全文获取类型
收费全文 | 2386篇 |
免费 | 164篇 |
国内免费 | 1篇 |
出版年
2023年 | 6篇 |
2022年 | 38篇 |
2021年 | 64篇 |
2020年 | 31篇 |
2019年 | 40篇 |
2018年 | 52篇 |
2017年 | 39篇 |
2016年 | 61篇 |
2015年 | 113篇 |
2014年 | 113篇 |
2013年 | 147篇 |
2012年 | 184篇 |
2011年 | 190篇 |
2010年 | 136篇 |
2009年 | 113篇 |
2008年 | 172篇 |
2007年 | 149篇 |
2006年 | 148篇 |
2005年 | 151篇 |
2004年 | 110篇 |
2003年 | 118篇 |
2002年 | 102篇 |
2001年 | 23篇 |
2000年 | 22篇 |
1999年 | 25篇 |
1998年 | 32篇 |
1997年 | 15篇 |
1996年 | 17篇 |
1995年 | 10篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 12篇 |
1989年 | 4篇 |
1988年 | 9篇 |
1987年 | 13篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1967年 | 2篇 |
1901年 | 2篇 |
1887年 | 1篇 |
1879年 | 1篇 |
排序方式: 共有2551条查询结果,搜索用时 15 毫秒
221.
The consequences of invasive species on ecosystem processes and ecological interactions remain poorly understood. Predator–prey
interactions are fundamental in shaping species evolution and community structure and can be strongly modified by species
introductions. To fully understand the ecological effects of invasive species on trophic linkages it is important to characterize
novel interactions between native predators and exotic prey and to identify the impacts of invasive species on the performance
of native predators. Although seaweed invasions are a growing global concern, our understanding of invasive algae—herbivore
interactions is still very limited. We used a series of feeding experiments between a native herbivore and four invasive algae
in the Mediterranean Sea to examine the potential of native sea urchins to consume invasive seaweeds and the impacts of invasive
seaweed on herbivore performance. We found that three of the four invasive species examined are avoided by native herbivores,
and that feeding behaviour in sea urchins is not driven by plant nutritional quality. On the other hand, Caulerpa racemosa is readily consumed by sea urchins, but may escape enemy control by reducing their performance. Recognizing the negative
impacts of C. racemosa on herbivore performance has highlighted an enemy escape mechanism that contributes to explaining how this widespread invasive
alga, which is preferred and consumed by herbivores, is not eradicated by grazing in the field. Furthermore, given the ecological
and economic importance of sea urchins, negative impacts of invasive seaweeds on their performance could have dramatic effects
on ecosystem function and services, and should be accounted for in sea urchin population management strategies. 相似文献
222.
Global patterns of leaf mechanical properties 总被引:1,自引:0,他引:1
Onoda Y Westoby M Adler PB Choong AM Clissold FJ Cornelissen JH Díaz S Dominy NJ Elgart A Enrico L Fine PV Howard JJ Jalili A Kitajima K Kurokawa H McArthur C Lucas PW Markesteijn L Pérez-Harguindeguy N Poorter L Richards L Santiago LS Sosinski EE Van Bael SA Warton DI Wright IJ Wright SJ Yamashita N 《Ecology letters》2011,14(3):301-312
Leaf mechanical properties strongly influence leaf lifespan, plant-herbivore interactions, litter decomposition and nutrient cycling, but global patterns in their interspecific variation and underlying mechanisms remain poorly understood. We synthesize data across the three major measurement methods, permitting the first global analyses of leaf mechanics and associated traits, for 2819 species from 90 sites worldwide. Key measures of leaf mechanical resistance varied c. 500-800-fold among species. Contrary to a long-standing hypothesis, tropical leaves were not mechanically more resistant than temperate leaves. Leaf mechanical resistance was modestly related to rainfall and local light environment. By partitioning leaf mechanical resistance into three different components we discovered that toughness per density contributed a surprisingly large fraction to variation in mechanical resistance, larger than the fractions contributed by lamina thickness and tissue density. Higher toughness per density was associated with long leaf lifespan especially in forest understory. Seldom appreciated in the past, toughness per density is a key factor in leaf mechanical resistance, which itself influences plant-animal interactions and ecosystem functions across the globe. 相似文献
223.
Leonard H Glasson E Nassar N Whitehouse A Bebbington A Bourke J Jacoby P Dixon G Malacova E Bower C Stanley F 《PloS one》2011,6(3):e17875
Background
Research findings investigating the sociodemographics of autism spectrum disorder (ASD) have been inconsistent and rarely considered the presence of intellectual disability (ID).Methods
We used population data on Western Australian singletons born from 1984 to 1999 (n = 398,353) to examine the sociodemographic characteristics of children diagnosed with ASD with or without ID, or ID without ASD compared with non-affected children.Results
The profiles for the four categories examined, mild-moderate ID, severe ID, ASD without ID and ASD with ID varied considerably and we often identified a gradient effect where the risk factors for mild-moderate ID and ASD without ID were at opposite extremes while those for ASD with ID were intermediary. This was demonstrated clearly with increased odds of ASD without ID amongst older mothers aged 35 years and over (odds ratio (OR) = 1.69 [CI: 1.18, 2.43]), first born infants (OR = 2.78; [CI: 1.67, 4.54]), male infants (OR = 6.57 [CI: 4.87, 8.87]) and increasing socioeconomic advantage. In contrast, mild-moderate ID was associated with younger mothers aged less than 20 years (OR = 1.88 [CI: 1.57, 2.25]), paternal age greater than 40 years (OR = 1.59 [CI: 1.36, 1.86]), Australian-born and Aboriginal mothers (OR = 1.60 [CI: 1.41, 1.82]), increasing birth order and increasing social disadvantage (OR = 2.56 [CI: 2.27, 2.97]). Mothers of infants residing in regional or remote areas had consistently lower risk of ASD or ID and may be linked to reduced access to services or under-ascertainment rather than a protective effect of location.Conclusions
The different risk profiles observed between groups may be related to aetiological differences or ascertainment factors or both. Untangling these pathways is challenging but an urgent public health priority in view of the supposed autism epidemic. 相似文献224.
Ahmed SB Ronksley PE Hemmelgarn BR Tsai WH Manns BJ Tonelli M Klarenbach SW Chin R Clement FM Hanly PJ 《PloS one》2011,6(4):e19029
Background
Although obstructive sleep apnea (OSA) is more common in patients with kidney disease, whether nocturnal hypoxia affects kidney function is unknown.Methods
We studied all adult subjects referred for diagnostic testing of sleep apnea between July 2005 and December 31 2007 who had serial measurement of their kidney function. Nocturnal hypoxia was defined as oxygen saturation (SaO2) below 90% for ≥12% of the nocturnal monitoring time. The primary outcome, accelerated loss of kidney function, was defined as a decline in estimated glomerular filtration rate (eGFR) ≥4 ml/min/1.73 m2 per year.Results
858 participants were included and followed for a mean study period of 2.1 years. Overall 374 (44%) had nocturnal hypoxia, and 49 (5.7%) had accelerated loss of kidney function. Compared to controls without hypoxia, patients with nocturnal hypoxia had a significant increase in the adjusted risk of accelerated kidney function loss (odds ratio (OR) 2.89, 95% confidence interval [CI] 1.25, 6.67).Conclusion
Nocturnal hypoxia was independently associated with an increased risk of accelerated kidney function loss. Further studies are required to determine whether treatment and correction of nocturnal hypoxia reduces loss of kidney function. 相似文献225.
Goldshmit Y Spanevello MD Tajouri S Li L Rogers F Pearse M Galea M Bartlett PF Boyd AW Turnley AM 《PloS one》2011,6(9):e24636
Upregulation and activation of developmental axon guidance molecules, such as semaphorins and members of the Eph receptor tyrosine kinase family and their ligands, the ephrins, play a role in the inhibition of axonal regeneration following injury to the central nervous system. Previously we have demonstrated in a knockout model that axonal regeneration following spinal cord injury is promoted in the absence of the axon guidance protein EphA4. Antagonism of EphA4 was therefore proposed as a potential therapy to promote recovery from spinal cord injury. To further assess this potential, two soluble recombinant blockers of EphA4, unclustered ephrin-A5-Fc and EphA4-Fc, were examined for their ability to promote axonal regeneration and to improve functional outcome following spinal cord hemisection in wildtype mice. A 2-week administration of either of these blockers following spinal cord injury was sufficient to promote substantial axonal regeneration and functional recovery by 5 weeks following injury. Both inhibitors produced a moderate reduction in astrocytic gliosis, indicating that much of the effect of the blockers may be due to promotion of axon growth. These studies provide definitive evidence that soluble inhibitors of EphA4 function offer considerable therapeutic potential for the treatment of spinal cord injury and may have broader potential for the treatment of other central nervous system injuries. 相似文献
226.
Riglar DT Richard D Wilson DW Boyle MJ Dekiwadia C Turnbull L Angrisano F Marapana DS Rogers KL Whitchurch CB Beeson JG Cowman AF Ralph SA Baum J 《Cell host & microbe》2011,9(1):9-20
- Download : Download high-res image (370KB)
- Download : Download full-size image
227.
Dabora SL Franz DN Ashwal S Sagalowsky A DiMario FJ Miles D Cutler D Krueger D Uppot RN Rabenou R Camposano S Paolini J Fennessy F Lee N Woodrum C Manola J Garber J Thiele EA 《PloS one》2011,6(9):e23379
Background
Tuberous sclerosis (TSC) related tumors are characterized by constitutively activated mTOR signaling due to mutations in TSC1 or TSC2.Methods
We completed a phase 2 multicenter trial to evaluate the efficacy and tolerability of the mTOR inhibitor, sirolimus, for the treatment of kidney angiomyolipomas.Results
36 adults with TSC or TSC/LAM were enrolled and started on daily sirolimus. The overall response rate was 44.4% (95% confidence intervals [CI] 28 to 61); 16/36 had a partial response. The remainder had stable disease (47.2%, 17/36), or were unevaluable (8.3%, 3/36). The mean decrease in kidney tumor size (sum of the longest diameters [sum LD]) was 29.9% (95% CI, 22 to 37; n = 28 at week 52). Drug related grade 1–2 toxicities that occurred with a frequency of >20% included: stomatitis, hypertriglyceridemia, hypercholesterolemia, bone marrow suppression (anemia, mild neutropenia, leucopenia), proteinuria, and joint pain. There were three drug related grade 3 events: lymphopenia, headache, weight gain. Kidney angiomyolipomas regrew when sirolimus was discontinued but responses tended to persist if treatment was continued after week 52. We observed regression of brain tumors (SEGAs) in 7/11 cases (26% mean decrease in diameter), regression of liver angiomyolipomas in 4/5 cases (32.1% mean decrease in longest diameter), subjective improvement in facial angiofibromas in 57%, and stable lung function in women with TSC/LAM (n = 15). A correlative biomarker study showed that serum VEGF-D levels are elevated at baseline, decrease with sirolimus treatment, and correlate with kidney angiomyolipoma size (Spearman correlation coefficient 0.54, p = 0.001, at baseline).Conclusions
Sirolimus treatment for 52 weeks induced regression of kidney angiomyolipomas, SEGAs, and liver angiomyolipomas. Serum VEGF-D may be a useful biomarker for monitoring kidney angiomyolipoma size. Future studies are needed to determine benefits and risks of longer duration treatment in adults and children with TSC.Trial Registration
Clinicaltrials.gov NCT00126672相似文献228.
There is a current need to develop novel non-toxic antifouling materials. The mechanisms utilized by marine organisms to prevent fouling of external surfaces are of interest in this regard. Biomimicry of these mechanisms and the ability to transfer the antifouling characteristics of these surfaces to artificial surfaces are a highly attractive prospect to those developing antifouling technologies. In order to achieve this, the mechanisms responsible for any antifouling ability must be elucidated from the study of the natural organism and the critical surface parameters responsible for fouling reduction. Dermal denticles of members of the shark family have been speculated to possess some natural, as yet unidentified antifouling mechanism related to the physical presence of denticles. In this study, the dermal denticles of one particular member of the slow-swimming sharks, Scyliorhinus canicula were characterized and it was found that a significant natural variation in denticle dimensions exists in this species. The degree of denticle surface contamination was quantified on denticles at various locations and it was determined that the degree of contamination of the dorsal surface of denticles varies with the position on the shark body. In addition, we successfully produced synthetic sharkskin samples using the real skin as a template. Testing of the produced synthetic skin in field conditions resulted in significant differences in material attachment on surfaces exhibiting denticles of different dimensions. 相似文献
229.
230.
LKB1 acts as a master upstream protein kinase regulating a number of kinases involved in diverse cellular functions. Recent studies have suggested a role for LKB1 in male fertility. Male mice with reduced total LKB1 expression, including the complete absence of the major splice variant in testis (LKB1(S)), are completely infertile. We sought to further characterise these mice and determine the mechanism underlying this infertility. This involved expression studies of LKB1 in developing germ cells, morphological analysis of mature spermatozoa and histological studies of both the testis and epididymis using light microscopy and transmission electron microscopy. We conclude that a defect in the release of mature spermatids from the seminiferous epithelium (spermiation) during spermatozoan development is a major cause of the infertility phenotype. We also present evidence that this is due, at least in part, to defects in the breakdown of the junctions, known as ectoplasmic specialisations, between the sertoli cells of the testis epithelium and the heads of the maturing spermatids. Overall this study uncovers a critical role for LKB1 in spermiation, a highly regulated, but poorly understood process vital for male fertility. 相似文献