全文获取类型
收费全文 | 2807篇 |
免费 | 277篇 |
国内免费 | 1篇 |
出版年
2023年 | 15篇 |
2022年 | 39篇 |
2021年 | 50篇 |
2020年 | 25篇 |
2019年 | 40篇 |
2018年 | 60篇 |
2017年 | 50篇 |
2016年 | 77篇 |
2015年 | 128篇 |
2014年 | 140篇 |
2013年 | 176篇 |
2012年 | 198篇 |
2011年 | 161篇 |
2010年 | 131篇 |
2009年 | 108篇 |
2008年 | 147篇 |
2007年 | 165篇 |
2006年 | 136篇 |
2005年 | 157篇 |
2004年 | 141篇 |
2003年 | 136篇 |
2002年 | 119篇 |
2001年 | 27篇 |
2000年 | 21篇 |
1999年 | 34篇 |
1998年 | 45篇 |
1997年 | 37篇 |
1996年 | 27篇 |
1995年 | 34篇 |
1994年 | 25篇 |
1993年 | 29篇 |
1992年 | 25篇 |
1991年 | 17篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 21篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 14篇 |
1984年 | 16篇 |
1983年 | 17篇 |
1982年 | 22篇 |
1981年 | 14篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1978年 | 18篇 |
1976年 | 18篇 |
1974年 | 12篇 |
1973年 | 17篇 |
1967年 | 10篇 |
排序方式: 共有3085条查询结果,搜索用时 15 毫秒
101.
This investigation was designed to explore the relationships between lichen symbionts (phycobiont and mycobiont) and the substrate on which they grow by examining the chemical and ultrastructural features of the lichen-soil interface. These lichens form an integral part of microbiotic soil crusts. Fragments of three different lichen biotypes growing over gypsum crystals and marls were fixed and embedded in resin. The lichen-substratum interface was then examined by scanning electron microscopy with backscattered electron imaging. In situ observation, microanalytical (EDS), and FT-Raman plus infrared spectroscopy of the lichen-substratum interface indicated that different ultrastructural features of the mycobiont were related to biogeochemical processes and Ca 2+ distribution in the soil crust. Phycobionts were observed to make direct contact with the substratum and to be surrounded by a nondifferentiated thallus structure. These observations suggest that they can grow outside the thallus in the early stages of lichen development in the semi-arid conditions of their habitat. The particular ultrastructural features of the lichen thallus and of the lichen-substratum interface appear to have marked effects on runoff phenomena and ponding generation of the surface. 相似文献
102.
Virginia Souza-Egipsy Angeles Aguilera Eva Mateo-Martí José Angel Martín-Gago Ricardo Amils 《Geomicrobiology journal》2013,30(8):692-706
This study examines fossil microorganisms found in iron-rich deposits in an extreme acidic environment, the Tinto River in SW Spain. Both electron microscopy (SEM and TEM) and non-destructive in situ microanalytical techniques (EDS, EMP and XPS) were used to determine the role of permineralization and encrustation in preserving microorganisms forming biofilms in the sediments. Unicellular algae were preserved by silica permineralization of their cell walls. Bacterial biofilms were preserved as molds by epicellular deposition of schwertmannite around them. In the case of fungi and filamentous algae, we observed permineralization of cell structures by schwertmannite in the sediments. The extracellular polymeric matrix around the cells was also preserved through permineralization of the fibrillar component. The process of permineralization and deposition of iron-rich precipitates present in the acidic waters of Rio Tinto served to preserve many microfossils in an oxidizing environment, in which organic compounds would not normally be expected to persist. Studies of microbial fossil formation mechanisms in modern extreme environments should focus on defining criteria to identify inorganic traces of microbial life in past environments on Earth or other planets. 相似文献
103.
104.
105.
Adel Shalata Maria C. Ramirez Robert J. Desnick Nolan Priedigkeit Christoph Buettner Claudia Lindtner Mohammed Mahroum Muhammad Abdul-Ghani Feng Dong Nazik Arar Olga Camacho-Vanegas Rui Zhang Sandra C. Camacho Ying Chen Mwafaq Ibdah Ralph DeFronzo Virginia Gillespie Kevin Kelley Brian D. Dynlacht Sehyun Kim Marc J. Glucksman Zvi U. Borochowitz John A. Martignetti 《American journal of human genetics》2013
106.
107.
Kate J. Howell Lucy A. Weinert Shi-Lu Luan Sarah E. Peters Roy R. Chaudhuri David Harris ?ystein Angen Virginia Aragon Julian Parkhill Paul R. Langford Andrew N. Rycroft Brendan W. Wren Alexander W. Tucker Duncan J. Maskell 《Journal of bacteriology》2013,195(18):4264-4273
Haemophilus parasuis is the causative agent of Glässer''s disease, a systemic disease of pigs, and is also associated with pneumonia. H. parasuis can be classified into 15 different serovars. Here we report, from the 15 serotyping reference strains, the DNA sequences of the loci containing genes for the biosynthesis of the group 1 capsular polysaccharides, which are potential virulence factors of this bacterium. We contend that these loci contain genes for polysaccharide capsule structures, and not a lipopolysaccharide O antigen, supported by the fact that they contain genes such as wza, wzb, and wzc, which are associated with the export of polysaccharide capsules in the current capsule classification system. A conserved region at the 3′ end of the locus, containing the wza, ptp, wzs, and iscR genes, is consistent with the characteristic export region 1 of the model group 1 capsule locus. A potential serovar-specific region (region 2) has been found by comparing the predicted coding sequences (CDSs) in all 15 loci for synteny and homology. The region is unique to each reference strain with the exception of those in serovars 5 and 12, which are identical in terms of gene content. The identification and characterization of this locus among the 15 serovars is the first step in understanding the genetic, molecular, and structural bases of serovar specificity in this poorly studied but important pathogen and opens up the possibility of developing an improved molecular serotyping system, which would greatly assist diagnosis and control of Glässer''s disease. 相似文献
108.
109.
110.
Virginia Sarropoulou Kortessa Dimassi-Theriou Ioannis Therios 《Plant biotechnology reports》2013,7(4):457-465
The effects of indole-3-butyric acid (IBA) alone and in combination with l-arginine on the morphogenic and biochemical responses in shoot tip explants of the cherry rootstock M × M 14 (Prunus avium × Prunus mahaleb) were examined. The maximum root number per rooted explant (16), root fresh (FW) and dry (DW) weights, as well as the rooting percentage (100 %) were recorded when 2 mg l?1 IBA (alone) were applied. Including the lowest IBA concentration (0.5 mg l?1) with the lowest and highest l-arginine concentrations (0.5 and 2 mg l?1, respectively) resulted in the greatest root length. The maximum leaf chlorophyll concentration and shoot length of the initial explant were recorded when 0.5 mg l?1 IBA plus 2 mg l?1 l-arginine were applied. In addition, l-arginine in combination with IBA (1 and 2 mg l?1) was found to suppress shoot FW and DW. On the other hand, l-arginine enhanced the promoting effect of IBA on both root length and leaf chlorophyll concentration. The carbohydrate and proline concentrations in leaves were not significantly altered with the application of IBA alone or in combination with l-arginine. On the other hand, the carbohydrate and proline concentrations in roots were decreased with the application of 1 and 2 mg l?1 IBA with l-arginine, resulting in the suppression of the promoting effects of IBA. It is clear from the findings that l-arginine has a direct effect on the in vitro rooting of M × M 14 explants, is involved in the function of the photosythetic apparatus, influences leaf chlorophyll content, participates in carbohydrate biosynthesis and metabolism, and is involved in proline accumulation both in leaves and roots. 相似文献