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   563篇
  免费   44篇
  国内免费   1篇
  2023年   3篇
  2022年   3篇
  2021年   7篇
  2020年   7篇
  2019年   12篇
  2018年   9篇
  2017年   7篇
  2016年   16篇
  2015年   25篇
  2014年   22篇
  2013年   25篇
  2012年   35篇
  2011年   24篇
  2010年   24篇
  2009年   18篇
  2008年   24篇
  2007年   34篇
  2006年   28篇
  2005年   21篇
  2004年   17篇
  2003年   20篇
  2002年   15篇
  2001年   22篇
  2000年   20篇
  1999年   14篇
  1998年   7篇
  1996年   6篇
  1995年   4篇
  1994年   7篇
  1993年   6篇
  1991年   13篇
  1990年   3篇
  1989年   5篇
  1988年   7篇
  1987年   3篇
  1986年   4篇
  1985年   8篇
  1984年   8篇
  1983年   4篇
  1981年   3篇
  1980年   4篇
  1979年   3篇
  1978年   3篇
  1977年   6篇
  1974年   3篇
  1973年   3篇
  1971年   3篇
  1970年   5篇
  1918年   3篇
  1892年   3篇
排序方式: 共有608条查询结果,搜索用时 41 毫秒
131.
Humpback whale use of areas off eastern Canada is poorly understood, a knowledge gap that could impact future conservation efforts. We describe the acoustic occurrence of humpback whales in and around the Gully Marine Protected Area (MPA), an eastern Scotian Shelf submarine canyon. Near‐continuous acoustic recordings sampling at 16 kHz were collected from the MPA and nearby slope areas from October 2012 to September 2014 using near‐bottom recorders. In an offshore region where humpbacks were thought to be rare, we observed calls from October to June with a peak in song and nonsong calls in December and January. This suggests that some individuals occur in Canadian waters in winter and the Gully region may be a North Atlantic humpback whale migratory corridor. Calls were predominantly songs indicating potential mating activities. Song and nonsong calls occurred more at sunset and during hours of darkness than during daylight. This study improves our understanding of the seasonal occurrence of humpback whales on the Scotian Slope and, more specifically, their use of an offshore protected area.  相似文献   
132.
This paper presents a Bayesian mixture model approach for detecting areas of habitat that are suitable for S. invicta infestation, aiding the ongoing surveillance for early detection of these exotic pests. We show that Landsat imagery is an affordable and valuable tool to assist in determining an informed surveillance strategy. In this paper, we use Landsat band 3 (visible red), Landsat band 6 (mid infrared) and a soil brightness index, in various combinations, to assess the probability that the area associated with each pixel is habitable terrain, either in a multivariate analysis, or as a univariate spatial temporal model. The multivariate analysis allows researchers to create meaningful clusters that reflect the sometimes complex combinations of conditions of conditions that form suitable habitat, rather then relying on single derived indices.  相似文献   
133.
Forty-nine clones derived by microdissection of a schizophrenia-associated t(1;11)(q42.1;q14.3) breakpoint region have been assigned by somatic cell hybrid mapping to seven discrete intervals on the long arm of human chromosome 11. Eleven of the clones were shown to map to a small region immediately distal to the translocation breakpoint on 11q.A 3-Mb contiguous clone map of this region was established by isolation of corresponding YAC recombinants. The contig was oriented and shown to traverse the translocation breakpoint by FISH and microsatellite marker analysis. This contig will facilitate the isolation of candidate sequences whose expression may be affected by the translocation.  相似文献   
134.
The goal of this research is to evaluate the relative strength of the influences of diet, size, and phylogenetic signal on dental geometric shape. Accurate comprehension of these factors and their interaction is important for reconstructing diet and deriving characters for a cladistic analysis in fossil primates. Geometric morphometric analysis is used to identify axes of shape variation in the lower second molars of (a) prosimian primates and (b) platyrrhines. Landmarks were placed on µCT‐generated surface renderings. Landmark configurations were aligned using generalized Procrustes analysis. Principal components analysis and phylogenetic principal components analysis (pPCA) were performed on species average landmark co‐ordinates. pPCs were examined with phylogenetic generalized least squares analysis for association with size and with diet. PCs from both phylogenetic and non‐phylogenetic analyses were sufficient to separate species by broad dietary categories, including insectivores and folivores. In neither analysis was pPC1 correlated with tooth size, but some other pPCs were significantly correlated with size. The pattern of association between pPCs and size altered when centroid size and dietary variables were combined in the model; effects of diet factors typically exceeded effects of size. These results indicate a dominant phylogenetic and dietary signal in molar shape but also show some shape change correlated with size in the absence of obvious dietary associations. Geometric morphometric analysis appears to be useful for tracking functional traits in molars, particularly in tracking differences between folivorous and insectivorous species.  相似文献   
135.
BackgroundType 2C and 2D limb girdle muscular dystrophies (LGMD) are a group of autosomal recessive limb girdle muscular dystrophies manifested by proximal myopathy, impaired respiratory muscle function and cardiomyopathy. The correlation and the prognostic impact of respiratory and heart impairment are poorly described. We aimed to describe the long-term cardiac and respiratory follow-up of these patients and to determine predictive factors of cardio-respiratory events and mortality in LGMD 2C and 2D.MethodsWe reviewed the charts of 34 LGMD patients, followed from 2005 to 2015, to obtain echocardiographic, respiratory function and sleep recording data. We considered respiratory events (acute respiratory failure, pulmonary sepsis, atelectasis or pneumothorax), cardiac events (acute heart failure, significant cardiac arrhythmia or conduction block, ischemic stroke) and mortality as outcomes of interest for the present analysis.ResultsA total of 21 patients had type 2C LGMD and 13 patients had type 2D. Median age was 30 years [IQR 24–38]. At baseline, median pulmonary vital capacity (VC) was 31% of predicted value [20–40]. Median maximal inspiratory pressure (MIP) was 31 cmH2O [IQR 20.25–39.75]. Median maximal expiratory pressure (MEP) was 30 cm H2O [20–36]. Median left ventricular ejection fraction (LVEF) was 55% [45–64] with 38% of patients with LVEF <50%. Over a median follow-up of 6 years, we observed 38% respiratory events, 14% cardiac events and 20% mortality. Among baseline characteristics, LVEF and left ventricular end diastolic diameter (LVEDD) were associated with mortality, whilst respiratory parameters (VC, MIP, MEP) and the need for home mechanical ventilation (HMV) were associated with respiratory events.ConclusionIn our cohort of severely respiratory impaired type 2C and 2D LGMD, respiratory morbidity was high. Cardiac dysfunction was frequent in particular in LGMD 2C and had an impact on long-term mortality.

Trial Registration

ClinicalTrials.gov NCT02501083  相似文献   
136.
Forest structural heterogeneity due to species composition, spatial relationships and tree size are widely studied patterns in forest systems, but their impacts on tree function are not as well documented. The objective of this study was to examine how stand composition, tree proximity relationships and tree size influence the leaf functional traits of aspen, an early successional species, and subalpine fir, a climax species. We measured foliar nutrients, nonstructural carbohydrates (aspen only), defense chemistry and xylem water potential of aspen and subalpine fir trees in three size classes growing in close proximity or independently from other trees under three stand conditions: aspen dominant, aspen-conifer mixed, and conifer dominant stands. Close proximity of subalpine fir to aspen reduced aspen’s storage of starch in foliar tissue by 17% suggesting that competition between these species may have small effects on carbon metabolism in aspen leaves. Simple sugar (glucose + sucrose) concentrations in aspen leaves were slightly higher in larger aspen trees than smaller trees. However, no differences were found in stem water potential, foliar concentrations of nitrogen, phosphorus, or secondary defense chemicals of aspen or subalpine fir across the gradients of stand composition, tree proximity or tree size. These results suggest that mechanisms of coexistence allow both aspen and subalpine fir to maintain leaf function across a wide range of stand structural characteristics. For aspen, resource sharing through its clonal root system and high resource storage capacity may partially contribute to its functional stability in mixed aspen-conifer stands.  相似文献   
137.
138.
139.
In evaluating the origin of autoantibodies, patterns of self-Ag recognition have been interpreted to reflect the relative role of Ag in stimulating a response. Few studies, however, have assessed whether human autoantibodies display patterns of autoantigen recognition similar to those of SLE-prone mice. In previous studies, anti-La antibodies from humans have been shown to bind multiple epitopes on recombinant human La Ag, including immunoreactivity with a large fragment, termed La C, representing the middle portion of the La sequence. We report herein for the first time that MRL-1pr mice also spontaneously produce antibodies to recombinant human La protein and resemble human autoantibodies in their reactivity with La C. To further investigate the fine specificity of this response, we tested for antibody binding to six synthetic La peptides representing sequences within La C. Whereas two of the synthetic La peptides reacted with MRL-1pr sera containing anti-La binding, low reactivity was observed with a large panel of human anti-La sera. Our results therefore show that patterns of La antigen recognition displayed by MRL-1pr antibodies differ from those of human autoantibodies, possibly reflecting differences between mouse and man in the induction of these responses.  相似文献   
140.
The triphosphate form of 9-[(2-hydroxyethoxy)-methyl]guanine (acyclovir), ACVTP, inactivates the herpes simplex virus type 1 DNA polymerase. ACVTP does not innately inactivate resting polymerase, but becomes an inactivator only while being processed as an alternative substrate. Pseudo first-order rates of inactivation were measured at varying concentrations of ACVTP and fixed concentrations of the natural substrate, deoxyguanosine triphosphate. These studies indicated that a reversible enzyme-ACVTP (Michaelis-type) complex is formed at the active site prior to inactivation. The formation of this complex was competitively retarded by deoxyguanosine triphosphate. An apparent dissociation constant (KD) of 3.6 +/- 0.2 (S.D.) nM was determined for ACVTP from this reversible complex. A second method for the estimation of the KD which used the extrapolated initial velocities produced a value of 5.9 +/- 0.4 (S.D.) nM. The rate of conversion of the reversible complex to the inactivated complex, at saturating ACVTP, was calculated to be 0.24 min-1. No reactivation of enzyme activity was detected following isolation of the inactivated complex by rapid desalting on Sephadex G-25. Under these conditions, an overall reactivation rate of 1.5 X 10(-5) min-1 could have been easily detected. Therefore, the overall inhibition constant must have been less than 3 pM. In contrast, when host DNA polymerase alpha was incubated with 14 microM ACVTP, only 60% inhibition of enzyme activity was observed, but inactivation was not detected. These data indicate that ACVTP functions as a suicide inactivator of the herpes simplex virus type 1 DNA polymerase, and is only a weak reversible inhibitor of DNA polymerase alpha.  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