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   361篇
  免费   7篇
  国内免费   2篇
  2015年   4篇
  2014年   7篇
  2013年   14篇
  2012年   3篇
  2011年   10篇
  2010年   24篇
  2009年   20篇
  2008年   20篇
  2007年   17篇
  2006年   8篇
  2005年   17篇
  2004年   5篇
  2003年   4篇
  2002年   3篇
  2000年   3篇
  1999年   3篇
  1998年   8篇
  1997年   11篇
  1996年   9篇
  1995年   4篇
  1994年   6篇
  1993年   5篇
  1992年   3篇
  1991年   3篇
  1990年   3篇
  1989年   6篇
  1988年   3篇
  1987年   4篇
  1986年   4篇
  1985年   6篇
  1983年   3篇
  1982年   5篇
  1980年   12篇
  1979年   13篇
  1978年   11篇
  1977年   4篇
  1976年   8篇
  1975年   3篇
  1972年   12篇
  1971年   7篇
  1970年   3篇
  1969年   3篇
  1968年   3篇
  1967年   3篇
  1966年   4篇
  1961年   2篇
  1958年   3篇
  1956年   3篇
  1955年   4篇
  1954年   3篇
排序方式: 共有370条查询结果,搜索用时 15 毫秒
41.
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   
42.
43.
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   
44.
45.
46.
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   
47.
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   
48.
Ixodes ricinus is the main vector for important infectious diseases in both humans and in animals. Microsatellite loci were isolated from a dinucleotide‐enriched library made from I. ricinus sampled in Norway. Seventeen polymorphic microsatellites were further characterized among 24 individuals sampled from an island in the Oslofjord region. The number of observed alleles ranged from two to 17 and the observed heterozygosities between 0.10 and 0.83. Analysis of family materials gives evidence of non‐Mendelian inheritance of several of the characterized loci, among which most could be explained by presence of null alleles.  相似文献   
49.
Multiple geographical regions have been proposed for the domestication of Equus caballus . It has been suggested, based on zooarchaeological and genetic analyses that wild horses from the Iberian Peninsula were involved in the process, and the overrepresentation of mitochondrial D1 cluster in modern Iberian horses supports this suggestion. To test this hypothesis, we analysed mitochondrial DNA from 22 ancient Iberian horse remains belonging to the Neolithic, the Bronze Age and the Middle Ages, against previously published sequences. Only the medieval Iberian sequence appeared in the D1 group. Neolithic and Bronze Age sequences grouped in other clusters, one of which (Lusitano group C) is exclusively represented by modern horses of Iberian origin. Moreover, Bronze Age Iberian sequences displayed the lowest nucleotide diversity values when compared with modern horses, ancient wild horses and other ancient domesticates using nonparametric bootstrapping analyses. We conclude that the excessive clustering of Bronze Age horses in the Lusitano group C, the observed nucleotide diversity and the local continuity from wild Neolithic Iberian to modern Iberian horses, could be explained by the use of local wild mares during an early Iberian domestication or restocking event, whereas the D1 group probably was introduced into Iberia in later historical times.  相似文献   
50.
Agricultural intensification is one of the main drivers of farmland bird declines, but effects on birds may be confounded with those of climate change. Here we examine the effects of intensification and climate change on a grassland breeding wader, the Black‐tailed Godwit Limosa l. limosa, in the Netherlands. Population decline has been linked to poor chick survival which, in turn, has been linked to available foraging habitat. Foraging habitat of the nidifugous chicks consists of uncut grasslands that provide cover and arthropod prey. Conservation measures such as agri‐environment schemes aim to increase the availability of chick foraging habitat but have not yet been successful in halting the decline. Field observations show that since the early 1980s, farmers advanced their first seasonal mowing or grazing date by 15 days, whereas Godwits did not advance their hatching date. Ringing data indicate that between 1945 and 1975 hatching dates advanced by about 2 weeks in parallel with the advancement of median mowing dates. Surprisingly, temperature sums at median mowing and hatching dates suggest that while the agricultural advancement before 1980 was largely due to agricultural intensification, after 1980 it was largely due to climate change. Examining arthropod abundance in a range of differently managed grasslands revealed that chick food abundance was little affected but that food accessibility in intensively used tall swards may be problematic for chicks. Our results suggest that, compared with 25 years ago, nowadays (1) a much higher proportion of clutches and chicks are exposed to agricultural activities, (2) there is little foraging habitat left when chicks hatch and (3) because of climate change, the vegetation in the remaining foraging habitat is taller and denser and therefore of lower quality. This indicates that for agri‐environment schemes to make a difference, they should not only be implemented in a larger percentage of the breeding area than the current maxima of 20–30% but they should also include measures that create more open, accessible swards.  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