全文获取类型
收费全文 | 327篇 |
免费 | 20篇 |
国内免费 | 1篇 |
专业分类
348篇 |
出版年
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 1篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 12篇 |
2015年 | 17篇 |
2014年 | 19篇 |
2013年 | 23篇 |
2012年 | 12篇 |
2011年 | 21篇 |
2010年 | 16篇 |
2009年 | 11篇 |
2008年 | 16篇 |
2007年 | 10篇 |
2006年 | 17篇 |
2005年 | 12篇 |
2004年 | 9篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2001年 | 14篇 |
2000年 | 3篇 |
1999年 | 17篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 13篇 |
1995年 | 6篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1966年 | 1篇 |
1964年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有348条查询结果,搜索用时 15 毫秒
61.
Coexistence of succulent tree aloes: partitioning of bird pollinators by floral traits and flowering phenology 总被引:1,自引:0,他引:1
Coexistence among species that lack genetic barriers to hybridization usually depends on pre-mating isolating barriers. It has been difficult to explain coexistence among African Aloe species because they readily hybridize, often flower simultaneously and are mostly bird-pollinated. Here we show that co-flowering aloes in a succulent thicket community in South Africa partition the fauna of flower-visiting birds. Aloe species with small amounts of concentrated nectar in long corolla tubes were pollinated primarily by long-billed sunbirds. These species co-flowered with species with large amounts of dilute nectar in short corolla tubes which were pollinated primarily by short-billed, generalist nectarivores. Aloe species which share pollinators tend to have divergent flowering times and differences in pollen placement on birds. Without these isolating barriers, genetic dissolution of sympatric Aloe species would be likely. 相似文献
62.
What predicts the richness of seeder and resprouter species in fire‐prone Cape fynbos: Rainfall reliability or vegetation density? 下载免费PDF全文
Richard M. Cowling Laure Gallien David M. Richardson Fernando Ojeda 《Austral ecology》2018,43(6):614-622
In ecosystems subject to regular canopy fires, woody species have evolved two general strategies of post‐fire regeneration. Seeder species are killed by fire and populations regenerate solely by post‐fire recruitment from a seed bank. Resprouter species survive fire and regenerate by vegetative regrowth from protected organs. Interestingly, the abundance of these strategies varies along environmental gradients and across regions. Two main hypotheses have been proposed to explain this spatial variation: the gap dependence and the environmental‐variability hypotheses. The gap‐dependence model predicts that seeders are favoured in sparse vegetation (vegetation gaps allowing effective post‐fire recruitment of seedlings), while resprouters are favoured in densely vegetated sites (seedlings being outcompeted by the rapid crown regrowth of resprouters). The environmental‐variability model predicts that seeders would prevail in reliable rainfall areas, whereas resprouters would be favoured in areas under highly variable rainfall that are prone to severe dry events (leading to high post‐fire seedling mortality). We tested these two models using distribution data, captured at the scale of quarter‐degree cells, for seeder and resprouter species of two speciose shrub genera (Aspalathus and Erica) common in fire‐prone fynbos ecosystems of the mediterranean‐climate part of the Cape Floristic Region. Contrary to the predictions of the gap‐dependence model, species number of both resprouters and seeders increased with values of the Normalized Difference Vegetation Index (a widely used surrogate for vegetation density), with a more marked increase for seeders. The predictions of the environmental‐variability hypothesis, by contrast, were not refuted by this study. Seeder and resprouter species of both genera showed highest richness in environments with high rainfall reliability. However, with decreasing reliability, seeder numbers dropped more quickly than those of resprouters. We conclude that the environmental‐variability model is better able to explain the abundance of woody seeder and resprouter species in Southern Hemisphere fire‐prone shrublands (fynbos and kwongan) than the gap‐dependence model. 相似文献
63.
Gibberellins control Arabidopsis hypocotyl growth via regulation of cellular elongation 总被引:8,自引:0,他引:8
The gibberellins (GAs) are endogenous regulators of plant growth.
Experiments are described here that test the hypothesis that GA regulates
hypocotyl growth by altering the extent of hypocotyl cell elongation. These
experiments use GA-deficient and altered GA-response mutants of
Arabidopsis thaliana (L.) Heyhn. It is shown that GA
regulates elongation, in both light- and dark-grown hypocotyls, by
influencing the rate and final extent of cellular elongation. However,
light- and dark-grown hypocotyls exhibit markedly different GA
dose-response relationships. The length of dark-grown hypocotyls is
relatively unaffected by exogenous GA, whilst light-grown hypocotyl length
is significantly increased by exogenous GA. Further analysis suggests that
GA control of hypocotyl length is close to saturation in dark-grown
hypocotyls, but not in light grown hypocotyls. The results show that a
large range of possible hypocotyl lengths is achieved via dose-dependent
GA-regulated alterations in the degree of elongation of individual
hypocotyl cells.Key words: Arabidopsis, cell
elongation, gibberellin (GA), GA mutants, hypocotyl.
相似文献
64.
65.
Changes in odor quality discrimination following recovery from olfactory nerve transection 总被引:4,自引:2,他引:2
Following recovery from olfactory nerve transection, animals regain their
ability to discriminate between odors. Odor discrimination is restored
after new neurons establish connections with the olfactory bulb. However,
it is not known if the new connections alter odor quality perception. To
address this question, 20 adult hamsters were first trained to discriminate
between cinnamon and strawberry odors. After reaching criterion (> or =
90% correct response), half of the animals received a bilateral nerve
transection (BTX) and half a surgical sham procedure. Animals were not
tested again until day 40, a point in recovery when connections are
re-established with the bulb. When BTX animals were tested without food
reinforcement, they could not perform the odor discrimination task. Sham
animals, however, could discriminate, demonstrating that the behavioral
response had not been extinguished during the 40 day period. When
reinforcement was resumed, BTX animals were able to discriminate between
cinnamon and strawberry after four test sessions. In addition, their
ability to discriminate between these two familiar odors was no different
than that of BTX and sham animals tested with two novel odors, baby powder
and coffee. These findings suggest that, after recovery from nerve
transection, there are alterations in sensory perception and that
restoration of odor quality discrimination requires that the animal must
again learn to associate individual odor sensations with a behavioral
response.
相似文献
66.
We examined patterns of richness and endemism in relation to phylogeny (family membership), growth form and habitat in the flora of an arid (<60 mm annual rainfall), sandy coastal belt between Port Nolloth and Alexander Bay on the north-west coast of Namaqualand, South Africa. This region forms part of the species-rich Succulent Karoo biome. In an area of 750 km2, 300 plant species were recorded with 24% endemnicity. This level of diversity and endemism is exceptionally high for a desert region. Plant distribution is strongly influenced by habitat type. Diversity, especially of endemic species, was concentrated on rocky or unusual (lag-gravel pavements) habitats. Endemics were not random phylogenetic nor biological assemblages. Dwarf leaf-succulent shrubs, especially Mesembryanthemaceae, were significantly over-represented amongst the endemic component. The results are discussed in terms of a model for plant diversification in an arid environment with relatively predictable sources of moisture. 相似文献
67.
Simulated effects of low atmospheric CO2 on structure and composition of North American vegetation at the Last Glacial Maximum 总被引:1,自引:1,他引:0
Cowling Sharon A. 《Global Ecology and Biogeography》1999,8(2):81-93
1. Physiological experiments have indicated that the lower CO2 levels of the last glaciation (200 μmol mol?1) probably reduced plant water-use efficiency (WUE) and that they combined with increased aridity and colder temperatures to alter vegetation structure and composition at the Last Glacial Maximum (LGM). 2. The effects of low CO2 on vegetation structure were investigated using BIOME3 simulations of leaf area index (LAI), and a two-by-two factorial experimental design (modern/LGM CO2, modern/LGM climate).3. Using BIOME3, and a combination of lowered CO2 and simulated LGM climate (from the NCAR-CCM1 model), results in the introduction of additional xeric vegetation types between open woodland and closed-canopy forest along a latitudinal gradient in eastern North America.4. The simulated LAI of LGM vegetation was 25–60% lower in many regions of central and eastern United States relative to modern climate, indicating that glacial vegetation was much more open than today.5. Comparison of factorial simulations show that low atmospheric CO2 has the potential to alter vegetation structure (LAI) to a greater extent than LGM climate.6. If the magnitude of LAI reductions simulated for glacial North America were global, then low atmospheric CO2 may have promoted atmospheric warming and increased aridity, through alteration of rates of water and heat exchange with the atmosphere. 相似文献
68.
69.
70.
Juliana M Sousa-Canavez Flavio C Canavez Kátia RM Leite Luiz H Camara-Lopes 《Genetic vaccines and therapy》2008,6(1):1-7