全文获取类型
收费全文 | 1316篇 |
免费 | 102篇 |
国内免费 | 225篇 |
出版年
2023年 | 18篇 |
2022年 | 33篇 |
2021年 | 44篇 |
2020年 | 54篇 |
2019年 | 59篇 |
2018年 | 66篇 |
2017年 | 61篇 |
2016年 | 61篇 |
2015年 | 55篇 |
2014年 | 50篇 |
2013年 | 146篇 |
2012年 | 56篇 |
2011年 | 60篇 |
2010年 | 55篇 |
2009年 | 74篇 |
2008年 | 62篇 |
2007年 | 78篇 |
2006年 | 72篇 |
2005年 | 53篇 |
2004年 | 52篇 |
2003年 | 56篇 |
2002年 | 45篇 |
2001年 | 34篇 |
2000年 | 30篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 21篇 |
1996年 | 8篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 9篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有1643条查询结果,搜索用时 125 毫秒
101.
广东省野生植物一新记录科——蜡梅科 总被引:10,自引:0,他引:10
报道了广东省野生植物一新记录科--蜡梅科(Calycanthaceae)、一新记录属--蜡梅属(Chimonanthus)及一新记录种--山蜡梅(Chimonanthus nitens Oliv.).标本存放于华南农业大学林学院树木标本室(CANT). 相似文献
102.
103.
Anne M. Sørensen Lars Stemmerik 《Palaeogeography, Palaeoclimatology, Palaeoecology》2007,251(2):198-209
Late Permian bryozoans from the Wegener Halvø, Ravnefjeld and Schuchert Formations in East Greenland have been investigated. 14 genera are recognised.Integration of the new bryozoan data from the Upper Permian of East Greenland with data on the distribution of Permian bryozoans along the northern margin of Pangea is used to test hypotheses concerning Late Palaeozoic evolution of the North Atlantic region. During the Permian, the Atlantic rift system formed a seaway between Norway and Greenland from the boreal Barents Shelf to the warm and arid Zechstein Basin. This seaway is considered to be the only marine connection to the Zechstein Basin and therefore the only possible migration route for bryozoans to enter the basin. The distribution of Permian bryozoans is largely in keeping with such a connection from the cool Barents Shelf past the East Greenland Basin to the warm Zechstein Basin and also corroborates the change in temperature through this connection. 相似文献
104.
Trauth MH Maslin MA Deino AL Strecker MR Bergner AG Dühnforth M 《Journal of human evolution》2007,53(5):475-486
The late Cenozoic climate of East Africa is punctuated by episodes of short, alternating periods of extreme wetness and aridity, superimposed on a regime of subdued moisture availability exhibiting a long-term drying trend. These periods of extreme climate variability appear to correlate with maxima in the 400-thousand-year (kyr) component of the Earth's eccentricity cycle. Prior to 2.7 Ma the wet phases appear every 400 kyrs, whereas after 2.7 Ma, the wet phases appear every 800 kyrs, with periods of precessional-forced extreme climate variability at 2.7-2.5 Ma, 1.9-1.7 Ma, and 1.1-0.9 Ma before present. The last three major lake phases occur at the times of major global climatic transitions, such as the onset of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker Circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1.0-0.7 Ma). High-latitude forcing is required to compress the Intertropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. These periods of extreme climate variability may have provided a catalyst for evolutionary change and driven key speciation and dispersal events amongst mammals and hominins in East Africa. 相似文献
105.
Temple DH 《American journal of physical anthropology》2007,133(4):1035-1046
Current archaeological evidence indicates that greater dietary reliance on marine resources is recorded among the eastern Jomon, while plant dependence prevailed in western/inland Japan. The hypothesis that the dietary choices of the western/inland Jomon will be associated with greater systemic stress is tested by comparing carious tooth and enamel hypoplasia frequencies between the eastern and western/inland Jomon. Demographic collapse coincides with climate change during the Middle to Late Jomon period, suggesting dwindling resource availability. It is hypothesized that this change was associated with greater systemic stress and/or dietary change among the Middle to Late Jomon. This hypothesis is tested by comparing enamel hypoplasia and carious tooth frequencies between Middle to Late and Late to Final Jomon foragers. Enamel hypoplasia was significantly more prevalent among the western/inland Jomon. Such findings are consistent with archaeological studies that argue for greater plant consumption and stresses associated with seasonal resource depletion among the western/inland Jomon. Approximately equivalent enamel hypoplasia frequencies between Middle to Late and Late to Final Jomon foragers argues against a demographic collapse in association with diminished nutritional returns. Significant differences in carious tooth frequencies are, however, observed between Middle to Late and Late to Final Jomon foragers. These results suggest a subsistence shift during the Middle to Late Jomon period, perhaps in response to a changed climate. The overall patterns of stress documented by this study indicate wide-spread environmentally directed biological variation among the prehistoric Jomon. 相似文献
106.
107.
108.
109.
Abundance,diversity and geographic distribution of cassava mosaic disease pandemic‐associated Bemisia tabaci in Tanzania 下载免费PDF全文
L. S. Tajebe S. B. Boni D. Guastella V. Cavalieri O. S. Lund C. P. Rugumamu C. Rapisarda J. P. Legg 《Journal of Applied Entomology》2015,139(8):627-637
Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), one of the most economically important agricultural pests worldwide, is the vector of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD). In East and Central Africa, a severe CMD pandemic that spread from Uganda in the late 1980s still continues to devastate cassava crops. To assess the association of distinct B. tabaci genetic groups with the CMD pandemic, mitochondrial cytochrome oxidase I gene sequences were analysed from whiteflies collected during surveys conducted from 2010 to 2013 in Tanzania. Four genetic groups – Sub‐Saharan Africa 1 (SSA1), Mediterranean, Indian Ocean and East Africa 1, and a group of unknown whitefly species were identified. SSA1 comprised four subgroups: SSA1‐SG1, SSA1‐SG2, SSA1‐SG1/2 and SSA1‐SG3. SSA1‐SG1 was confined to the pandemic‐affected north‐western parts of Tanzania whilst SSA1‐SG2 and SSA1‐SG3 were found in the central and eastern parts not yet affected by the pandemic. The CMD pandemic front was estimated to lie in Geita Region, north‐western Tanzania, and to be spreading south‐east at a rate of ca 26 km/year. The pandemic‐associated B. tabaci SSA1‐SG1 predominated up to 180 km ahead of the CMD front indicating that changes in whitefly population characteristics precede changes in disease characteristics. 相似文献
110.
Diversity of symbiotic bacteria associated with Bemisia tabaci (Homoptera: Aleyrodidae) in cassava mosaic disease pandemic areas of Tanzania 下载免费PDF全文
L.S. Tajebe D. Guastella V. Cavalieri S.E. Kelly M.S. Hunter O.S. Lund J.P. Legg C. Rapisarda 《The Annals of applied biology》2015,166(2):297-310
All Bemisia tabaci individuals harbour an obligate bacterial symbiont (Portiera aleyrodidarum), and many also harbour non‐essential facultative symbionts. The association of symbiotic bacteria with the various genetic groups of B. tabaci remains unknown for East Africa. This study aimed to assess any association between the various whitefly genetic groups and the endosymbionts they harbour; to investigate if a unique endosymbiont is associated with super‐abundant whiteflies, and to provide baseline information on endosymbionts of whiteflies for a part of East Africa. Whiteflies collected during surveys in Tanzania were genotyped and screened for the presence of the obligate and six secondary symbionts (SS): Rickettsia (R), Hamiltonella (H), Arsenophonus (A), Wolbachia (W), Cardinium (C) and Fritschea (F). The results revealed the presence of Mediterranean (MED), East Africa 1 (EA1), Indian Ocean (IO) and Sub‐Saharan Africa 1 (SSA1) genetic groups of Bemisia tabaci, with SSA1 further clustered into four sub‐groups: SSA1‐SG1, SSA1‐SG2, SSA1‐SG1/2 and SSA1‐SG3. F was completely absent from all of the whiteflies tested while R was always found in double or multiple infections. In general, no particular symbiont appeared to be associated with the super‐abundant SSA1‐SG1 B. tabaci, although A or AC infections were common among infected individuals. The most striking feature of these super‐abundant whiteflies, dominating cassava mosaic disease pandemic areas, was the high prevalence of individuals uninfected by any of the six SS tested. This study of the endosymbionts of B. tabaci in East Africa showed contrasting patterns of infection in crop and weed hosts. 相似文献