全文获取类型
收费全文 | 3131篇 |
免费 | 148篇 |
国内免费 | 1篇 |
出版年
2023年 | 19篇 |
2021年 | 42篇 |
2020年 | 35篇 |
2019年 | 38篇 |
2018年 | 88篇 |
2017年 | 52篇 |
2016年 | 79篇 |
2015年 | 86篇 |
2014年 | 105篇 |
2013年 | 125篇 |
2012年 | 196篇 |
2011年 | 207篇 |
2010年 | 114篇 |
2009年 | 72篇 |
2008年 | 185篇 |
2007年 | 144篇 |
2006年 | 174篇 |
2005年 | 140篇 |
2004年 | 126篇 |
2003年 | 125篇 |
2002年 | 99篇 |
2001年 | 75篇 |
2000年 | 76篇 |
1999年 | 52篇 |
1998年 | 17篇 |
1995年 | 16篇 |
1994年 | 22篇 |
1993年 | 17篇 |
1992年 | 19篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1988年 | 20篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1984年 | 27篇 |
1983年 | 20篇 |
1982年 | 26篇 |
1981年 | 21篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 22篇 |
1977年 | 14篇 |
1976年 | 20篇 |
1975年 | 17篇 |
1974年 | 34篇 |
1973年 | 17篇 |
1972年 | 23篇 |
1968年 | 17篇 |
1959年 | 19篇 |
排序方式: 共有3280条查询结果,搜索用时 343 毫秒
61.
Aims
Shallow soils on acidic bedrock in dry areas of Central Europe support dry grasslands and heathlands that were formerly used as extensive pastures. These habitats are of high conservation value, but their abandonment in the 20th century triggered slow natural succession that poses a threat to specialized plant species. We asked how this vegetation and its plant diversity have changed over the past three decades and whether protected areas have positively affected habitat quality.Location
Southwestern and central Moravia, Czech Republic.Methods
In 2018–2019, we resurveyed 94 vegetation plots first sampled in 1986–1991 at 47 acidic dry grassland and heathland sites. We compared the number of all vascular plant species, Red List species and alien species per plot using parametric and non-parametric tests, life-form spectra using the chi-square test, species composition using detrended correspondence analysis, and indicator values using a permutation test. We also compared these changes between sites within and outside protected areas.Results
Vegetation changes over the past three decades have been relatively small. However, we detected a decrease in total species richness, the number of Red List species and the number of characteristic species of dry grasslands. Neophytes were infrequent, while archaeophytes increased slightly. The competitive tall grass Arrhenatherum elatius, annual species and young woody plants increased in abundance or newly established at many sites. Indicator values did not change except for a slight increase in nutrient values. These negative trends occurred both within and outside protected areas but were more pronounced outside.Conclusions
Formerly grazed acidic dry grasslands and heathlands in Moravia are slowly losing habitat specialists, including threatened plant species, and are increasingly dominated by Arrhenatherum elatius. Conservation management, especially cutting in protected areas, slows down the negative trends of decline in plant diversity and habitat quality but is insufficient to halt these processes completely. 相似文献62.
63.
Łukasz Wejnerowski Ewa Poniecka Jakub Buda Piotr Klimaszyk Agnieszka Piasecka Marcin Krzysztof Dziuba Gianmarco Mugnai Nozomu Takeuchi Krzysztof Zawierucha 《Journal of phycology》2023,59(5):939-949
Cryoconite, the dark sediment on the surface of glaciers, often aggregates into oval or irregular granules serving as biogeochemical factories. They reduce a glacier's albedo, act as biodiversity hotspots by supporting aerobic and anaerobic microbial communities, constitute one of the organic matter (OM) sources on glaciers, and are a feeder for micrometazoans. Although cryoconite granules have multiple roles on glaciers, their formation is poorly understood. Cyanobacteria are ubiquitous and abundant engineers of cryoconite hole ecosystems. This study tested whether cyanobacteria may be responsible for cryoconite granulation as a sole biotic element. Incubation of Greenlandic, Svalbard, and Scandinavian cyanobacteria in different nutrient availabilities and substrata for growth (distilled water alone and water with quartz powder, furnaced cryoconite without OM, or powdered rocks from glacial catchment) revealed that cyanobacteria bind mineral particles into granules. The structures formed in the experiment resembled those commonly observed in natural cryoconite holes: they contained numerous cyanobacterial filaments protruding from aggregated mineral particles. Moreover, all examined strains were confirmed to produce extracellular polymeric substances (EPS), which suggests that cryoconite granulation is most likely due to EPS secretion by gliding cyanobacteria. In the presence of water as the only substrate for growth, cyanobacteria formed mostly carpet-like mats. Our data empirically prove that EPS-producing oscillatorialean cyanobacteria isolated from the diverse community of cryoconite microorganisms can form granules from mineral substrate and that the presence of the mineral substrate increases the probability of the formation of these important and complex biogeochemical microstructures on glaciers. 相似文献
64.
L. Dugovičová L. Fojtík V. Novak Z. Gálová Z. Gálová M. Lejko I. Michalík J. Honza J. Minář J. Zehnálek A. Kostrej I. Kousalová M. Kováčová M. Lexa D. Miklovič M. Kováčová L. Nátr Z. Nátrová V. Novák Y. Kumar M. Novotná F. Plhák E. Tománková J. Zehnálek J. Černohorská M. Dvořák D. Harmancová 《Biologia Plantarum》1992,34(1):555
65.
Copy numbers of the streptomycete plasmid vector pRS410 and five other recombinant plasmid derivatives of the original cryptic
streptomycete plasmid pSLG33 were determined using calibrated laser densitometry. DNA preparations, electrophoretically separated
on agarose gels, were stained with ethidium bromide, photographed and the negatives were subsequently scanned in a laser densitometer.
The pSLG33 replicon is very stable, as no effect of the selective pressure was observed. It is a multicopy plasmid with up
to 220 detected copies per chromosome. The use of deletion and/or insertion mutants allowed us to define two regions of the
pSLG33 molecule involved in the control of plasmid replication. 相似文献
66.
Witold Lasek Anna Wańkowicz Katarzyna Kuc Wojciech Feleszko Jakub Golab Adam Giermasz Wiesŀaw Wiktor-J/cedrzejczak Marek Jakóbisiak 《Cancer immunology, immunotherapy : CII》1995,40(5):315-321
The efficacy of systemic infusion of recombinant human macrophage-colony-stimulating factor (M-CSF) in combination with local treatment with human recombinant tumor necrosis factor (TNF) and mouse recombinant interferon (IFN) was studied in vivo on a subclone of B16 melanoma (MmB16) in mice. Short-term intravenous administration of M-CSF at a dose of 106 units daily had no antitumor effect in vivo. Similarly, local treatment of tumor with TNF (5 g daily) did not produce any therapeutic effect. However, simultaneous administration of the same dose of TNF with IFN (1000 units daily) resulted in a synergistic effects manifested by the retardation of tumor growth. Addition of systemic infusion of M-CSF to the local therapy with TNF and IFN induced further augmentation of antitumor efficacy and delayed progression of MmB16 melanoma. The strengthened antitumor effect of combination therapy including M-CSF, TNF and IFN was most probably due to the increased release of monocytes from the bone marrow, their recruitment into the site of tumor growth and subsequent local stimulation of their antitumor activity. 相似文献
67.
A. E. Retief J. A. Van Zyl R. Menkveld M. F. Fox G. M. Kotzè J. Brusnický 《Human genetics》1984,66(2-3):162-164
Summary Chromosome studies were performed on 106 men with azoospermia and 390 men with oligozoospermia (consistant sperm count below 10 million/ml). Constitutional chromosome abnormalities were found in 14.1% of the azoospermia group and in 5.1% of the oligozoospermia group. An overall incidence of 7.1% constitutional abnormalities indicates that this criterion of selection may be advisable for routine chromosome analysis of infertile men. A reduction of 25% in the workload increases the yield of chromosome abnormalities in the group of infertile men to 10–14 times above that expected in the normal population. 相似文献
68.
Zusammenfassung In der vorgelegten Arbeit wurde die sogenante deduktive Klassifikationsmethode der anthropogenen Pflanzengesellschaften behandelt. Diese Methode geht vom System der höheren Vegetationseinheiten der Braun-Blanquet Schule aus. Sie ermöglicht die syntaxonomische Bearbeitung zahlreicher anthropogener Gesellschaften unserer Kulturlandschaft, die nur aus Arten mit breiterer ökologischer und zönologischer Amplitude zusammengesetzt sind, und zwar ohne unerwünschte Verwechslung der Bestimmungsmerkmale einzelner Rangstufen des Systems (Klassen-, Ordnungs-, Verbands- und Assoziationskenn-und trennarten). Die Grundprinzipien der Methode wurden schon früher von Kopecký & Hejný (1971, 1973, 1974) und Kopecký (1976) erläutert. Die vorliegende Arbeit bringt eine präzisierte Auslegung der Methode auf Grund neuer Erfahrungen, die bei der Bearbeitung der spontan entstandenen strassenbegleitenden Gesellschaften derMolinio-Arrhenatheretea Tx. 1937 in Nordostböhmen gewonnen wurden. Aus dem reichen Aufnahmematerial werden nur einige Aufnahmen der Basalgesellschaften mitAlopecurus pratensis und der von ihnen abgeleiteten Derivatgesellschaften mitDactylis glomerata undAgropyron repens fur die anschauliche Demonstration der Methode beigegeben (Tab. 1 und 2). Die Aufnahmen der übrigen Gesellschafts-typen werden in einer anderen Arbeit (Kopecký 1978) veröffentlicht.
Summary Use of a so-called deductive method in syntaxonomic classification of roadside plant communities in NE Bohemia.The present paper explains the so-called deductive method of syntaxonomic classification of plant communities, derived from the system of syntaxa of higher rank of the Braun-Blanquet approach. This method enables the synthesis of vegetations units of the varied anthropogenic plant communities in the man-made landscape, and their arrangement within the existing system, without changing the classification and determination criteria of both the higher and lower syntaxa in this system. Basic principles of this method have been outlined earlier by Kopecký & Hejný (1971, 1973, 1974) and Kopecký (1976). Following the newly obtained data and experience, this paper brings further details and more precise explanation of the method. Recently, the deductive method has been applied to the classification of turf communities which spontaneously develop along the roads in NE Bohemia. In order to illustrate the principles of the method, selected relevés of basal communities withAlopecurus pratensis (Table 1), and respective communities derived from these basal communities (Table 2) are presented. A complete set of vegetation relevés of all other roadside communities will be published elsewhere (Kopecký 1978).
Die wissenschaftlichen Pflanzennamen sind nach Rothmaler et al. (1966) angeführt. 相似文献
69.
70.