全文获取类型
收费全文 | 768篇 |
免费 | 64篇 |
出版年
2023年 | 4篇 |
2022年 | 2篇 |
2021年 | 9篇 |
2020年 | 6篇 |
2019年 | 10篇 |
2018年 | 15篇 |
2017年 | 9篇 |
2016年 | 16篇 |
2015年 | 31篇 |
2014年 | 22篇 |
2013年 | 61篇 |
2012年 | 63篇 |
2011年 | 39篇 |
2010年 | 32篇 |
2009年 | 31篇 |
2008年 | 42篇 |
2007年 | 56篇 |
2006年 | 55篇 |
2005年 | 47篇 |
2004年 | 48篇 |
2003年 | 40篇 |
2002年 | 40篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 12篇 |
1998年 | 14篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 8篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 7篇 |
1975年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有832条查询结果,搜索用时 171 毫秒
91.
E. R. Jasper Wubs G. Arjen de Groot Heinjo J. During Johannes C. Vogel Michael Grundmann Piet Bremer Harald Schneider 《Annals of botany》2010,106(4):583-590
Background and Aims
Human-mediated environmental change is increasing selection pressure for the capacity in plants to colonize new areas. Habitat fragmentation combined with climate change, in general, forces species to colonize areas over longer distances. Mating systems and genetic load are important determinants of the establishment and long-term survival of new populations. Here, the mating system of Asplenium scolopendrium, a diploid homosporous fern species, is examined in relation to colonization processes.Methods
A common environment experiment was conducted with 13 pairs of sporophytes, each from a different site. Together they constitute at least nine distinct genotypes, representing an estimated approx. 95 % of the non-private intraspecific genetic variation in Europe. Sporophyte production was recorded for gametophytes derived from each parent sporophyte. Gametophytes were grown in vitro in three different ways: (I) in isolation, (II) with a gametophyte from a different sporophyte within the same site or (III) with a partner from a different site.Key Results
Sporophyte production was highest in among-site crosses (III), intermediate in within-site crosses (II) and was lowest in isolated gametophytes (I), strongly indicating inbreeding depression. However, intragametophytic selfing was observed in most of the genotypes tested (eight out of nine).Conclusions
The results imply a mixed mating system in A. scolopendrium, with outcrossing when possible and occasional selfing when needed. Occasional intragametophytic selfing facilitates the successful colonization of new sites from a single spore. The resulting sporophyte, which will be completely homozygous, will shed large amounts of spores over time. Each year this creates a bed of gametophytes in the vicinity of the parent. Any unrelated spore which arrives is then selectively favoured to reproduce and contribute its genes to the new population. Thus, while selfing facilitates initial colonization success, inbreeding depression promotes genetically diverse populations through outcrossing. The results provide further evidence against the overly simple dichotomous distinction of fern species as either selfing or outcrossing. 相似文献92.
Lankhaar JW Westerhof N Faes TJ Marques KM Marcus JT Postmus PE Vonk-Noordegraaf A 《American journal of physiology. Heart and circulatory physiology》2006,291(4):H1731-H1737
Right ventricular (RV) afterload is commonly defined as pulmonary vascular resistance, but this does not reflect the afterload to pulsatile flow. The purpose of this study was to quantify RV afterload more completely in patients with and without pulmonary hypertension (PH) using a three-element windkessel model. The model consists of peripheral resistance (R), pulmonary arterial compliance (C), and characteristic impedance (Z). Using pulmonary artery pressure from right-heart catheterization and pulmonary artery flow from MRI velocity quantification, we estimated the windkessel parameters in patients with chronic thromboembolic PH (CTEPH; n = 10) and idiopathic pulmonary arterial hypertension (IPAH; n = 9). Patients suspected of PH but in whom PH was not found served as controls (NONPH; n = 10). R and Z were significantly lower and C significantly higher in the NONPH group than in both the CTEPH and IPAH groups (P < 0.001). R and Z were significantly lower in the CTEPH group than in the IPAH group (P < 0.05). The parameters R and C of all patients obeyed the relationship C = 0.75/R (R(2) = 0.77), equivalent to a similar RC time in all patients. Mean pulmonary artery pressure P and C fitted well to C = 69.7/P (i.e., similar pressure dependence in all patients). Our results show that differences in RV afterload among groups with different forms of PH can be quantified with a windkessel model. Furthermore, the data suggest that the RC time and the elastic properties of the large pulmonary arteries remain unchanged in PH. 相似文献
93.
Bareetseng AS Kock JL Pohl CH Pretorius EE Strauss CJ Botes PJ Van Wyk PW Nigam S 《Systematic and applied microbiology》2006,29(6):446-449
The distribution of 3-hydroxy oxylipins in Saturnispora saitoi was mapped using immunofluorescence microscopy. Fluorescence was observed on aggregating ascospores, indicating the presence of 3-hydroxy oxylipins on the surface or between ascospores. The oxylipin was identified as 3-hydroxy 9:1 using gas chromatography mass spectrometry. Furthermore, ultrastructural studies using scanning and transmission electron microscopy on ascospores revealed a clear equatorial ledge surrounding oval-shaped ascospores. 相似文献
94.
95.
Gambaryan S Butt E Tas P Smolenski A Allolio B Walter U 《American journal of physiology. Endocrinology and metabolism》2006,290(3):E423-E433
Aldosterone production in zona glomerulosa (ZG) cells of adrenal glands is regulated by various extracellular stimuli (K(+), ANG II, ACTH) that all converge on two major intracellular signaling pathways: an increase in cAMP production and calcium (Ca(2+)) mobilization. However, molecular events downstream of the increase in intracellular cAMP and Ca(2+) content are controversial and far from being completely resolved. Here, we found that Ca(2+)/calmodulin-dependent protein kinases (CaMKs) play a predominant role in the regulation of aldosterone production stimulated by ANG II, ACTH, and cAMP. The specific CaMK inhibitor KN93 strongly reduced ANG II-, ACTH-, and cAMP-stimulated aldosterone production. In in vitro kinase assays and intact cells, we could show that cAMP-induced activation of CaMK, using the adenylate cyclase activator forskolin or the cAMP-analog Sp-5,6-DCI-cBIMPS (cBIMPS), was not mediated by PKA. Activation of the recently identified cAMP target protein Epac (exchange protein directly activated by cAMP) by 8-pCPT-2'-O-Me-cAMP had no effect on CaMK activity and aldosterone production. Furthermore, we provide evidence that cAMP effects in ZG cells do not involve Ca(2+) or MAPK signaling. Our results suggest that ZG cells, in addition to PKA and Epac/Rap proteins, contain other as yet unidentified cAMP mediator(s) involved in regulating CaMK activity and aldosterone secretion. 相似文献
96.
97.
Piet F. M. Verdonschot 《Hydrobiologia》2006,565(1):39-58
The Rhine ecosystem is highly influenced by anthropogenic stresses from pollution, intensive shipping and increased connectivity
with other large European rivers. Canalization of the Rhine resulted in a reduction of heterogeneity to two main biotopes:
sandy streambeds and riverbanks consisting of groyne stones. Both biotopes are heavily subjected to biological invasions,
affecting the rivers food web structure. The Ponto-Caspian amphipods, Chelicorophium curvispinum and Dikerogammarus villosus, have exerted the highest impact on this food web. The filterfeeding C. curvispinum dominated the Rhine food web on the stones in 1998, swamping the stone substrata with mud. However, in 2001 it decreased
in numbers, most likely due to top-down regulation caused by increased parasitic and predatory pressure of other more recently
invaded Ponto-Caspian species. D. villosus showed a fast population increase after its invasion and particularly influenced the macroinvertebrate community on the stones
by predaceous omnivory. This species seemed to have maintained its predatory level after its population established. Effects
of these mass invaders on the macroinvertebrate community of sandy streambeds in the Rhine are unclear. Here, low densities
of macroinvertebrates were observed with the Asiatic clam, Corbicula fluminea, as most abundant species. Stable isotope values of food webs from the stones and sand in 2001 were similar. Aquatic macrophytes
are nearly absent and the food web is fuelled by phytoplankton and particulate organic matter, originating from riparian vegetation
as indicated by similar δ13C values. Omnivores, filter-, deposit-, and detritus-feeders are the primary and secondary macroinvertebrate consumers and
function as keystone species in transferring energy to higher trophic levels. Invaders comprise 90% of the macroinvertebrate
numbers, and can be considered ecosystem engineers determining the functional diversity and food web structure of the Rhine
by either bottom-up or top-down regulation. 相似文献
98.
This short paper summarises the information developed in the EU funded research project STAR on autecology databases, metrics,
multimetrics and community approaches. For Europe the WFD implementation gave an important stimulus for the development of
ecology based assessment techniques. Along with the development of metrics and multimetrics indices taxalists and autecological
information were strongly improved. Recommendations are given to further develop ecological assessment in European streams
and rivers. 相似文献
99.
Despite the widespread application of periphytic diatoms to water quality assessment at a regional level, there is no standard
European sampling protocol or associated assessment metrics. Furthermore, relatively little is known about the uncertainty
in the results of such assessments. One of the objectives of the European project for the Standardisation of River Classifications
(STAR) is to improve and standardise diatom assessment methods. An extensive diatom ring test, together with an audit of the
project results, provided a better understanding and quantification of the uncertainty in quality assessment of running waters
using diatoms. The variation in multimetric analysis shows that the choice of site and substrate for sampling, the inter-operator
differences in diatom taxonomy and the counting techniques are the primary sources of uncertainty. To some extent, this variation
also reveals the robustness of specific metrics in relation to the sources of uncertainty. Of the three most common substrate
types tested (stone, macrophyte and sediment), macrophytes emerge as the most preferred substrate for diatom sampling when
performing multimetric water quality assessment. 相似文献
100.