全文获取类型
收费全文 | 13334篇 |
免费 | 398篇 |
国内免费 | 1篇 |
出版年
2021年 | 121篇 |
2020年 | 105篇 |
2019年 | 133篇 |
2018年 | 288篇 |
2017年 | 251篇 |
2016年 | 350篇 |
2015年 | 370篇 |
2014年 | 432篇 |
2013年 | 585篇 |
2012年 | 564篇 |
2011年 | 590篇 |
2010年 | 372篇 |
2009年 | 279篇 |
2008年 | 416篇 |
2007年 | 431篇 |
2006年 | 438篇 |
2005年 | 410篇 |
2004年 | 325篇 |
2003年 | 364篇 |
2002年 | 319篇 |
2001年 | 282篇 |
2000年 | 230篇 |
1999年 | 180篇 |
1998年 | 91篇 |
1992年 | 237篇 |
1991年 | 228篇 |
1990年 | 185篇 |
1989年 | 221篇 |
1988年 | 237篇 |
1987年 | 242篇 |
1986年 | 220篇 |
1985年 | 251篇 |
1984年 | 205篇 |
1983年 | 169篇 |
1980年 | 87篇 |
1979年 | 241篇 |
1978年 | 156篇 |
1977年 | 137篇 |
1976年 | 136篇 |
1975年 | 172篇 |
1974年 | 270篇 |
1973年 | 261篇 |
1972年 | 253篇 |
1971年 | 248篇 |
1970年 | 258篇 |
1969年 | 248篇 |
1968年 | 192篇 |
1967年 | 197篇 |
1966年 | 173篇 |
1965年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
ObjectivesJust as people with the same weight can have different body builds, woods with the same wood density can have different anatomies. Here, our aim was to assess the magnitude of anatomical variation within a restricted range of wood density and explore its potential ecological implications.MethodsTwig wood of 69 angiosperm tree and shrub species was analyzed. Species were selected so that wood density varied within a relatively narrow range (0.38–0.62 g cm-3). Anatomical traits quantified included wood tissue fractions (fibres, axial parenchyma, ray parenchyma, vessels, and conduits with maximum lumen diameter below 15 μm), vessel properties, and pith area. To search for potential ecological correlates of anatomical variation the species were sampled across rainfall and temperature contrasts, and several other ecologically-relevant traits were measured (plant height, leaf area to sapwood area ratio, and modulus of elasticity).ResultsDespite the limited range in wood density, substantial anatomical variation was observed. Total parenchyma fraction varied from 0.12 to 0.66 and fibre fraction from 0.20 to 0.74, and these two traits were strongly inversely correlated (r = -0.86, P < 0.001). Parenchyma was weakly (0.24 ≤|r|≤ 0.35, P < 0.05) or not associated with vessel properties nor with height, leaf area to sapwood area ratio, and modulus of elasticity (0.24 ≤|r|≤ 0.41, P < 0.05). However, vessel traits were fairly well correlated with height and leaf area to sapwood area ratio (0.47 ≤|r|≤ 0.65, all P < 0.001). Modulus of elasticity was mainly driven by fibre wall plus vessel wall fraction rather than by the parenchyma component.ConclusionsOverall, there seem to be at least three axes of variation in xylem, substantially independent of each other: a wood density spectrum, a fibre-parenchyma spectrum, and a vessel area spectrum. The fibre-parenchyma spectrum does not yet have any clear or convincing ecological interpretation. 相似文献
2.
The advent of publishing anonymized call detail records opens the door for temporal and spatial human dynamics studies. Such studies, besides being useful for creating universal models for mobility patterns, could be also used for creating new socio-economic proxy indicators that will not rely only on the local or state institutions. In this paper, from the frequency of calls at different times of the day, in different small regional units (sub-prefectures) in Côte d''Ivoire, we infer users'' home and work sub-prefectures. This division of users enables us to analyze different mobility and calling patterns for the different regions. We then compare how those patterns correlate to the data from other sources, such as: news for particular events in the given period, census data, economic activity, poverty index, power plants and energy grid data. Our results show high correlation in many of the cases revealing the diversity of socio-economic insights that can be inferred using only mobile phone call data. The methods and the results may be particularly relevant to policy-makers engaged in poverty reduction initiatives as they can provide an affordable tool in the context of resource-constrained developing economies, such as Côte d''Ivoire''s. 相似文献
3.
B. Oderfeld-Nowak W. Jegliski M. Skup J. Skangiel-Kramska M. Zaremba D. Koczyk 《Journal of neurochemistry》1993,61(1):116-119
Abstract: The glial fibrillary acidic protein (GFAP) content was investigated using immunoblotting techniques in the septum and hippocampus of the rat after bilateral lateral fimbria transection. Seven days after surgery GFAP content increased significantly both in the septum (140% of control) and hippocampus (120% in dorsal, the less denervated, and 145% in the most denervated ventral part), indicating the occurrence of reactive gliosis. The GM1 treatment caused statistically significant attenuation of GFAP increment in all hippocampal parts. In contrast, GM1 treatment has no influence on the increase of GFAP content in the septum. Results suggest a differential effect of GM1 on the two gliotic reactions formed as a consequence of the lesion at the level of the source of innervation (septum) and the target (hippocampus). 相似文献
4.
J Marcinkiewicz B M Chain E Olszowska S Olszowski J M Zgliczyński 《The International journal of biochemistry》1991,23(12):1393-1395
1. Chlorination of ovalbumin results in its enhanced immunogenic properties. 2. This has been evaluated by the interleukin-2 production after incubation of the modified protein with antigen presenting cells and T helper cells. 相似文献
5.
Plant Ecology - The impact of exotic annual Impatiens glandulifera on invaded European vegetation is ambiguous; there are studies reporting considerable negative as well as weak or even no impact... 相似文献
6.
Using a simple model of linear response of polarizable centers to an electric field, function delta is defined to characterize structural organization of protein polar groups. The function makes it possible to detect specific structures of nonuniformly distributed and mutually coupled groups that can transmit local structural and charge density perturbations, induced by an ion over long distances to functionally active sites of a protein molecule. In 9 kd calbinding (a small protein from the troponin C superfamily) two structural chains have been demonstrated that link together both Ca2+ ions coordinated by the protein. The chains form a rigid structure stabilized by coordination of one of the ions, so that binding of the other is promoted. Such a structure is probably common to all members of the superfamily and plays an important role in the mechanism of calcium binding by these proteins. 相似文献
7.
Gro O. Nygaard Elisabeth G. Celius Sigrid A. de Rodez Benavent Piotr Sowa Marte W. Gustavsen Anders M. Fjell Nils I. Landr? Kristine B. Walhovd Hanne F. Harbo 《PloS one》2015,10(8)
New treatment options may make “no evidence of disease activity” (NEDA: no relapses or disability progression and no new/enlarging MRI lesions, as opposed to “evidence of disease activity” (EDA) with at least one of the former), an achievable goal in relapsing-remitting multiple sclerosis (RRMS). The objective of the present study was to determine whether early RRMS patients with EDA at one-year follow-up had different disability, cognition, treatment and gray matter (GM) atrophy rates from NEDA patients and healthy controls (HC). RRMS patients (mean age 34 years, mean disease duration 2.2 years) were examined at baseline and one-year follow-up with neurological (n = 72), neuropsychological (n = 56) and structural MRI (n = 57) examinations. Matched HC (n = 61) were retested after three years. EDA was found in 46% of RRMS patients at follow-up. EDA patients used more first line and less second line disease modifying treatment than NEDA (p = 0.004). While the patients groups had similar disability levels at baseline, they differed in disability at follow-up (p = 0.010); EDA patients progressed (EDSS: 1.8–2.2, p = 0.010), while NEDA patients improved (EDSS: 2.0–1.7, p<0.001). Cognitive function was stable in both patient groups. Subcortical GM atrophy rates were higher in EDA patients than HC (p<0.001). These results support the relevance of NEDA as outcome in RRMS and indicate that pathological neurodegeneration in RRMS mainly occur in patients with evidence of disease activity. 相似文献
8.
A. Śliwińska O. Olszowska M. Furmanowa A. Nosov 《In vitro cellular & developmental biology. Plant》2008,44(2):69-77
Efficient plant regeneration through somatic embryogenesis was achieved in Polyscias filicifolia. Embryogenic calluses were induced on Murashige and Skoog (MS) basal medium supplemented with 0.5 mg l−1 2,4-dichlorophenoxyacetic acid (2,4-D) and 1.0 mg l−1 benzylaminopurine (BAP; type I callus) and on MS medium with 2.0 mg l−1 2,4-D and 0.01 mg l−1 kinetin (type II callus) from leaf explants of a 2-yr-old plant. Primary somatic embryos (PSEs) developed after four passages
of suspension culture established from embryogenic callus when cultured in liquid half-strength MS medium (1/2 MS) without
growth regulators. PSEs in the cotyledonary stage were multiplied by adventitious embryogenesis. Single secondary somatic
embryos (SSEs) or their clusters developed at the base of PSE hypocotyls and regenerated into plantlets in a one-step process
on plant growth regulator-free 1/2 MS medium. Low sucrose concentration of 15 g l−1 promoted development of normal SSEs. All SSEs regenerated into single, well-rooted plantlets on a Nitsch and Nitsch medium
supplemented with 0.5 mg l−1 kinetin, 0.1 mg l−1 indole-3-butyric acid, and 10 mg l−1 adenine sulfate. Subsequent two subculture cycles on the same medium were necessary to obtain plantlets sufficiency developed
to allow successful transfer to the soil. Rooted plantlets were established in a peat mixture with 90% survival, with the
plants showing normal morphological characteristics. 相似文献
9.
Mikołaj Świerczyński Agata Szymaszkiewicz Jakub Fichna Marta Zielińska 《生物化学与生物物理学报:癌评论》2021,1875(1):188460
Colorectal cancer (CRC) is one of the most common cause of death among neoplasms around the world. The environmental factors, like diet and obesity, are crucial in CRC pathogenesis by creating cancer-favorable microenvironment and hormonal changes. Adiponectin, the adipose tissue-specific hormone, is generally considered to negatively correlate with CRC development. The interleukin 6 (IL-6) is one of the most important pro-inflammatory cytokine connected with CRC, which is strongly inflammation-associated. The opioids are variable group substantially correlated with cancers - the endogenous opioids affect immune system and cell cycle including proliferation and cell death whereas exogenous opioids are leading clinically used analgesics in terminal cancer patients. In this review we discuss the involvement of adiponectin, IL-6 and opioids in CRC pathogenesis, their link with obesity, possible cross-talk and potential novel therapeutic approach in CRC treatment. 相似文献
10.
The vegetables germplasm conservation programme in Poland was started in 1982, activities under this program resulted up to
this date in building up a tomato germplasm collection of 782 accessions that are available for distribution to breeders and
researchers in Poland and abroad. Between 1990 and 1999 we distributed 651 tomato seed samples to breeders and scientists
in Poland, and 95 samples to users abroad. During 1990 – 1999 the 652 tomato accessions were evaluated for 43 traits of plants
and fruits. Work is in progress to expand the collection and identify useful characteristics of the collected material, and
to make use of them in breeding and other research works. 相似文献