全文获取类型
收费全文 | 426篇 |
免费 | 35篇 |
国内免费 | 6篇 |
出版年
2023年 | 6篇 |
2022年 | 4篇 |
2021年 | 11篇 |
2019年 | 4篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 9篇 |
2015年 | 10篇 |
2014年 | 13篇 |
2013年 | 18篇 |
2012年 | 24篇 |
2011年 | 19篇 |
2010年 | 11篇 |
2009年 | 11篇 |
2008年 | 16篇 |
2007年 | 12篇 |
2006年 | 23篇 |
2005年 | 18篇 |
2004年 | 15篇 |
2003年 | 17篇 |
2002年 | 18篇 |
2001年 | 5篇 |
2000年 | 8篇 |
1999年 | 16篇 |
1998年 | 6篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1993年 | 4篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 12篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1971年 | 3篇 |
1970年 | 8篇 |
1968年 | 3篇 |
1966年 | 3篇 |
1958年 | 3篇 |
1946年 | 3篇 |
排序方式: 共有467条查询结果,搜索用时 15 毫秒
1.
Tryptophan, tryptamine, or indolepyruvic acid were applied to 2 systems: a bacterial (pea stem sections containing the epiphytic bacteria) and a plant system (pea stem sections under sterile conditions). In the plant system, the production of indoleacetic acid and indoleethanol (tryptophol) from each applied indole derivative is clearly reduced by the aldehyde reagents bisulfite and dimedon, respectively. Indoleacetaldehyde is chromatographically detected after alkaline liberation from its bisulfite addition product. In the bacterial system, the production of indoleacetic acid and indoleethanol is likewise reduced by bisulfite and dimedon. However, after tryptophan or tryptamine application, we could not detect indoleacetaldehyde in the described way. In one case only, namely tryptamine application to the bacterial system, indoleethanol production (contrary to indoleacetic acid production) is scarcely reduced by the aldehyde reagents. This indicates a bacterial pathway tryptamine → indoleethanol which bypasses indoleacetaldehyde. 相似文献
2.
Summary An analysis was made of the effects of different sampling and extraction techniques on the amounts and pattern of monoterpenoids isolated from needles of Norway spruce. The following isolation and analysis procedure was finally adopted: liquid nitrogen-cooled needles were pulverized by a microdismembrator, extracted with pentane overnight at 2°–3°C and concentrated to a volume not less than 3 ml/g fresh weight on a Vigreux column. The crude extract was injected splitless (with solvent split) onto a cold programmed temperature vaporized (PTV) precolumn of a gas chromatograph and the vaporizable compounds heated to a capillary column. This method was tested for production of artefacts and quantitative extraction and applied to needles of eleven 80-year-old spruce trees. 相似文献
3.
4.
Conjugation to an antifibrin monoclonal antibody enhances the fibrinolytic potency of tissue plasminogen activator in vitro 总被引:1,自引:0,他引:1
Tissue plasminogen activator (tPA) was covalently linked by disulfide bonds to a monoclonal antibody specific for the amino terminus of the beta chain of fibrin (antibody 59D8). The activity of the tPA-59D8 conjugate was compared with that of tPA, urokinase (UK), and a UK-59D8 conjugate. For lysis of fibrin monomer, tPA was 10 times as potent as UK, whereas both UK-59D8 and tPA-59D8 conjugates were 100 times as potent as UK and 10 times as potent as tPA. Conjugation of tPA or UK to antibody 59D8 produced a 3.2-4.5-fold enhancement in clot lysis in human plasma over that of the respective unconjugated plasminogen activator. However, the UK-59D8 conjugate was only as potent as tPA alone. Antibody-conjugated tPA or UK consumed less fibrinogen, alpha 2-antiplasmin, and plasminogen than did the unconjugated activators, at equipotent fibrinolytic concentrations. Antibody targeting thus appears to increase the concentration of tPA in the vicinity of a fibrin deposit, which thereby leads to enhanced fibrinolysis. 相似文献
5.
Fritz Runge 《Plant Ecology》1969,17(1):151-156
Summary Following the dry year of 1959 considerable damage was done to a number of beeches in a Melico-Fagetum wood in the Drübel area near Brilon (Sauerland). The effects of this damage coupled with the subsequent removal of the affected trees were studied over an 8-year period with a permanent quadrat and revealed the following vegetation cycle: pure or almost pure beechwood — dying or cutting down of single beeches and with it an opening up of the tree canopy — an increase in the abundance and flowering of wild strawberry (Fragaria vesca) and other lightloving species — an increase in the density and growth of sycamore (Acer) and ash (Fraxinus) saplings — a decrease in the amount of wild strawberry and indeed of other light-loving species too because of the increasing shade — development of beech seedlings into young trees under the shadow of the other shrubs and trees — probably a sycamoreash-beech stand — probably a pure or almost pure becchwood again.
Zusammenfassung Das Dürrejahr 1959 verursachte in einem Melico-Fagetum des Drübels bei Brilon erhebliche Schäden an mehreren Buchen. Diese Schäden und die folgende Abholzung der erkrankten Bäume lösten, wie 8jährige Untersuchungen in einem Dauerquadrat ergaben, folgenden Vegetationsyklus aus: Reiner oder fast reiner Buchen-Hochwald — Absterben bzw. Schlag einzelner Buchen und damit Auflockerung der Baumschicht — Vermehrung bzw. Blüte der Walderdbeere und anderer lichtliebender Arten — Verdichtung und schnelles Emporwachsen des Ahorn- und Eschen-Jungwuchses — Abnahme der Erdbeere, wohl auch anderer licht-liebender Arten infolge wieder zunehmender Beschattung — Heranwachsen von Buchen-Keimlingen zu Jungpflanzen, und zwar im Schatten der anderen Sträucher und Bäume — voraussichtlich Ahorn-Eschen-Buchen-Bestand — voraussichtlich wieder reiner oder fast reiner Buchen-Hochwald.相似文献
6.
Substrate-dependent transport of the NADPH:protochlorophyllide oxidoreductase into isolated plastids. 总被引:13,自引:3,他引:10 下载免费PDF全文
The key regulatory enzyme of chlorophyll biosynthesis in higher plants, the light-dependent NADPH:protochlorophyllide oxidoreductase (POR), is a nuclear-encoded plastid protein. Its post-translational transport into plastids is determined by its substrate. The precursor of POR (pPOR) is taken up and processed to mature size by plastids only in the presence of protochlorophyllide (Pchlide). In etioplasts, the endogenous level of Pchlide saturates the demands for pPOR translocation. During the light-induced transformation of etioplasts into chloroplasts, the Pchlide concentration declined drastically, and isolated chloroplasts rapidly lost the ability to import the precursor enzyme. The chloroplasts' import capacity for the pPOR, however, was restored when their intraplastidic level of Pchlide was raised by incubating the organelles in the dark with delta-aminolevulinic acid, a common precursor of tetrapyrroles. Additional evidence for the involvement of intraplastidic Pchlide in regulating the transport of pPOR into plastids was provided by experiments in which barley seedlings were grown under light/dark cycles. The intraplastidic Pchlide concentration in these plants underwent a diurnal fluctuation, with a minimum at the end of the day and a maximum at the end of the night period. Chloroplasts isolated at the end of the night translocated pPOR, whereas those isolated at the end of the day did not. Our results imply that the Pchlide-dependent transport of the pPOR into plastids might be part of a novel regulatory circuit by which greening plants fine tune both the enzyme and pigment levels, thereby avoiding the wasteful degradation of the imported pPOR as well as photodestruction of free Pchlide. 相似文献
7.
8.
Seasonal fluctuations in the diversity and compositional stability of phytoplankton communities in small lakes in upper Bavaria 总被引:2,自引:2,他引:0
Roswitha Holzmann 《Hydrobiologia》1993,249(1-3):101-109
A study of diversity and compositional stability of phytoplankton communities during one vegetation period was carried out in small lakes in upper Bavaria. Shannon-Weaver diversity index was calculated on the base of number of individuals and on the base of biomass. On average, the diversity (annual mean) was highest in mesotrophic lakes. A comparison of three morphologically different (esp. exposure to wind, surface area and mean depth) lakes (Pelhamer See, Thalersee and Kautsee) sought to find out how the phytoplankton community structure reacts to events of intermediate disturbance, in terms of diversity- or biomass changes. Principal Component Analysis was used to measure the persistence of the phytoplankton association. The examples given in this paper led to these conclusions: High diversity or increase in diversity occur in compositionally instable communities, in high wind-stress events, with small algae and with high grazing. Low diversity or decrease in diversity occur in compositionally stable periods, when conditions select few species, as large forms dominate and as grazing by zooplankton takes effect. Gradual seasonal changes are observed in structurally stable lakes. In lakes exposed to frequent disturbances, seasonal changes may be dominated by intermediate responses. 相似文献
9.
Mohammadjavad Haghighatnia Antonin Machac Roswitha Schmickl Clément Lafon Placette 《Biological reviews of the Cambridge Philosophical Society》2023,98(6):1928-1944
Sexual selection is considered one of the key processes that contribute to the emergence of new species. While the connection between sexual selection and speciation has been supported by comparative studies, the mechanisms that mediate this connection remain unresolved, especially in plants. Similarly, it is not clear how speciation processes within plant populations translate into large-scale speciation dynamics. Here, we review the mechanisms through which sexual selection, pollination, and mate choice unfold and interact, and how they may ultimately produce reproductive isolation in plants. We also overview reproductive strategies that might influence sexual selection in plants and illustrate how functional traits might connect speciation at the population level (population differentiation, evolution of reproductive barriers; i.e. microevolution) with evolution above the species level (macroevolution). We also identify outstanding questions in the field, and suitable data and tools for their resolution. Altogether, this effort motivates further research focused on plants, which might potentially broaden our general understanding of speciation by sexual selection, a major concept in evolutionary biology. 相似文献
10.
Maarten Jaap Erik Broekman Selwyn Hoeks Rosa Freriks Merel M. Langendoen Katharina M. Runge Ecaterina Savenco Ruben ter Harmsel Mark A. J. Huijbregts Marlee A. Tucker 《Global Ecology and Biogeography》2023,32(2):198-205