全文获取类型
收费全文 | 142篇 |
免费 | 0篇 |
专业分类
丛书文集 | 1篇 |
教育与普及 | 1篇 |
现状及发展 | 33篇 |
研究方法 | 18篇 |
综合类 | 83篇 |
自然研究 | 6篇 |
出版年
2020年 | 2篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2014年 | 1篇 |
2013年 | 1篇 |
2012年 | 4篇 |
2011年 | 12篇 |
2008年 | 5篇 |
2007年 | 6篇 |
2006年 | 9篇 |
2005年 | 9篇 |
2004年 | 3篇 |
2003年 | 1篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 9篇 |
1999年 | 2篇 |
1995年 | 1篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 6篇 |
1969年 | 6篇 |
1968年 | 1篇 |
1967年 | 4篇 |
1966年 | 2篇 |
1965年 | 3篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有142条查询结果,搜索用时 15 毫秒
41.
42.
43.
Berger MF Lawrence MS Demichelis F Drier Y Cibulskis K Sivachenko AY Sboner A Esgueva R Pflueger D Sougnez C Onofrio R Carter SL Park K Habegger L Ambrogio L Fennell T Parkin M Saksena G Voet D Ramos AH Pugh TJ Wilkinson J Fisher S Winckler W Mahan S Ardlie K Baldwin J Simons JW Kitabayashi N MacDonald TY Kantoff PW Chin L Gabriel SB Gerstein MB Golub TR Meyerson M Tewari A Lander ES Getz G Rubin MA Garraway LA 《Nature》2011,470(7333):214-220
44.
Overfishing of large-bodied benthic fishes and their subsequent population collapses on the Scotian Shelf of Canada's east coast and elsewhere resulted in restructuring of entire food webs now dominated by planktivorous, forage fish species and macroinvertebrates. Despite the imposition of strict management measures in force since the early 1990s, the Scotian Shelf ecosystem has not reverted back to its former structure. Here we provide evidence of the transient nature of this ecosystem and its current return path towards benthic fish species domination. The prolonged duration of the altered food web, and its current recovery, was and is being governed by the oscillatory, runaway consumption dynamics of the forage fish complex. These erupting forage species, which reached biomass levels 900% greater than those prevalent during the pre-collapse years of large benthic predators, are now in decline, having outstripped their zooplankton food supply. This dampening, and the associated reduction in the intensity of predation, was accompanied by lagged increases in species abundances at both lower and higher trophic levels, first witnessed in zooplankton and then in large-bodied predators, all consistent with a return towards the earlier ecosystem structure. We conclude that the reversibility of perturbed ecosystems can occur and that this bodes well for other collapsed fisheries. 相似文献
45.
46.
In autumn 1998 stream metabolism was measured in the Hassayampa River, Arizona, a Sonoran Desert stream, using single-station diel oxygen curves and an oxygen mass balance model. Oxygen consumption rates of parafluvial and channel sediments were determined with respiration chambers. Bedload of channel sediments (sand) prevented significant primary production by benthic autotrophs, despite favorable nutrient, light, and temperature conditions. Ecosystem respiration was relatively low (1.50 g O 2 m -2 d -1 ) and presumably fueled by production in the riparian zone and riverine marshes. Respiration rates in the parafluvial zone and in channel sediments ranged from 0.6 to 1.4 g O 2 m -3 sediment h -1 . Sediment organic matter (ash-free dry mass) was 4.0 ± 1.8 kg m -3 sediment and did not significantly differ between the channel and the parafluvial zone. Results indicate that heterotrophic processes may dominate the metabolism of desert stream segments over extended periods of time if unstable sandy bed sediments prevail. 相似文献
47.
Biologically diverse molecular variants within a single HIV-1 isolate 总被引:55,自引:0,他引:55
A G Fisher B Ensoli D Looney A Rose R C Gallo M S Saag G M Shaw B H Hahn F Wong-Staal 《Nature》1988,334(6181):444-447
AIDS is a disorder characterized by a slow progressive impairment of immune function and by infection of human immunodeficiency viruses (HIV-1, HIV-2). Our knowledge of how these viruses cause disease in man, or how the related lentiviruses (visna and equine infectious anaemia virus) cause disease in animals, is still fragmentary. In particular, the significance of genetic variation in HIV-1, occurring within populations, within individuals and over periods of time, and the mechanisms of viral persistence remain unclear. To address these issues we prepared a series of proviral clones of HIV-1 originating from a single patient and compared their biological properties. Here we show that hybrid genomes (in which the envelope region of six viral clones were separately substituted into a prototype HIV-1 genome) generated viruses with widely differing capacity to grow in human T cells, cell lines and monocytoid cultures. These data suggest that extensive biological variation exists in vivo within an infected individual and is in part determined at the level of the viral envelope. 相似文献
48.
A molecular clone of HTLV-III with biological activity 总被引:5,自引:0,他引:5
Acquired immune deficiency syndrome (AIDS) is an epidemic immunosuppressive disease characteristically associated with a depletion of T lymphocytes of the helper/inducer phenotype. Numerous converging lines of research have implicated a human T-cell lymphotropic retrovirus, HTLV-III, in the pathogenesis of AIDS. Recently, several distinct forms of the HTLV-III genome were molecularly cloned in phage and extensively characterized. In the present study, a clone containing full-length HTLV-III proviral DNA was inserted into a plasmid and used to transfect cord blood T cells from normal newborn humans. We demonstrate that this molecular clone is infectious in vitro and causes marked cytopathic effects on T-cell cultures. This is the first direct evidence that the HTLV-III genome, rather than a minor component of the virus complex, is cytopathic for T cells. Using this biologically competent clone and mutants derived from it, it should now be possible to localize the subgenomic regions that contribute to the biological effects of HTLV-III. 相似文献
49.
F. E. Vega P. Barbosa H. L. Kuo-Sell D. B. Fisher T. C. Nelsen 《Cellular and molecular life sciences : CMLS》1995,51(3):293-299
Insect-transmitted plant pathogens can have positive, negative or no effects on the vector insect. Effects could be direct (caused by the pathogen) or indirect (mediated by changes in the plant). Pathogen effects on non-vector insects are less well known.Peregrinus maidis (Ashmead) (Homoptera: Delphacidae), an insect that is not a vector of the corn stunt spiroplasma (CSS), weighed less 15 days after feeding on CSS-infected corn plants relative to insects feeding on healthy corn plants. Fecundity of non-vector insects that were removed from diseased plants was higher than for insects coming from healthy plants. ForDalbulus maidis (DeLong & Wolcott) (Homoptera: Cicadellidae), an insect that transmits CSS, there were no differences in weight, longevity, eggs per day, weekly or total fecundity after feeding on healthy or on CSS-infected corn plants. Significant differences in some phloem amino acids were detected between healthy and CSS-infected plants. Infected plants also showed an increased phloem acidity as disease symptoms progressed. Feeding on CSS-infected corn plants by an insect that does not vector the pathogen infecting the plant can have favorable consequences as evidenced by an increased fecundity.Scientific aritcle no. 8819, contribution no. A-6605 of the Maryland Agricultural Experiment Station. 相似文献
50.
D. Robeson G. Strobel G. K. Matusumoto E. L. Fisher M. H. Chen J. Clardy 《Cellular and molecular life sciences : CMLS》1984,40(11):1248-1250
Summary The phytopathogenic fungusAlternaria eichorniae attacks water hyacinth, an economically significant aquatic weed. The novel phytotoxin alteichin was isolated from liquid cultures of this fungus and its structure was deduced by X-ray crystallographic analysis. Altheichin is a doubly hydrated form of 4,9-dihydroxy perylene-3,10-quinone. A single step dehydration of alteichin to anhydroalteichin is catalyzed both by acid and by a crude enzyme preparation from water hyacinth.We thank Dr R. N. Strange for the protoplast assay. Funding was provided by BARD 1-31-79, NIH-CA 24487, Bristol Laboratories, the Montana Agricultural Expt. Station, and NSF. 相似文献