全文获取类型
收费全文 | 6271篇 |
免费 | 530篇 |
国内免费 | 5篇 |
专业分类
6806篇 |
出版年
2023年 | 39篇 |
2022年 | 96篇 |
2021年 | 175篇 |
2020年 | 90篇 |
2019年 | 97篇 |
2018年 | 140篇 |
2017年 | 128篇 |
2016年 | 221篇 |
2015年 | 400篇 |
2014年 | 439篇 |
2013年 | 428篇 |
2012年 | 622篇 |
2011年 | 568篇 |
2010年 | 335篇 |
2009年 | 292篇 |
2008年 | 401篇 |
2007年 | 398篇 |
2006年 | 388篇 |
2005年 | 292篇 |
2004年 | 312篇 |
2003年 | 212篇 |
2002年 | 219篇 |
2001年 | 45篇 |
2000年 | 28篇 |
1999年 | 48篇 |
1998年 | 42篇 |
1997年 | 45篇 |
1996年 | 29篇 |
1995年 | 20篇 |
1994年 | 35篇 |
1993年 | 19篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1974年 | 4篇 |
1973年 | 6篇 |
1970年 | 4篇 |
1960年 | 3篇 |
排序方式: 共有6806条查询结果,搜索用时 0 毫秒
991.
992.
The perceptual and chemical bases of egg discrimination in communally nesting greater anis Crotophaga major
下载免费PDF全文
Mark E. Hauber Miri Dainson Daniel T. Baldassarre Marouf Hossain Mande Holford Christina Riehl 《Journal of avian biology》2018,49(8)
The eggshells of communally breeding greater anis Crotophaga major consist of a blue‐green pigmented calcite matrix overlaid by a chalky white layer of vaterite, both of which are polymorphs of calcium carbonate. The white vaterite layer is intact in freshly laid eggs and may function in protecting the eggs from mechanical damage, but it also abrades during incubation to reveal the blue calcite shell underneath. Previous research has shown that this color change serves a visual signaling function: nesting greater anis can discriminate between eggs that are freshly laid and those that have already been incubated, which allows them to reject asynchronous eggs laid by extra‐group parasites. Here we use avian visual modeling and pigment extraction to assess the perceptual and chemical bases of such egg recognition. We found that there was no overlap between the avian perceptual space occupied by ani eggshells with and without vaterite, and that vaterite lacked both of the pigments found in the eggshell's calcite matrix, bililverdin and protoporphyrin. The visual contrast between the unpigmented vaterite and the blue‐pigmented calcite appears to pre‐date the evolution of the signaling function, since the related guira cuckoo Guira guira, also a communal breeder, lays similarly structured and pigmented eggs but does not use the visual contrast as a signal to detect parasitism. 相似文献
993.
Marina Mikhaylova Julia Bär Bas van Bommel Philipp Schätzle PingAn YuanXiang Rajeev Raman Johannes Hradsky Anja Konietzny Egor Y. Loktionov Pasham Parameshwar Reddy Jeffrey Lopez-Rojas Christina Spilker Oliver Kobler Syed Ahsan Raza Oliver Stork Casper C. Hoogenraad Michael R. Kreutz 《Neuron》2018,97(5):1110-1125.e14
994.
995.
996.
Christina M. Laukaitis Elizabeth S. Critser Robert C. Karn 《Evolution; international journal of organic evolution》1997,51(6):2000-2005
We wanted to determine whether the microevolution of the mouse salivary androgen-binding protein (ABP) Alpha subunit gene (Abpa) could mediate sexual selection and thereby have a potential role in maintaining gene pool integrity where radiating mouse subspecies make secondary contact. This hypothesis is based upon previous work in this laboratory, which has shown that each subspecies apparently has its own allele and that these alleles have a 25-fold excess of nonsynonymous/synonymous base substitutions compared to an average protein under purifying selection. We provide direct evidence for ABP-assortative mate selection in a laboratory setting: Mus musculus domesticus and M. m. musculus female mice recognize and discriminate between the territories of male mice that essentially differ solely in their Abpa genotype and, when the males are present, the female prefers to mate with the one of her own ABP type. The observation that females could differentiate between the territories of the two males when those mice were absent suggests that the males marked their territories with ABP. In this study, we also detected ABP on the pelts of male mice and in their environment. It is likely that the animals apply the protein to their pelts by licking and that it is then deposited in their surroundings. We suggest that females of the two subspecies are able to discriminate between males of those subspecies on the basis of this protein molecule. Mouse salivary ABP might present a worthwhile system with which to study a prezygotic isolation mechanism in a mammal. 相似文献
997.
A molecular comparison of culturable aerobic heterotrophic bacteria and 16S rDNA clones derived from a deep subsurface sediment 总被引:9,自引:0,他引:9
998.
Karin M. Jger Christina Johansson Ulrike Kunz H. Lehmann 《Plant biology (Stuttgart, Germany)》1997,110(2):151-157
Element analysis using electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS) was performed in a symbiotic Nostoc sp. strain found in the upper stem tissue of Gunnera manicata, and in Nostoc PCC 9229, a free-living heterocyst-forming cyanobacterium able to enter into symbiosis with the angiosperm Gunnera in reconstitution experiments. ESI and EELS unequivocally identified the four elements nitrogen (N), sulphur (S), phosphorus (P) and oxygen (O) in different inclusion bodies of these biological specimens. High amounts of nitrogen were solely detected in huge cyanophycin granules in vegetative cells of the symbiotic Nostoc strain, whereas large polyphosphate bodies, containing high amounts of phosphorus, sulphur and oxygen, could be seen in the free-living Nostoc PCC 9229. The latter were usually not present or, when found, very small in vegetative cells of the cyanobiont. 相似文献
999.
Produce Isolates of the Escherichia coli Ont:H52 Serotype That Carry both Shiga Toxin 1 and Stable Toxin Genes 下载免费PDF全文
Steven R. Monday Christina Keys Patricia Hanson Yuelian Shen Thomas S. Whittam Peter Feng 《Applied microbiology》2006,72(4):3062-3065
Produce isolates of the Escherichia coli Ont:H52 serotype carried Shiga toxin 1 and stable toxin genes but only expressed Stx1. These strains had pulsed-field gel electrophoresis profiles that were 90% homologous to clinical Ont:H52 strains that had identical phenotypes and genotypes. All Ont:H52 strains had identical single nucleotide polymorphism profiles that are suggestive of a unique clonal group. 相似文献
1000.
Storage behavior of Typha latifolia pollen at low water contents: Interpretation on the basis of water activity and glass concepts 总被引:5,自引:0,他引:5
Julia Buitink Christina Walters Folkert A. Hoekstra Jennifer Crane 《Physiologia plantarum》1998,103(2):145-153
Germplasm must be stored under optimal conditions to maximize longevity and efficiently maintain genetic resources. In order to identify optimal storage conditions, we investigated the effects of temperature (−5 to 45°C) and water content (<0.17 g H2 O g−1 dry weight) on longevity of Typha latifolia L. pollen. Longevity was highest at water contents corresponding to storage relative humidity (RH) of 11‐15% which corresponded to the shoulder of water sorption isotherms. Also coinciding with this shoulder were abrupt changes in heat capacity of water present in the pollen. Consistent with changes in isotherms with temperature and the concept of critical RH for storage, optimum water contents increased with decreasing temperature. An attempt was made to explain the aging behavior according to the glass concept. The water content‐temperature combinations of optimal storage were found to be below the glass transition curve, indicating that optimum storage conditions are achieved when intracellular glasses are present. We also found a change in activation energy of aging in Arrhenius plots around Tg , demonstrating a change in aging kinetics when the glassy state is lost. We concluL that Tg curves cannot be used solely to predict precise conditions of optimum storage, but might be useful for predictions of storage longevity above optimum water contents. The data imply that too much drying reduces longevity and should be avoided, particularly when cryogenic storage is considered. 相似文献