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   432篇
  免费   10篇
  国内免费   3篇
  2015年   4篇
  2014年   7篇
  2013年   14篇
  2012年   5篇
  2011年   16篇
  2010年   27篇
  2009年   23篇
  2008年   23篇
  2007年   21篇
  2006年   8篇
  2005年   18篇
  2004年   6篇
  2003年   4篇
  2000年   4篇
  1998年   9篇
  1997年   13篇
  1996年   12篇
  1995年   4篇
  1994年   6篇
  1993年   6篇
  1992年   5篇
  1991年   6篇
  1990年   4篇
  1989年   6篇
  1988年   4篇
  1987年   6篇
  1986年   8篇
  1985年   7篇
  1984年   3篇
  1983年   4篇
  1982年   6篇
  1980年   14篇
  1979年   16篇
  1978年   12篇
  1977年   4篇
  1976年   10篇
  1975年   4篇
  1974年   3篇
  1972年   12篇
  1971年   9篇
  1970年   3篇
  1969年   3篇
  1968年   3篇
  1967年   3篇
  1966年   4篇
  1958年   6篇
  1956年   3篇
  1955年   6篇
  1954年   3篇
  1952年   4篇
排序方式: 共有445条查询结果,搜索用时 230 毫秒
41.
Abstract: We document a new record dispersal for wolves worldwide. The natal straight-line dispersal distance of a Global Positioning System-collared female wolf from the Scandinavian population was 1,092 km from southeast Norway to northeast Finland, with a multistage actual travel distance of >10,000 km. Natural gene flow to the isolated, inbred Scandinavian wolf population may occur if survival of dispersers is improved.  相似文献   
42.
Leaf photosynthetic CO2 responses can provide insight into how major nutrients, such as phosphorus (P), constrain leaf CO2 assimilation rates (Anet). However, triose‐phosphate limitations are rarely employed in the classic photosynthesis model and it is uncertain as to what extent these limitations occur in field situations. In contrast to predictions from biochemical theory of photosynthesis, we found consistent evidence in the field of lower Anet in high [CO2] and low [O2] than at ambient [O2]. For 10 species of trees and shrubs across a range of soil P availability in Australia, none of them showed a positive response of Anet at saturating [CO2] (i.e. Amax) to 2 kPa O2. Three species showed >20% reductions in Amax in low [O2], a phenomenon potentially explained by orthophosphate (Pi) savings during photorespiration. These species, with largest photosynthetic capacity and Pi > 2 mmol P m?2, rely the most on additional Pi made available from photorespiration rather than species growing in P‐impoverished soils. The results suggest that rarely used adjustments to a biochemical photosynthesis model are useful for predicting Amax and give insight into the biochemical limitations of photosynthesis rates at a range of leaf P concentrations. Phosphate limitations to photosynthetic capacity are likely more common in the field than previously considered.  相似文献   
43.
44.
Physical disorders are, compared to the general population, more prevalent in people with severe mental illness (SMI). Although this excess morbidity and mortality is largely due to modifiable lifestyle risk factors, the screening and assessment of physical health aspects remains poor, even in developed countries. Moreover, specific patient, provider, treatment and system factors act as barriers to the recognition and to the management of physical diseases in people with SMI. Psychiatrists can play a pivotal role in the improvement of the physical health of these patients by expanding their task from clinical psychiatric care to the monitoring and treatment of crucial physical parameters. At a system level, actions are not easy to realize, especially for developing countries. However, at an individual level, even simple and very basic monitoring and treatment actions, undertaken by the treating clinician, can already improve the problem of suboptimal medical care in this population. Adhering to monitoring and treatment guidelines will result in a substantial enhancement of physical health outcomes. Furthermore, psychiatrists can help educate and motivate people with SMI to address their suboptimal lifestyle, including smoking, unhealthy diet and lack of exercise. The adoption of the recommendations presented in this paper across health care systems throughout the world will contribute to a significant improvement in the medical and related psychiatric health outcomes of patients with SMI.  相似文献   
45.
46.
TORE LARSEN 《Ibis》2000,142(3):476-481
The Bar-tailed Godwit Limosa lapponica rarely defends its nest aggressively against predators, but rather associates with other more aggressive species for protection. I studied the distribution of Bar-tailed Godwit nests relative to nests of the aggressive Longtailed Skua Stercorarius longicaudus, as well as to nests of two slightly less aggressive alternative 'protective umbrella' species, the Whimbrel Numenius phaeopus and the Grey Plover Pluvialis squatarola. Rodents are the main prey of the Long-tailed Skua but, in years when numbers of rodents are low, Long-tailed Skuas also eat eggs and chicks of other birds. Data on rodent densities and nest distribution were collected on Finnmarksvidda, Northern Norway, in 1987, 1989, 1990, 1992 and 1997, and on Taimyr, Siberia, in 1994 and 1995. It is concluded that the Bar-tailed Godwit is able to evaluate the probability of nest predation from Long-tailed Skuas, based on direct or indirect information about the status of local rodent populations. This information is used to choose between Long-tailed Skuas and other potential species for protection against nest predators.  相似文献   
47.
48.
The ecology and social systems of Red Sea butterflyfish were studied along the Sinai coast. Distribution, population density and sociography indicate a preponderance of monogamy in all species, independent of reef zonation and seasonal or long-term changes of the reef environment. The social and mating systems are mutual monogamy; partners remain together for up to 6 years. Links between the monogamy and benthic habits of these fish are indicated by their feeding ecology and by a comparison with Indopacific plankton-feeding species, which usually occur in groups. Territories are defended pairwise. Pair partners nearly always remain within visual contact, but a species-typical distance apart. Pair swimming is independent of season or time of day. Experimental partner removals led to speedy replacement of mates. Possible causes of high partner permanency with little partner fidelity are discussed. In good habitats, single territorial adults do not occur. They would be unable to defend and maintain a territory over long periods. While cooperating in defence and maintenance of a territory, each individual can exploit the resources of the territory including availability of a mate. Pair swimming with mutual partner guarding is considered a joint territorial advertisement similar to duetting in birds; it minimizes agonistic interactions between territory holders. As butterflyfish have planktonic larvae without any broodcare, monogamy could not have evolved in response to broodcare activities as in many other vertebrate groups.  相似文献   
49.
50.
It has been reported in the literature that onion leaf stomatatend to close at noon (Loftfield, 1921) and that the minimumcarbon dioxide concentration () in onion leaf tissue rises from0.012 per cent to 0.024 per cent. When the leaf temperatureincreases from 30°C. to 35°C (light intensity 900.f.c.)(Heath and Orchard, 1957). Experiments were therefore carried out of test the hypothesisthat raising the left temperature above about 30°C. causesstomatal closing movements in onion leaves (e.g. mid-day closure)by increasing the carbon dioxide concentration in the leaf tissue.This hypothesis has been supported by the results of these experiments.Another and diametrically opposed effect of temperature on stomatalmovements in onion leaves has, however, been discovered, Namelythat, provided the final width of stomatal pores.  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