全文获取类型
收费全文 | 6092篇 |
免费 | 88篇 |
国内免费 | 6篇 |
出版年
2013年 | 132篇 |
2012年 | 126篇 |
2011年 | 182篇 |
2010年 | 277篇 |
2009年 | 260篇 |
2008年 | 258篇 |
2007年 | 327篇 |
2006年 | 183篇 |
2005年 | 205篇 |
2004年 | 102篇 |
2003年 | 65篇 |
2002年 | 46篇 |
2001年 | 53篇 |
2000年 | 53篇 |
1999年 | 73篇 |
1998年 | 122篇 |
1997年 | 169篇 |
1996年 | 141篇 |
1995年 | 126篇 |
1994年 | 107篇 |
1993年 | 126篇 |
1992年 | 107篇 |
1991年 | 125篇 |
1990年 | 129篇 |
1989年 | 131篇 |
1988年 | 114篇 |
1987年 | 131篇 |
1986年 | 107篇 |
1985年 | 100篇 |
1984年 | 89篇 |
1983年 | 101篇 |
1982年 | 68篇 |
1981年 | 79篇 |
1980年 | 83篇 |
1979年 | 106篇 |
1978年 | 83篇 |
1977年 | 74篇 |
1976年 | 60篇 |
1975年 | 85篇 |
1974年 | 73篇 |
1973年 | 103篇 |
1972年 | 121篇 |
1971年 | 107篇 |
1970年 | 46篇 |
1969年 | 47篇 |
1968年 | 42篇 |
1957年 | 41篇 |
1955年 | 40篇 |
1954年 | 47篇 |
1953年 | 43篇 |
排序方式: 共有6186条查询结果,搜索用时 15 毫秒
1.
1. Few detailed long-term data sets exist for fresh waters with which to examine large-scale temporal changes in community composition. Consequently, insight into community persistence has been restricted to a few, contingent case studies. We collated and analysed data for the aquatic macroinvertebrate community of Broadstone Stream in south-east England, spanning three decades. The pH of this naturally acid stream has risen progressively since the 1970s, and we sought to examine the potential effects of this environmental change upon the community.
2. Persistence within Broadstone was high when compared with other systems that have been analysed using similar methods. The stream was characterised by a `core' community of eight taxa that were always present, and contributed 75–97% of total invertebrate abundance, with a trailing limb of progressively rarer and more acid-sensitive taxa. There was little species turnover, although the time-series exceeded 20 generations for most species.
3. Despite this high persistence, a long-term response to rising pH was detected: species indicating profound acidity (identified a priori from independent studies) have declined since the 1970s, whereas indicators of moderate acidity increased. The structure of the community food web has also changed since the 1970s, with increased predator diversity and abundance, and a lengthening of food chains following the invasion of a new top predator.
4. These changes in the community appeared to be driven by an interaction between pH and climate. The unusually hot, dry summers characteristic of the 1990s may have raised pH during the more sensitive (i.e. early) stages of the life-cycle, and thus provided a window of opportunity for less acid-tolerant taxa to colonise and become established. Changes in pH appeared to set the boundaries of the available local species pool, within which biotic interactions ultimately shaped the community. 相似文献
2. Persistence within Broadstone was high when compared with other systems that have been analysed using similar methods. The stream was characterised by a `core' community of eight taxa that were always present, and contributed 75–97% of total invertebrate abundance, with a trailing limb of progressively rarer and more acid-sensitive taxa. There was little species turnover, although the time-series exceeded 20 generations for most species.
3. Despite this high persistence, a long-term response to rising pH was detected: species indicating profound acidity (identified a priori from independent studies) have declined since the 1970s, whereas indicators of moderate acidity increased. The structure of the community food web has also changed since the 1970s, with increased predator diversity and abundance, and a lengthening of food chains following the invasion of a new top predator.
4. These changes in the community appeared to be driven by an interaction between pH and climate. The unusually hot, dry summers characteristic of the 1990s may have raised pH during the more sensitive (i.e. early) stages of the life-cycle, and thus provided a window of opportunity for less acid-tolerant taxa to colonise and become established. Changes in pH appeared to set the boundaries of the available local species pool, within which biotic interactions ultimately shaped the community. 相似文献
2.
THE TIMING OF DIVISION IN CHLAMYDOMONAS 总被引:3,自引:2,他引:1
3.
Sequential medical trials involving paired data 总被引:2,自引:0,他引:2
4.
Drought-stressed flatpea (Lathyrus sylvestris L.) plants from8 to 22 weeks old were analysed for nitrogen, soluble proteinand free amino acids. An increase in nitrogen and free aminoacid concentrations and a decrease in soluble protein levelwere observed in roots of plants up to 16 weeks old. The cumulativeconcentration of free amino acids increased with drought stress.Tissue concentrations of 2, 4-diaminobutyric acid (1.62.6%of the dry weight) were highest in leaves. Levels increasedsteadily, nearly doubling, in leaves and stems between weeks10 and 16. Levels in drought-stressed leaves were, on average,11.9% higher than those of controls. Estimated concentrationsof a mixture of 4-aminobutyric acid and an unknown amino acidwere highest in stems, increased in this tissue with age andtended to increase in stems and leaves and decrease in rootsin response to water deficit. Levels of the mixture of homoserineand another unidentified amino acid were not influenced by ageor water status of the plants. Root concentrations of asparagine,arginine, glutamine, aspartate, and another prominent, unidentifiedamino acid increased with plant age and reached a peak at thetime of flowering (14 to 18 weeks). Only the concentration ofthe unknown compound was elevated following drought stress.Concentrations of valine, isoleucine, leucine, phenylalanine,and methionine also increased during this period and were elevatedin drought-stressed plants. Proline levels increased with plantage and drought stress, but proline accounted for only about10% of the total free amino acids in the drought-stressed plants. Key words: 2, 4-Diaminobutyric acid, drought, flatpea 相似文献
5.
6.
The presence of the internal parasite (Aphidius matricariae) of the aphid Myzus persicae can be identified by electrophoresis, and staining of several enzyme systems, of which malate dehydrogenase is recommended as the most reliable. It is suggested that the technique could be extended to other small insects, and that pest populations can be screened for percentage parasitism as an adjunct to insecticide and integrated control field trials. 相似文献
7.
8.
ROBIN M. OVERSTREET WILLIAM E. HAWKINS JOHN W. FOURNIE 《The Journal of eukaryotic microbiology》1984,31(2):332-339
Calyptospora n. g. was erected for Eimeria funduli because the sporocyst of that species lacks Stieda and sub-Stieda bodies, has a veil supported by sporopodia, and has an anterior apical opening. A suture may be present, but it does not completely divide the sporocyst into two valves. Because C. funduli has an obligatory invertebrate intermediate host, we established Calyptosporidae n. fam. to accommodate Calyptospora and tentatively to accept Goussia. A new subgenus, Plagula, is erected for species of Goussia with a sporocystic veil not supported by sporopodia. We consider the family more primitive than Eimeriidae, Sarcocystidae, and possibly Lankesterellidae. 相似文献
9.
NITROGEN METABOLISM IN POTERIUM SANGUISORBA DURING WATER STRESS 总被引:1,自引:1,他引:0
10.
Decoupled evolution of display morphology and display behaviour in phrynosomatid lizards 总被引:1,自引:0,他引:1
JOHN J. WIENS 《Biological journal of the Linnean Society. Linnean Society of London》2000,70(4):597-612
A widespread trend in animals is the evolution of morphological ornaments and behaviours that are involved in aggressive and courtship displays. These display traits are important from the standpoint of communication, sexual selection, and speciation. Previous authors have suggested that the evolution of display morphology and display behaviour should be closely linked. In this study, I tested for this association with behavioural and morphological data for 59 taxa of phrynosomatid lizards using phylogenetic comparative methods (Mad-dison's concentrated changes test and Felsenstein's independent contrasts). The results showed little significant association between features of display morphology and behaviour, suggesting that the evolution of these traits is not tightly coupled. This decoupling is particularly evident in the genus Sceloporus , in which several species have lost the display coloration but retain unmodified display behaviour. The results also suggest that display morphology is more evolutionarily labile than display behaviour in this group. 相似文献