全文获取类型
收费全文 | 4337篇 |
免费 | 340篇 |
国内免费 | 1篇 |
出版年
2023年 | 7篇 |
2022年 | 21篇 |
2021年 | 68篇 |
2020年 | 43篇 |
2019年 | 51篇 |
2018年 | 83篇 |
2017年 | 61篇 |
2016年 | 124篇 |
2015年 | 218篇 |
2014年 | 227篇 |
2013年 | 321篇 |
2012年 | 396篇 |
2011年 | 378篇 |
2010年 | 194篇 |
2009年 | 173篇 |
2008年 | 275篇 |
2007年 | 282篇 |
2006年 | 238篇 |
2005年 | 255篇 |
2004年 | 217篇 |
2003年 | 201篇 |
2002年 | 187篇 |
2001年 | 48篇 |
2000年 | 38篇 |
1999年 | 51篇 |
1998年 | 44篇 |
1997年 | 49篇 |
1996年 | 40篇 |
1995年 | 34篇 |
1994年 | 32篇 |
1993年 | 25篇 |
1992年 | 34篇 |
1991年 | 27篇 |
1990年 | 20篇 |
1989年 | 20篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 25篇 |
1985年 | 11篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1969年 | 4篇 |
1965年 | 4篇 |
1927年 | 4篇 |
1925年 | 4篇 |
排序方式: 共有4678条查询结果,搜索用时 15 毫秒
31.
Petra Sumasgutner Susan J. Cunningham Arne Hegemann Arjun Amar Hannah Watson Johan F. Nilsson Martin N. Andersson Caroline Isaksson 《Global Change Biology》2023,29(9):2399-2420
Climate change and urbanisation are among the most pervasive and rapidly growing threats to biodiversity worldwide. However, their impacts are usually considered in isolation, and interactions are rarely examined. Predicting species' responses to the combined effects of climate change and urbanisation, therefore, represents a pressing challenge in global change biology. Birds are important model taxa for exploring the impacts of both climate change and urbanisation, and their behaviour and physiology have been well studied in urban and non-urban systems. This understanding should allow interactive effects of rising temperatures and urbanisation to be inferred, yet considerations of these interactions are almost entirely lacking from empirical research. Here, we synthesise our current understanding of the potential mechanisms that could affect how species respond to the combined effects of rising temperatures and urbanisation, with a focus on avian taxa. We discuss potential interactive effects to motivate future in-depth research on this critically important, yet overlooked, aspect of global change biology. Increased temperatures are a pronounced consequence of both urbanisation (through the urban heat island effect) and climate change. The biological impact of this warming in urban and non-urban systems will likely differ in magnitude and direction when interacting with other factors that typically vary between these habitats, such as resource availability (e.g. water, food and microsites) and pollution levels. Furthermore, the nature of such interactions may differ for cities situated in different climate types, for example, tropical, arid, temperate, continental and polar. Within this article, we highlight the potential for interactive effects of climate and urban drivers on the mechanistic responses of birds, identify knowledge gaps and propose promising future research avenues. A deeper understanding of the behavioural and physiological mechanisms mediating species' responses to urbanisation and rising temperatures will provide novel insights into ecology and evolution under global change and may help better predict future population responses. 相似文献
32.
Jessica N. Hightower Dolly L. Crawford Wayne E. Thogmartin Kyle R. Aldinger Sara Barker Swarthout David A. Buehler John Confer Christian Friis Jeffery L. Larkin James D. Lowe Martin Piorkowski Ronald W. Rohrbaugh Kenneth V. Rosenberg Curtis Smalling Petra B. Wood Rachel Vallender Amber M. Roth 《Diversity & distributions》2023,29(2):254-271
Aim
Climate change is affecting the distribution of species and subsequent biotic interactions, including hybridization potential. The imperiled Golden-winged Warbler (GWWA) competes and hybridizes with the Blue-winged Warbler (BWWA), which may threaten the persistence of GWWA due to introgression. We examined how climate change is likely to alter the breeding distributions and potential for hybridization between GWWA and BWWA.Location
North America.Methods
We used GWWA and BWWA occurrence data to model climatically suitable conditions under historical and future climate scenarios. Models were parameterized with 13 bioclimatic variables and 3 topographic variables. Using ensemble modeling, we estimated historical and modern distributions, as well as a projected distribution under six future climate scenarios. We quantified breeding distribution area, the position of and amount of overlap between GWWA and BWWA distributions under each climate scenario. We summarized the top explanatory variables in our model to predict environmental parameters of the distributions under future climate scenarios relative to historical climate.Results
GWWA and BWWA distributions are projected to substantially change under future climate scenarios. GWWA are projected to undergo the greatest change; the area of climatically suitable breeding season conditions is expected to shift north to northwest; and range contraction is predicted in five out of six future climate scenarios. Climatically suitable conditions for BWWA decreased in four of the six future climate scenarios, while the distribution is projected to shift east. A reduction in overlapping distributions for GWWA and BWWA is projected under all six future climate scenarios.Main Conclusions
Climate change is expected to substantially alter the area of climatically suitable conditions for GWWA and BWWA, with the southern portion of the current breeding ranges likely to become climatically unsuitable. However, interactions between BWWA and GWWA are expected to decline with the decrease in overlapping habitat, which may reduce the risk of genetic introgression. 相似文献33.
Petra Gevers Anthony J. M. Coenen Henk Schipper Henri W. J. Stroband Lucy P. M. Timmermans 《Development genes and evolution》1993,202(3):152-158
Summary The present report firstly describes a pilot study in which, during early development of embryos of the common carp, Cyprinus carpio, the cellular adhesion to fibronectin (FN) was blocked by administration of GRGDS peptide (which binds to the FN-receptor). As this treatment resulted in developmental aberrations, suggesting a functional role for FN, the major part of the work was focussed on the distribution of reactivity of anti-FN antibodies during epiboly and gastrulation. GRGDS treatment had a concentration dependent effect on development. Incubation of embryos in 1.5 mg/ml from the 32-cell stage onwards caused a retardation of epiboly, which did not proceed beyond 60%. The embryos did not show involution, as was confirmed by histological study. These preliminary results suggest that FN is involved in both epiboly and gastrulation of carp embryos. During cleavage, no specific extracellular binding of anti-FN antiserum could be observed. However, binding to a number of cell membranes took place from early epiboly onwards. After the onset of gastrulation, we observed a gradually increasing number of the deepest epiblast cells, showing immunostaining on part of their surface, facing the yolk syncytial layer (YSL) or the involuted cells. During early epiboly, anti-FN binding was restricted to areas in front of the migratory hypoblast cells. Later on, binding was found at the border of hypoblast and epiblast cells. At 100% epiboly, some contact areas of epiblast and hypoblast showed a discontinuous lining of reactivity, whilst other areas appeared devoid of anti-FN binding sites. The results indicate that FN is involved in the migration and guidance of hypoblast cells during gastrulation in carp.
Correspondence to: P. Gevers 相似文献
34.
35.
MICROBIAL COLONIZATION ON NATURAL AND ARTIFICIAL MACROPHYTES IN A PHOSPHORUS-LIMITED,HARDWATER LAKE1
Epiphyte communities in a phosphorus-limited hard-water lake were compared over a 14-week period from Potamogeton illinoensis and structurally similar artificial plants of different leaf ages. Artificial plants were serially incubated in the lake to simulate the age of natural leaves. The physiognomy of loosely attached epiphytes appeared similar on the two substrata. Algal cell number and biovolume were 15-fold and 17-fold higher, respectively, on artificial leaves early in the growing season, but total algal density gradually became similar on natural and artificial plants. In contrast, the taxonomic composition of loosely attached algae became increasingly distinct, and mean cell biovolume on natural leaves was twice that on artificial leaves. Adnate epiphytes on both substrata developed from sparse populations of bacteria on new leaves to a community of diatoms, blue-green algae and numerous bacteria on mature and senescent leaves. Adnate community succession on natural leaves in late senescence/death differed from that on artificial leaves colonized for comparable periods in having (1) a marked increase in filamentous blue-green algae, (2) a subsequent decrease in all algae, and (3) a final fungi-dominated stage. The trends in colonization indicate that macrophytes in this oligotrophic lake provided a distinct habitat from that of artificial substrata for epiphytes throughout the growing season. 相似文献
36.
Alexander Christmann Jacqueline Christmann Petra Schiller Burkhard Frenzel 《Trees - Structure and Function》1996,10(5):331-338
Levels of indole-3-acetic acid (IAA) were determined in needles from silver fir (Abies alba Mill.) trees in the northern Black Forest. IAA was quantified by gas chromatography (GC) as 1-heptafluorobutyryl-IAA-methylester
(HFB-IAA-ME) using electron capture detection. Prior to GC analysis, extensive purification of needle extracts was performed
employing two HPLC steps. Peak identity of HFB-IAA-ME was confirmed by combined gas chromatography-mass spectrometry in selected
samples. Levels of IAA in needles belonging to different needle age-classes exhibited a cyclic seasonal pattern with highest
concentrations in winter and lowest levels in spring when bud-break occurred. Such a cyclic seasonal pattern of IAA levels
was also observed in needles from declining fir trees or fir trees suffering from a strong sulfur impact (S-impact) in the
field due to a local SO2 source. Levels of IAA increased with increasing needle age. This age dependency of IAA concentrations was most pronounced
in late autumn when IAA levels were high and nearly disappeared in spring when IAA levels reached their minimum. In needles
from declining fir trees or fir trees suffering from a strong S-impact in the field, IAA levels hardly increased with increasing
needle age. It is suggested that in healthy trees high levels of IAA protect older needles from abscission and that the considerable
losses of older needles of declining fir trees or of fir trees under S-impact are a consequence of the low levels of IAA found
in older needles of such trees.
Received: 4 May 1995 / Accepted: 29 August 1995 相似文献
37.
Petra R. Moog Tom A. W. van der Kooij Wolfgang Brüggemann John W. Schiefelbein Pieter J. C. Kuiper 《Planta》1995,195(4):505-513
Arabidopsis thaliana (L.) Heynh. Columbia wild type and a root hair-less mutant RM57 were grown on iron-containing and iron-deficient nutrient solutions. In both genotypes, ferric chelate reductase (FCR) of intact roots was induced upon iron deficiency and followed a Michaelis-Menten kinetic with a K
m of 45 and 54 M FeIII-EDTA and a V
max of 42 and 33 nmol Fe2+·(g FW)–1·min–1 for the wild type and the mutant, respectively. The pH optimum for the reaction was around pH 5.5. The approximately four fold stimulation of FCR activity was independent of formation of root hairs and/or transfer cells induced by iron deficiency. Iron-deficiency-induced chlorosis and the development of a rigid root habit disappeared when ferric chelate was applied to the leaves, while FCR activity remained unchanged. The time course of the responses to iron deficiency showed that morphological and physiological responses were controlled separately.Abbreviations FCR
ferric chelate reductase
- FW
fresh weight
Thanks are due to Klaas Sjollema (Department of Electronmicroscopy, University of Groningen, The Netherlands) for help with the electron microscopy sample preparation and especially to Dr. Uwe Santore (Heinrich-Heine-University for electron microscopy. This work was supported by the SCIENCE programm of the European community; P.R.M.) and a Personal Research Grant by the Ministerium für Wissenschaft und Forschung of Nordrhein-Westfalen (P.R.M.) and last, not least by the productive discussions in ECOTRANS B.V. 相似文献
38.
Petra Düx Brian Whitehead Rolf Boelens Robert Kaptein Geerten W. Vuister 《Journal of biomolecular NMR》1997,10(3):301-306
A modified HNHB experiment is presented that allows thedetermination of J(NH) coupling constants directly from the ratio ofcross-peak to diagonal-peak intensities. The experiment was applied to thephotoactive yellow protein (PYP) and yielded the magnitude of 1173J(NH) coupling constants. In addition, 293J(NH(i–1)) coupling constantscould be measured, providing information about the backbone angle .These data, in conjunction with the magnitudes of the3J(HNH) coupling constantsobtained from the HNHA spectrum, effectively discriminate the twopossibilities for the stereospecific assignment of theH resonances in glycine residues. For all eight glycineresidues in PYP that were not subject to conformational averaging and hadnon-degenerate H resonance frequencies, the J-couplingdata, together with limited NOE data, yielded the stereospecific assignmentof the H resonances for these residues. In addition,reliable and precise , dihedral constraints were also derived forthese residues from the J-coupling data. 相似文献
39.
40.
Eight compounds exuded from young roots of black locust (Robinia pseudoacacia) were separated by two-dimensional HPTLC, by HPLC and GC, and were identified by spectroscopic methods (ultraviolet/visible
spectroscopy and mass spectrometry) as 4′,7-dihydroxyflavone, apigenin, naringenin, chrysoeriol and isoliquiritigenin. Structural
assignments were confirmed by comparison with authentic standards. The capacity to induce β-galactosidase activity in Rhizobium sp. NGR234 containing a nod box::lacZ fusion on plasmid pA27 identified these flavonoids and the chalcone as nod gene inducers. This indicates the important role of these compounds in nodulation of this legume tree.
Received: 26 July 1996 / Accepted: 9 September 1996 相似文献