric Cambisol). Low P uptake was associated with a tendency towards higher relative root growth rates. In terms of the concept of steady state nutrition the high relative root growth rate in some treatments may be interpreted as an acclimation to low P supply. The P limitation seemed to be related to interactions among Fe, Al and organic compounds of the soil solution.FAX no: +4646104423 相似文献
14.
We conducted a set of in situ incubations to evaluate patterns of N availability among dominant land uses in the shortgrass
steppe region of Colorado, USA, and to assess recovery of soil fertility in abandoned fields. Replicated 30 d incubations
were performed in 3 sets of native (never cultivated), abandoned (cultivated until 1937), and currently cultivated, fallow
fields. Net N mineralization and the percentage of total N that was mineralized increased in the order: native, abandoned,
cultivated. Higher soil water content in fallow fields is the most likely reason for greater mineralization in cultivated
fields, while higher total organic C and C/N ratios in native and abandoned fields may explain differences in mineralization
between these land uses. Recovery of soil organic matter in abandoned fields appears to involve accumulation of soil C and
N under perennial plants, but probable methodological artifacts complicate evaluation of the role of individual plants in
recovery of N availability. Higher N mineralization and turnover in cultivated fields may make them more susceptible to N
losses; recovery of N cycling in abandoned fields appears to involve a return to slower N turnover and tighter N cycling similar
to native shortgrass steppe. 相似文献
15.
Liedtke Christa; Polsakiewicz Monika; Hartmann Ingrid; Peters Petra; Volkmann Dieter; Scherer Gnther F.E. 《Journal of experimental botany》1997,48(6):1215-1221
The vacuolar membrane, the tonoplast, is a proteinrich membranehitherto only few proteins in it have been identified. As anapproach for the identification of tonoplast proteins by monoclonalantibodies (MABs), purified tonoplast from cress roots (Lepidiumsativum L.) were used for immunization and plasma membranesas a control membrane to test the absence of antigen. The MABTOP 35 identified a glycoprotein of about 35 kDa in purifiedtonoplast of cress roots. Triton X-114 phase separation showedthat it was a hydrophobic integral membrane protein. In immunocytochemistrythe MAB TOP 35 strongly labelled the vacuolar membrane. Theabsence of cell wall or plasma membrane labelling by TOP 35indicates a distinct biosynthetic pathway of this protein tothe vacuolar membrane in plants. Key words: Immnocytochemistry, Lepidium sativum, monoclonal antibody, secretion, vacuole 相似文献
16.
The species- and genus-specific DNA content, average base composition of nuclear DNA, presence or absence of satellite DNA, the percentage of heterochromatin and other characteristics of nuclear DNA and nuclear structure allow to deduce the molecular changes which accompanied, or more probably caused, cladogenesis in the orchids studied. It is suggested that saltatory replication (generative amplification) of certain DNA sequenes, diversification of reiterated DNA sequences, and loss of DNA play an important role in the evolution of orchids.—The relationship between changes of genome composition and of nuclear structure and ultrastructure is discussed on the basis of cot curves, heterochromatin staining with Giemsa (C banding), electron microscopy of nuclei, and molecular hybridization in situ.Some aspects of this paper have been presented at the Helsinki Chromosome Conference, August 1977 (Nagl & Capesius 1977). 相似文献
17.
18.
19.
Lars-Åke Fransson Anders Malmström Ingrid Sjöberg Thomas N. Huckerby 《Carbohydrate research》1980,80(1):131-145
Heparin, heparan sulphate, and various derivatives thereof have been oxidised with periodate at pH 3.0 and 4° and at pH 7.0 and 37°. Whereas oxidation under the latter conditions destroys all of the nonsulphated uronic acids, treatment with periodate at low pH and temperature causes selective oxidation of uronic acid residues. The reactivity of uronic acid residues depends on the nature of neighbouring 2-amino-2-deoxyglucose residues. d-Glucuronic acid residues are susceptible to oxidation when flanked by N-acetylated amino sugars, but resistant when adjacent residues are either unsubstituted or N-sulphated. L-Iduronic acid residues in their natural environment (2-deoxy-2-sulphoamino-d-glucose) are resistant to oxidation, whereas removal of N-sulphate groups renders a portion of these residues periodate-sensitive. Oxidised uronic acid residues in heparin-related glycans may be cleaved by alkali, producing a series of oligosaccharide fragments. Thus, periodate oxidation-alkaline elimination provides an additional method for the controlled degradation of heparin. 相似文献
20.