全文获取类型
收费全文 | 562篇 |
免费 | 63篇 |
专业分类
625篇 |
出版年
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 12篇 |
2020年 | 10篇 |
2019年 | 16篇 |
2018年 | 16篇 |
2017年 | 19篇 |
2016年 | 15篇 |
2015年 | 32篇 |
2014年 | 25篇 |
2013年 | 40篇 |
2012年 | 46篇 |
2011年 | 23篇 |
2010年 | 27篇 |
2009年 | 28篇 |
2008年 | 22篇 |
2007年 | 32篇 |
2006年 | 22篇 |
2005年 | 20篇 |
2004年 | 24篇 |
2003年 | 16篇 |
2002年 | 27篇 |
2001年 | 13篇 |
2000年 | 21篇 |
1999年 | 10篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 10篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有625条查询结果,搜索用时 15 毫秒
601.
Food supply is a major source of variation in breeding success for predators, and to what extent individuals are able to cope with temporal variability in food availability remains an outstanding question in life-history studies. We confronted the natural variation in clutch size and breeding success with results from a food supplementation experiment during egg formation, conducted over several contrasted years of natural food supply in an avian specialist predator, the Montagu's harrier Circus pygargus . This raptor mainly preys on common vole Microtus arvalis a cyclic microtine under temperate latitudes. Vole abundance together with timing of breeding accounted for most of the variance in clutch size and number of fledglings. Results from empirical and experimental data were overall in agreement. Fed pairs consistently increased clutch size compared with controls in all experimental years, whereas no effect of food supplementation on egg volume was detected. Supplemented pairs, however, did not fledge significantly more chicks than controls. The costs entailed by the increase in clutch size appear nevertheless to be limited compared with previous studies. Food supply seemed therefore to display sufficient predictability throughout a breeding season to afford individuals the opportunity to adjust their breeding effort to an optimal number of offspring, in agreement with Lack's anticipation hypothesis. 相似文献
602.
Non‐congruent fossil and phylogenetic evidence on the evolution of climatic niche in the Gondwana genus Nothofagus
下载免费PDF全文
![点击此处可从《Journal of Biogeography》网站下载免费的PDF全文](/ch/ext_images/free.gif)
603.
The breeding systems, reproductive efficacies and population densities of 75 species of trees, shrubs, perennial herbs and
vines in a montane tropical cloud forest in Venezuela are investigated. 56.96% and 44.32% of the trees, versus the other life
forms considered, respectively possess obligate outbreeding mechanisms. Two shrubs are non-pseudogamous apomicts. The percentage
of dioecy among tree species (31%) is among the highest recorded in tropical forests. Reproductive efficacy is similar under
all breeding systems in the forest interior. Obligately outbred taxa are slower recolonizers of a disturbed border as compared
with non-obligate outbreeders. Explanations are advanced for the high incidence of dioecy combined with a low level of self-incompatibility
among hermaphroditic species. It is concluded that the breeding system spectrum reflects an unpredictable pollination pattern,
rather than insufficient pollinator servicing. 相似文献
604.
Elisa E. Figueroa-Angulo Francisco J. Rendón-Gandarilla Jonathan Puente-Rivera Jaeson S. Calla-Choque Rosa E. Cárdenas-Guerra Jaime Ortega-López Laura I. Quintas-Granados M. Elizbeth Alvarez-Sánchez Rossana Arroyo 《Microbes and infection / Institut Pasteur》2012,14(15):1411-1427
This review focused on potential regulatory mechanisms of Trichomonas vaginalis virulence properties, cytoadherence, cytotoxicity, phagocytosis, hemolysis, induction of apoptosis, and immune evasion in response to environmental factors of the human urogenital tract, iron, zinc, and polyamines. Understanding the multifactorial nature of trichomonal pathogenesis and its regulation may help to unravel the survival strategies of trichomonads and to implement prevention policies, opportune diagnosis, and alternative treatments for control of trichomoniasis. 相似文献
605.
606.
Frédéric Jiguet Paul Dufour Kevin J. Kardynal Keith A. Hobson José Luis Copete Jose Luis Arroyo Raymond W. Lee Hamid Rguibi-Idrissi Petr Procházka 《Ibis》2023,165(1):204-213
The Iberian and North African populations of reed warblers have been described recently as a separate taxon, ambiguus, forming a sister clade to the Sahelian subspecies minor of African Reed Warbler Acrocephalus baeticatus. Although the breeding range of ambiguus has been identified, the migration strategy of its populations remained unknown. We deployed geolocators and sampled the innermost primary from breeding adults in Spain for stable hydrogen (δ2H) analyses and also analysed stable carbon (δ13C) and nitrogen (δ15N) isotopes in feathers collected in two reed warbler taxa (Acrocephalus scirpaceus and Acrocephalus baeticatus ambiguus) in Morocco, to identify the moulting and wintering sites of these populations. Ring recoveries, geolocator tracks and probabilistic assignments to origin from δ2H values indicate that Spanish ambiguus are likely to moult south of the Sahara and winter in West Africa, probably from Mauretania to southern Mali and Ivory Coast. Moroccan ambiguus, however, undergo post-breeding moult north of the Sahara, and possibly then migrate to West Africa. With other populations of ambiguus breeding from Algeria to Libya and probably wintering further east in the Sahelian belt, the Barbary Reed Warbler can therefore be considered a trans-Saharan migrant, with a post-breeding moult strategy that varies between populations, and probably structured according to breeding latitude. 相似文献
607.
Placental morphology in two sympatric Andean lizards of the genus Liolaemus (Reptilia: Liolaemidae)
下载免费PDF全文
![点击此处可从《Journal of morphology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
César Aguilar Michael R. Stark Juan A. Arroyo Michael D. Standing Shary Rios Trevor Washburn Jack W. Sites Jr. 《Journal of morphology》2015,276(10):1205-1217
Viviparity is a remarkable feature in squamate sauropsids and it has evolved multiple times in parallel with the formation of a placenta. One example of this repeated evolution of viviparity and placentation occurs in the species‐rich South American genus Liolaemus with at least six independent origins of viviparity. However, evolutionary studies of placentation in this genus are limited by a lack of data on placental morphology. The aim of this study is to describe and compare the microanatomy and vessel diameter (Dv, a function of blood flow) of the placenta using scanning electron microscopy (SEM) and confocal laser scanning microscopy (cLSM) in two sympatric Andean viviparous but highly divergent species, Liolaemus robustus and Liolaemus walkeri. We found interspecific differences in cell types in the chorion, allantois, and omphalopleure that may be explained by divergent phylogenetic history. Time elapsed since divergence may also explain the pronounced interspecific differences in vessel diameter, and within each species, there are strong differences in Dv between tissue locations. Both species show features to improve gas exchange in the chorioallantoic placenta including absence of eggshell, large Dv in the allantois (L. robustus) or embryonic side of the uterus (L. walkeri), and when present, microvillous cells in the allantois (L. walkeri). Both species also show features that suggest transfer of nutrients or water in the omphaloplacenta, including an almost complete reduction of the eggshell, secretive material (L. robustus), or vesicles (L. walkeri) on cell surface uterus, and when present specialized cells in the omphalopleure (L. walkeri). No statistical differences in Dv were found among stages 32–39 in each species, suggesting that a different mechanism, other than enhanced blood flow, might satisfy the increased oxygen demand of the developing embryos in the hypoxic environments of the high Andes. J. Morphol. 276:1205–1217, 2015. © 2015 Wiley Periodicals, Inc. 相似文献
608.
The cytological effects have been studied of methotrexate administered in single doses on a hypertriploid ascites tumour in the mouse, which seems to be sensitive to this drug. The drug produced almost total stoppage of the mitotic index, which was maintained for 50 h after administration. The occurrence of fractures in both chromosomes and chromatids in metaphase was the most frequently observed anomaly, sometimes reaching an incidence of 100%. Chromosomal bridges were found in the anaphase in up to 60% of the cells. 相似文献
609.
P Shiyanov S Bagchi G Adami J Kokontis N Hay M Arroyo A Morozov P Raychaudhuri 《Molecular and cellular biology》1996,16(3):737-744
610.
Flowering-time, plant longevity and size of capitulum were studied in a common garden experiment on seven populations of annual Chaetanthera moenchioides derived from a strong latitudinal aridity gradient in the mediterranean climate area of Chile. Populations derived from the drier northern part of the latitudinal gradient showed significantly shorter flowering and fruiting phenology and smaller capitula under uniform growing conditions. Water stress experiments performed in the greenhouse induced further phenological hastening and a reduction in number of ray florets per capitulum. Population differentiation and the reaction norms in C. moenchioides agree with the predictions of genetic-assimilation given that the plastic response of the species under water stress mimics phenotypic differentiation that has evolved along the environmental gradient. 相似文献