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   1519篇
  免费   67篇
  国内免费   41篇
  2022年   13篇
  2021年   13篇
  2020年   20篇
  2019年   23篇
  2018年   28篇
  2017年   25篇
  2016年   39篇
  2015年   31篇
  2014年   51篇
  2013年   55篇
  2012年   32篇
  2011年   47篇
  2010年   41篇
  2009年   77篇
  2008年   74篇
  2007年   69篇
  2006年   55篇
  2005年   53篇
  2004年   52篇
  2003年   44篇
  2002年   33篇
  2001年   28篇
  2000年   55篇
  1999年   27篇
  1998年   29篇
  1997年   25篇
  1996年   19篇
  1995年   25篇
  1994年   21篇
  1993年   27篇
  1992年   36篇
  1991年   21篇
  1990年   23篇
  1989年   33篇
  1988年   29篇
  1987年   29篇
  1986年   20篇
  1985年   27篇
  1984年   36篇
  1983年   23篇
  1982年   35篇
  1981年   21篇
  1980年   34篇
  1979年   29篇
  1978年   20篇
  1977年   12篇
  1976年   15篇
  1975年   11篇
  1974年   12篇
  1972年   8篇
排序方式: 共有1627条查询结果,搜索用时 15 毫秒
281.
282.
283.
To establish a relation between biologically effective erythemal radiation (EER) and global solar radiation, the hourly and daily clear-sky broadband (310–2,800 nm) global solar radiation (G) and spectral ultraviolet radiation incident on a horizontal surface at Esfahan, Iran (32°37′N, 51°40′E) were measured during the period 2001–2005. Good correlations at statistically significant levels between the daily values of EER and the daily G were found. The seasonal variability of EER/G is also discussed and the correction factors are determined for inclusion of vertical column ozone and solar zenith angle (SZA) cycles. The comparison of the estimated daily EER against the independent observed EER revealed that under clear sky conditions the estimations are accurate to 10% or better over SZA of 10–60° and column ozone of 250–350 Dobson. The comparison of the results with the similar works that have used shorter period of experimental data showed more accurate estimates. The deduced relations could be used to a rough estimate of the daily EER from G in arid climate regions, where there is no measured UV radiation or there are instrumental and other difficulties encountered in measuring UV radiation.  相似文献   
284.
Doug Bruce  John Biggins 《BBA》1985,810(3):295-301
Linear-dichroism spectra of Anacystis nidulans at 77 K were determined for whole cells chemically fixed in light State 1 and light State 2. Whole cells were oriented by the squeezed gel technique using 5% gelatin 2.2 M sucrose gels. Peaks with positive dichroism were observed at 638 nm and 688 nm with shoulders at approx. 650 nm and 700 nm. The amplitude of the 650 nm shoulder was greater for cells in State 2 than those in State 1, and the State-2-minus-State 1 difference spectrum had a single peak at 656 nm. The linear dichroism spectrum of phycobilisomes isolated from A. nidulans showed peaks at 635 nm (phycocyanin) and 656 nm (allophycocyanin). The spectrum for thylakoid membranes free of phycobilisomes had one peak at 685 nm with a shoulder at 698 nm. We suggest that the change in dichroism at 656 nm between cells in State 1 and State 2 results from a change in orientation of the allophycocyanin core of the phycobilisome. This result is discussed in the context of our model for the light-state transition in phycobilisome-containing organisms.  相似文献   
285.
Recently developed DNA fingerprinting techniques employing “minisatellite” hypervariable regions of DNA proved useful for investigating male reproductive success in Japanese macaques (Macaca fuscata), for which other conventional behavioral or biochemical methods were impracticable. The identified paternity in a captive group indicated that inbreeding was avoided within the same maternal lineage and that females did not tend to give birth to offspring fathered by the same males during their life. It also revealed the possibility of a correlation between male dominance rank and number of offspring.  相似文献   
286.
S. Halle 《Oecologia》1988,75(3):451-455
Summary Pellets of diurnal avian predators (mainly kestrels and buzzards) were analysed to prove the hypothesis of selective predation for a mixed population of small rodents. It was found that voles heavily predominated as prey over mice (up to a factor of 19 during winter). Within both prey species, predation focussed on distinct parts of the populations: during winter the heaviest specimens were preferentially captured, during summer the subadults were in an exposed position. In the voles, an additional predominance of males occurred (up to a factor of 5.2 during summer). These findings verify the assumption of Errington (1956) that speciments of a low social rank are susceptible to the highest risk of predation.  相似文献   
287.
288.
Helichrysum genus consists of about 600 species widespread throughout the world, especially in South Africa and in the Mediterranean area. In this study the aroma profile (HS‐SPME) and the EO compositions of seven Helichrysum species (Hcymosum, Hodoratissimum, Hpetiolare, Hfontanesii, Hsaxatile, Hsanguineum, and Htenax) were evaluated. All the plants were grown in Italy under the same growth conditions. The volatile constituents, particularly monoterpenes, depended by the plant's genotype and ecological adaptation. This study represents the first headspace evaluation on the selected plants and the results evidenced that monoterpenes represented the main class of constituents in five of the seven species analysed (from 59.2% to 95.0%). The higher content in sesquiterpene hydrocarbons was observed in the Mediterranean species of Hsanguineum (68.0%). Only Hsaxatile showed relative similar abundance of monoterpenes and sesquiterpene hydrocarbons. The essential oil composition of the majority of examined species are characterised by high percentage of sesquiterpenes (especially β‐caryophyllene and δ‐cadinene) ranging from 51.3% to 92.0%, except for Hcymosum, Htenax, and Hsanguineum leaves where monoterpenes predominated (from 51.7% to 74.7%).  相似文献   
289.
PurposeInterlaced beams have previously been proposed for delivering proton grid therapy. This study aims to assess dose-averaged LET (LETd) and RBE-weighted dose (DRBE) distributions of such beam geometries, and compare them with conventional intensity modulated proton therapy (IMPT).MethodsIMPT plans and four different interlaced proton grid therapy plans were generated for five patient cases (esophagus, lung, liver, prostate, anus). The constant RBE = 1.1 was assumed for optimization. The LETd was subsequently Monte Carlo calculated for each plan and used as input for two LET-dependent variable RBE models. The fulfilment of clinical goals, along with DVH and spatial distribution evaluations, were then assessed and compared.ResultsAll plans fulfilled the clinical target goals assuming RBE = 1.1. The target coverage was slightly compromised for some grid plans when assuming the variable RBE models. All IMPT plans, and 18 of 20 grid plans, fulfilled all clinical goals for the organs at risk when assuming RBE = 1.1, whereas most plans failed at least one goal when assuming the variable RBE models. Compared with the IMPT plans, the grid plans demonstrated substantially different LETd distributions due to the fundamentally different beam geometries. However, DRBE distributions in the target were similar.ConclusionsDespite the unconventional beam geometries of interlaced proton grid plans, with resulting alternating dose and LETd patterns, the fulfillment of realistic clinical goals seems to be comparable to regular IMPT plans, both assuming RBE = 1.1 and variable RBE models. In addition, the alternating grid patterns do not seem to give rise to unexpected DRBE hot-spots.  相似文献   
290.

Questions

Management practices implemented on road verges are partly established to preserve biodiversity in agricultural landscapes. Their evaluation was primarily based on the analysis of the taxonomic structure and composition of communities. What is the relationship between management practices and the functional characteristics of road–field plants within elements?

Location

West‐central France.

Methods

We sampled the berm, the embankment and the field margin of 40 road–field boundaries located in west‐central France, an area where delayed mowing of some berms has been practised since 2009 for biodiversity reasons. We characterized management practices implemented on the different elements, i.e., the frequency and timing of mowing (early summer or late summer), the frequency of herbicide treatment in field margins and the N input rate. We retrieved from databases seven functional traits and types known to be influenced by management practices. To identify relationships between traits or types and environmental variables we first performed partial RLQ analyses to remove any potential confounding effect of the landscape context studied. We then computed fourth‐corner statistics to quantify relationships between traits or types, environmental variables and partial RLQ axes.

Results

Late mowing of the berm promoted nitrophilous species within berms and competitive rather than ruderal species within arable field margins. The frequency of herbicide treatment in field margins promoted broad‐leaf species within this element and, to a lesser extent, within embankments. Finally, the functional characteristics of communities of the three elements were not influenced by the level of N input in field margins.

Conclusions

In our environmental context, managing road verges affected the functional structure of plant assemblages both within them and within their adjacent arable field margins. We suggest a single early mowing of berms as a valuable practice for both conservation purposes and weed risk control in adjacent field margins.  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