首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6424篇
  免费   403篇
  国内免费   1篇
  2023年   22篇
  2022年   12篇
  2021年   88篇
  2020年   76篇
  2019年   84篇
  2018年   131篇
  2017年   101篇
  2016年   180篇
  2015年   265篇
  2014年   350篇
  2013年   497篇
  2012年   561篇
  2011年   496篇
  2010年   327篇
  2009年   280篇
  2008年   427篇
  2007年   371篇
  2006年   368篇
  2005年   325篇
  2004年   337篇
  2003年   338篇
  2002年   314篇
  2001年   62篇
  2000年   59篇
  1999年   61篇
  1998年   65篇
  1997年   60篇
  1996年   51篇
  1995年   45篇
  1994年   61篇
  1993年   41篇
  1992年   41篇
  1991年   28篇
  1990年   23篇
  1989年   19篇
  1988年   19篇
  1987年   15篇
  1986年   11篇
  1985年   16篇
  1984年   15篇
  1983年   22篇
  1982年   15篇
  1981年   26篇
  1980年   23篇
  1979年   13篇
  1978年   6篇
  1977年   17篇
  1976年   6篇
  1975年   7篇
  1974年   9篇
排序方式: 共有6828条查询结果,搜索用时 15 毫秒
111.
When a fossil vanishes to a private collection, it must be considered lost to science because, frequently, it is no longer available for study. Fortunately some fossils occasionally are regained. We had the opportunity to recoup an interesting footprint-bearing slab that was part of a private collection in Italy. The specimen, found in 1992 near Seligman, Arizona (USA) was described, before disappearing, as one of the best fossil examples of vertebrate (Chelichnus [Laoporus])-on-invertebrate (Octopodichnus) predation. After a careful re-examination of the slab, the primary conclusions of the former describers are demonstrably groundless. The reanalysis of the tracks, as well as peculiar sedimentary structures associated with the tracks, allowed obtaining new information about the depositional environment and the complex interactions between the type of substrate and trackmaker behavior. The re-examination of the specimen also revealed interesting aspects about trackmaker biomechanics.  相似文献   
112.
Abstract

Anemophylus dispersal of pollen grains in the Po Valley.—Results of phenological and aerobiological measurements with Castanea sativa pollen are presented. The data concerning the dispersal of this pollen in the Po Valley, show some features of the transport over hundred kilometer distances and give useful suggestions for application research.  相似文献   
113.
Paolo Meletti 《Plant biosystems》2013,147(3-5):372-384
Abstract

New perspectives in the study of factors which control seed germination. — Seedlings of Triticum durum, cv. « Cappelli », coming from unirradiated embryos grafted on to irradiated endosperms (EM(u)/EN(i) of presoaked seeds (in distilled water for 24 h. at 20[ddot]C.) (treatment: X-rays, doses, 2,4,6,8,10 and 20 Kr), grow more than seedlings of control EM(u)/EN(u) (dose 0) (fig. 1). To have this reaction, it is necessary that the used seeds be after-ripe; at the various stages of seed ripening, be ginning from the milk stage, the phenomenon is not present.

On this basis, the author has thought that a natural inhibitor occurs in the after-ripe endosperm of « Cappelli », which is neutralized or destroyed by X-rays.

As an experimental demonstration, some trials have been made of growing wheat seedlings in Petri dish, on moistened (distilled water) filter paper, together with excised embryos or isolated endosperms (fig. 2): the after-ripe endosperm is able to depress the seedling growth (fig. 3). In the same experimental conditions, X-rays, dose 6 Kr, neutralize the inhibition effect given by the endosperm. (fig. 4).

A completely different situation occurs in wheat seed, during its ripening: endosperm is inactive, embryo produces inhibition effects on the seedling growth, which, also in this case, are reduced by X-rays.

These phenomena, put in relation with dormancy in Triticum durum, cv. « Cappelli », which is a relative dormancy, having its maximum at the milk stage, have led the author to the general conclusion that, during dormancy, a germination inhibitor occurs in the embryo of wheat seed; when dormancy is finished, the inhibitor appears in the endosperm, in a situation which becomes stable and definitive.  相似文献   
114.
Abstract

Megagametophytes of Macrozamia communis were incubated in White's Basal Medium and in White's Basal Medium modified with 2,4-D and kinetin. On the medium enriched with growth substances, regeneration of coralloid roots was induced. These are morphologically identical to sporophytic coralloid roots, without any endosymbiont and displaying negative geotropism. These results confirm the fact that coralloid roots represent an inherent feature of the root system of the Cycadales rather than being the result of induction by microbial factors. Therefore it is possible to suggest that coralloid roots represent vestigial pneumatophores.  相似文献   
115.
Critical processes of B-cell physiology, including immune signaling through the B-cell receptor (BcR) and/or Toll-like receptors (TLRs), are targeted by microRNAs. With this in mind and also given the important role of BcR and TLR signaling and microRNAs in chronic lymphocytic leukemia (CLL), we investigated whether microRNAs could be implicated in shaping the behavior of CLL clones with distinct BcR and TLR molecular and functional profiles. To this end, we examined 79 CLL cases for the expression of 33 microRNAs, selected on the following criteria: (a) deregulated in CLL versus normal B-cells; (b) differentially expressed in CLL subgroups with distinct clinicobiological features; and, (c) if meeting (a) + (b), having predicted targets in the immune signaling pathways. Significant upregulation of miR-150, miR-29c, miR-143 and miR-223 and downregulation of miR-15a was found in mutated versus unmutated CLL, with miR-15a showing the highest fold difference. Comparison of two major subsets with distinct stereotyped BcRs and signaling signatures, namely subset 1 [IGHV1/5/7-IGKV1(D)-39, unmutated, bad prognosis] versus subset 4 [IGHV4-34/IGKV2-30, mutated, good prognosis] revealed differences in the expression of miR-150, miR-29b, miR-29c and miR-101, all down-regulated in subset 1. We were also able to link these distinct microRNA profiles with cellular phenotypes, importantly showing that, in subset 1, miR-101 downregulation is associated with overexpression of the enhancer of zeste homolog 2 (EZH2) protein, which has been associated with clinical aggressiveness in other B-cell lymphomas. In conclusion, specific miRNAs differentially expressed among CLL subgroups with distinct BcR and/or TLR signaling may modulate the biological and clinical behavior of the CLL clones.  相似文献   
116.
117.
The contribution that oxidative damage to DNA and/or RNA makes to the aging process remains undefined. In this study, we used the hMTH1‐Tg mouse model to investigate how oxidative damage to nucleic acids affects aging. hMTH1‐Tg mice express high levels of the hMTH1 hydrolase that degrades 8‐oxodGTP and 8‐oxoGTP and excludes 8‐oxoguanine from both DNA and RNA. Compared to wild‐type animals, hMTH1‐overexpressing mice have significantly lower steady‐state levels of 8‐oxoguanine in both nuclear and mitochondrial DNA of several organs, including the brain. hMTH1 overexpression prevents the age‐dependent accumulation of DNA 8‐oxoguanine that occurs in wild‐type mice. These lower levels of oxidized guanines are associated with increased longevity and hMTH1‐Tg animals live significantly longer than their wild‐type littermates. Neither lipid oxidation nor overall antioxidant status is significantly affected by hMTH1 overexpression. At the cellular level, neurospheres derived from adult hMTH1‐Tg neural progenitor cells display increased proliferative capacity and primary fibroblasts from hMTH1‐Tg embryos do not undergo overt senescence in vitro. The significantly lower levels of oxidized DNA/RNA in transgenic animals are associated with behavioral changes. These mice show reduced anxiety and enhanced investigation of environmental and social cues. Longevity conferred by overexpression of a single nucleotide hydrolase in hMTH1‐Tg animals is an example of lifespan extension associated with healthy aging. It provides a link between aging and oxidative damage to nucleic acids.  相似文献   
118.

Introduction

Microcirculation dysfunction is a typical feature of systemic sclerosis (SSc) and represents the earliest abnormality of primary myocardial involvement. We assessed coronary microcirculation status by combining two functional tests in SSc patients and estimating its impact on disease outcome.

Methods

Forty-one SSc patients, asymptomatic for coronary artery disease, were tested for coronary flow velocity reserve (CFR) by transthoracic-echo-Doppler with adenosine infusion (A-TTE) and for left ventricular wall motion abnormalities (WMA) by dobutamine stress echocardiography (DSE). Myocardial multi-detector computed tomography (MDCT) enabled the presence of epicardial stenosis, which could interfere with the accuracy of the tests, to be excluded. Patient survival rate was assessed over a 6.7- ± 3.5-year follow-up.

Results

Nineteen out of 41 (46%) SSc patients had a reduced CFR (≤2.5) and in 16/41 (39%) a WMA was observed during DSE. Furthermore, 13/41 (32%) patients showed pathological CFR and WMA. An inverse correlation between wall motion score index (WMSI) during DSE and CFR value (r = -0.57, P <0.0001) was observed; in addition, CFR was significantly reduced (2.21 ± 0.38) in patients with WMA as compared to those without (2.94 ± 0.60) (P <0.0001). In 12 patients with abnormal DSE, MDCT was used to exclude macrovasculopathy. During a 6.7- ± 3.5-year follow-up seven patients with abnormal coronary functional tests died of disease-related causes, compared to only one patient with normal tests.

Conclusions

A-TTE and DSE tests are useful tools to detect non-invasively pre-clinical microcirculation abnormalities in SSc patients; moreover, abnormal CFR and WMA might be related to a worse disease outcome suggesting a prognostic value of these tests, similar to other myocardial diseases.  相似文献   
119.
The genus Cleptes Latreille, 1802 from China is revised and illustrated for the first time. Seventeen species of Cleptes are recorded. Nine species are new to science, Cleptes albonotatus sp. n., Cleptes eburnecoxis sp. n., Cleptes flavolineatus sp. n., Cleptes helanshanus sp. n., Cleptes niger sp. n., Cleptes shengi sp. n., Cleptes sinensis sp. n., Cleptes tibetensis sp. n., and Cleptes villosus sp. n.,and two species are reported as new to China, Cleptes metallicorpus Ha, Lee & Kim, 2011, and Cleptes seoulensis Tsuneki, 1959.  相似文献   
120.
The X-ray repair cross-complementing group 3 gene (XRCC3) belongs to a family of genes responsible for repairing DNA double-strand breaks caused by normal metabolic processes and exposure to ionizing radiation. Polymorphisms in DNA repair genes may alter an individual's capacity to repair damaged DNA and may lead to genetic instability and contribute to malignant transformation. We examined the role of a polymorphism in the XRCC3 gene (rs861529; codon 241: threonine to methionine change) in determining breast cancer risk in Thai women. The study population consisted of 507 breast cancer cases and 425 healthy women. The polymorphism was analysed by fluorescence-based melting curve analysis. The XRCC3 241Met allele was found to be uncommon in the Thai population (frequency 0.07 among cases and 0.05 among controls). Odds ratios (OR) adjusted for age, body mass index, age at menarche, family history of breast cancer, menopausal status, reproduction parameters, use of contraceptives, tobacco smoking, involuntary tobacco smoking, alcohol drinking, and education were calculated for the entire population as well as for pre- and postmenopausal women. There was a significant association between 241Met carrier status and breast cancer risk (OR 1.58, 95% confidence interval (CI) 1.02–2.44). Among postmenopausal women, a slightly higher OR (1.82, 95% CI 0.95–3.51) was found than among premenopausal women (OR 1.48, 95% CI 0.82–2.69). Our findings suggest that the XRCC3 Thr241Met polymorphism is likely to play a modifying role in the individual susceptibility to breast cancer among Thai women as already shown for women of European ancestry.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号