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   481篇
  免费   17篇
  498篇
  2022年   3篇
  2020年   3篇
  2019年   6篇
  2018年   7篇
  2017年   3篇
  2016年   11篇
  2015年   23篇
  2014年   26篇
  2013年   27篇
  2012年   32篇
  2011年   27篇
  2010年   17篇
  2009年   19篇
  2008年   19篇
  2007年   24篇
  2006年   29篇
  2005年   22篇
  2004年   26篇
  2003年   20篇
  2002年   13篇
  2001年   6篇
  2000年   6篇
  1999年   4篇
  1998年   5篇
  1997年   5篇
  1996年   4篇
  1995年   7篇
  1994年   3篇
  1993年   8篇
  1992年   8篇
  1989年   3篇
  1988年   3篇
  1987年   3篇
  1986年   6篇
  1985年   2篇
  1984年   4篇
  1983年   4篇
  1982年   5篇
  1981年   11篇
  1980年   3篇
  1979年   5篇
  1978年   3篇
  1976年   3篇
  1973年   3篇
  1971年   3篇
  1970年   3篇
  1966年   2篇
  1965年   2篇
  1958年   2篇
  1938年   2篇
排序方式: 共有498条查询结果,搜索用时 0 毫秒
41.
1 The pine processionary moth Thaumetopoea pityocampa is expanding its geographical range in Europe, as a consequence of enhanced winter survival under a warmer climate. A combination of daytime nest temperatures and night air temperatures determines the number of hours larvae are able to feed (hours above realized feeding threshold, RFT). 2 We tested the RFT‐based model for survival across multiple areas of the insect’s range in the Italian Alps over a 2‐year period. In a series of translocation experiments using natural temperature gradients as spatial analogues for global warming, we transferred colonies of T. pityocampa larvae to sites within zones of historical distribution, recent distribution, and outside the present range. The sites included traditional (Pinus nigra, Pinus sylvestris) as well as novel (Pinus mugo, Pinus uncinata, Pinus cembra) hosts. 3 Survival during precold (August to November), cold (December to February) and postcold (March to May) periods were analysed against climatic variables (temperature and rainfall) and predictors developed by the model. 4 Host species did not significantly affect final survival, with the exception of slower larval development, and resulting lower cold tolerance, on P. cembra than on P. mugo at the same site. 5 Across all the sites and hosts, final survival of colonies depended on the number of feeding hours during the cold period (RFT), which explained 82% of the variance in a regression model. We recommend using RFT, or its surrogate daily mean minimum temperature when nest temperature is not available, in predictive models of range expansion of T. pityocampa under climate change scenarios.  相似文献   
42.
The annual routines and seasonal ecology of herbivorous zooplankton species are relatively well known due to their tight coupling with their pulsed food source, the primary production. For higher trophic levels of plankton, these seasonal interactions are less well understood. Here, we study the mid-winter feeding of chaetognaths in high-Arctic fjord ecosystems. Chaetognaths are planktivorous predators which comprise high biomass in high-latitude seas. We investigated the common species Parasagitta elegans around the Svalbard archipelago (78–81°N) during the winters of 2012 and 2013. Our samples consisted of individuals (body lengths 9–55 mm) from three fjords, which were examined for gut contents (n = 903), stable isotopes, fatty acid composition, and maturity status (n = 352). About a quarter of the individuals contained gut contents, mainly lipid droplets and chitinous debris, whilst only 4 % contained identifiable prey, chiefly the copepods Calanus spp. and Metridia longa. The δ15N content of P. elegans, and its average trophic level of 2.9, confirmed its carnivorous position and its fatty acid profile [in particular its high levels of 20:1(n-9) and 22:1(n-11)] confirmed carnivory on Calanus. Observations of undeveloped gonads in many of the larger P. elegans, and the absence of small individuals <10 mm, suggested that reproduction had not started this early in the year. Its average feeding rate across fjords and years was 0.12 prey ind.?1 day?1, which is low compared to estimates of spring and summer feeding in high-latitude environments. Our findings suggest reduced feeding activity during winter and that predation by P. elegans had little impact on the mortality of copepods.  相似文献   
43.
Blooms of domoic acid (DA) producing Pseudo-nitzschia, regularly occur off the coast of California. Although it has been hypothesized that these blooms are increasing in frequency, the lack of historical records limits our understanding of potential causal mechanisms. In this study, an 15-year time-series (1993–2008) of sediment trap samples collected from the Santa Barbara Basin (SBB) at 540 m were analyzed for Pseudo-nitzschia (n = 196, microscopy and SEM) and DA (n = 206, LC–MS/MS) concentrations and fluxes. Results suggest that there was an abrupt shift towards greater frequency and higher magnitude Pseudo-nitzschia blooms and toxic DA flux events in the SBB after the year 2000. SEM analysis of sediment trap material indicates that these events were mainly blooms of P. australis, with cell fluxes increasing by an order of magnitude from a maximum of 4.5 × 106 cells m−2 d−1 pre-2000, to as high as 3.2 × 108 cells m−2 d−1 thereafter. Similarly, sediment trap DA fluxes increased by an average of 13.4 μg m−2 d−1, with only one large event (>5 μg m−2 d−1) from 1993 to 1999 versus 16 large DA events from 2000 to 2008. While the causes of this abrupt shift remain ambiguous, we suggest that this shift may be related to natural climate variability associated with a change in phase of the North Pacific Gyre Oscillation (NPGO) and its potential influence on the composition and magnitude of waters that are upwelled into the SBB.  相似文献   
44.

Background

Network inference methods reconstruct mathematical models of molecular or genetic networks directly from experimental data sets. We have previously reported a mathematical method which is exclusively data-driven, does not involve any heuristic decisions within the reconstruction process, and deliveres all possible alternative minimal networks in terms of simple place/transition Petri nets that are consistent with a given discrete time series data set.

Results

We fundamentally extended the previously published algorithm to consider catalysis and inhibition of the reactions that occur in the underlying network. The results of the reconstruction algorithm are encoded in the form of an extended Petri net involving control arcs. This allows the consideration of processes involving mass flow and/or regulatory interactions. As a non-trivial test case, the phosphate regulatory network of enterobacteria was reconstructed using in silico-generated time-series data sets on wild-type and in silico mutants.

Conclusions

The new exact algorithm reconstructs extended Petri nets from time series data sets by finding all alternative minimal networks that are consistent with the data. It suggested alternative molecular mechanisms for certain reactions in the network. The algorithm is useful to combine data from wild-type and mutant cells and may potentially integrate physiological, biochemical, pharmacological, and genetic data in the form of a single model.  相似文献   
45.
This is not a book that you probably start reading from thefirst page. Its 24 review chapters have informative titles andare organised in seven sections that together with a four-pagelocation index and an eight-page subject index invites you todive for your favourite subjects. The editors have succeededto structure the book in ways that makes easy navigation amongstits  相似文献   
46.
A non-invasive, digital technique was used to measure and quantify intrauterine pressure (IUP) changes in early postpartum dairy cows kept under farm conditions in order to document physiological changes in uterine contractility after uncomplicated calvings. In addition, possible relationships between characteristics of uterine contractility and blood ionized calcium (Ca(2+))-concentrations were investigated. Recordings of uterine contractility were made by using a transcervically inserted open tip catheter in 12 healthy cows during their first 48h after calving. The IUP recording technique appeared easily applicable under farm conditions. Although mean frequency (FREQ), amplitude (AMP) and area under the curve (AUC) of the myometrial contractions significantly decreased due to time, untreated early postpartum cows showed a high variability in characteristics of uterine contractility. There was no correlation between blood Ca2+ -concentrations and any of the contractility parameters.  相似文献   
47.
The management of boars to ensure good sperm production under differing environmental conditions is a major concern for pig keepers in both tropical countries and countries where there are extreme environmental changes. Such changes create stress in animals and influence the production of spermatozoa. High temperatures during hot summer months may result in lower feed consumption and create stresses that result in the inhibition of spermatogenesis. Although tropical countries do not have a problem with major variations in day length, this can cause problems such as decreased litter size and infertility in other regions of the world. Evaporative cooling systems built into boar accommodation are often used to reduce fluctuations in both temperature and humidity during the hot and humid months seen in tropical countries. The system has become popular in AI boar studs, where it is reported to reduce stress and improve feed consumption. Other management factors, such as housing comfort, social contact, mating conditions and the frequency of mating, are also very important boar management aids that assist good quality semen production; these will be covered briefly in this review. This review will consider primarily those management factors, for example, the management of temperature and humidity using evaporative cooling systems and other techniques that enable AI boar studs to maximize sperm fertility through adjustments to the environment.  相似文献   
48.
The myocytes of the adult mammalian heart are considered unable to divide. Instead, mitogens induce cardiomyocyte hypertrophy. We have investigated the effect of adenoviral overexpression of cyclin D2 on myocyte proliferation and morphology. Cardiomyocytes in culture were identified by established markers. Cyclin D2 induced DNA synthesis and proliferation of cardiomyocytes and impaired hypertrophy induced by angiotensin II and serum. At the molecular level, cyclin D2 activated CDK4/6 and lead to pRB phosphorylation and downregulation of the cell cycle inhibitors p21Waf1/Cip1 and p27Kip1. Expression of the CDK4/6 inhibitor p16 inhibited proliferation and cyclin D2 overexpressing myocytes became hypertrophic under such conditions. Inhibition of hypertrophy by cyclin D2 correlated with downregulation of p27Kip1. These data show that hypertrophy and proliferation are highly related processes and suggest that cardiomyocyte hypertrophy is due to low amounts of cell cycle activators unable to overcome the block imposed by cell cycle inhibitors. Cell cycle entry upon hypertrophy may be converted to cell division by increased expression of activators such as cyclin D2.  相似文献   
49.
While cardiac hypertrophy elicited by pathological stimuli eventually leads to cardiac dysfunction, exercise-induced hypertrophy does not. This suggests that a beneficial hypertrophic phenotype exists. In search of an underlying molecular substrate we used microarray technology to identify cardiac gene expression in response to exercise. Rats exercised for seven weeks on a treadmill were characterized by invasive blood pressure measurements and echocardiography. RNA was isolated from the left ventricle and analysed on DNA microarrays containing 8740 genes. Selected genes were analysed by quantitative PCR. The exercise program resulted in cardiac hypertrophy without impaired cardiac function. Principal component analysis identified an exercise-induced change in gene expression that was distinct from the program observed in maladaptive hypertrophy. Statistical analysis identified 267 upregulated genes and 62 downregulated genes in response to exercise. Expression changes in genes encoding extracellular matrix proteins, cytoskeletal elements, signalling factors and ribosomal proteins mimicked changes previously described in maladaptive hypertrophy. Our most striking observation was that expression changes of genes involved in beta-oxidation of fatty acids and glucose metabolism differentiate adaptive from maladaptive hypertrophy. Direct comparison to maladaptive hypertrophy was enabled by quantitative PCR of key metabolic enzymes including uncoupling protein 2 (UCP2) and fatty acid translocase (CD36). DNA microarray analysis of gene expression changes in exercise-induced cardiac hypertrophy suggests that a set of genes involved in fatty acid and glucose metabolism could be fundamental to the beneficial phenotype of exercise-induced hypertrophy, as these changes are absent or reversed in maladaptive hypertrophy.  相似文献   
50.
Hypersensitivity responses (HR) play a major role in plant resistance to pathogens. It is often claimed that HR is also important in plant resistance to insects, although there is little unambiguous documentation. Large genotypic variation in resistance against the gall midge Dasineura marginemtorquens is found in Salix viminalis. Variation in larval performance and induced responses within a full-sib S. viminalis family is reported here; 36 sibling plants were completely resistant (larvae died within 48 h after egg hatch, no gall induction), 11 plants were totally susceptible, 25 plants were variable (living and dead larvae present on the same plant). Resistance was associated with HR, but to different degrees; 21 totally resistant genotypes showed typical HR symptoms (many distinct necrotic spots) whereas the remaining 15 genotypes showed no, or very few, such symptoms. Hydrogen peroxide, used as a marker for HR, was induced in genotypes expressing HR symptoms but not in resistant genotypes without symptoms, or in susceptible genotypes. These data suggest that production of hydrogen peroxide, and accompanying cell death, cannot explain larval mortality in the symptomless reaction. Another, as yet unknown, mechanism of resistance may be present. If so, then it is possible that this unknown mechanism also contributes to resistance in plants displaying HR. The apparent complexity observed in this interaction, with both visible and invisible plant responses associated with resistance against an adapted insect species, may have implications for the study of resistance factors in other plant-insect interactions.  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