全文获取类型
收费全文 | 24447篇 |
免费 | 53篇 |
国内免费 | 71篇 |
专业分类
系统科学 | 135篇 |
丛书文集 | 205篇 |
教育与普及 | 71篇 |
理论与方法论 | 140篇 |
现状及发展 | 9849篇 |
研究方法 | 1100篇 |
综合类 | 12541篇 |
自然研究 | 530篇 |
出版年
2013年 | 177篇 |
2012年 | 391篇 |
2011年 | 940篇 |
2010年 | 163篇 |
2008年 | 464篇 |
2007年 | 475篇 |
2006年 | 489篇 |
2005年 | 505篇 |
2004年 | 541篇 |
2003年 | 459篇 |
2002年 | 435篇 |
2001年 | 675篇 |
2000年 | 677篇 |
1999年 | 470篇 |
1992年 | 428篇 |
1991年 | 319篇 |
1990年 | 349篇 |
1989年 | 358篇 |
1988年 | 364篇 |
1987年 | 374篇 |
1986年 | 341篇 |
1985年 | 489篇 |
1984年 | 372篇 |
1983年 | 277篇 |
1982年 | 257篇 |
1981年 | 251篇 |
1980年 | 294篇 |
1979年 | 695篇 |
1978年 | 524篇 |
1977年 | 513篇 |
1976年 | 477篇 |
1975年 | 515篇 |
1974年 | 637篇 |
1973年 | 591篇 |
1972年 | 566篇 |
1971年 | 713篇 |
1970年 | 951篇 |
1969年 | 740篇 |
1968年 | 718篇 |
1967年 | 701篇 |
1966年 | 686篇 |
1965年 | 474篇 |
1964年 | 175篇 |
1959年 | 267篇 |
1958年 | 480篇 |
1957年 | 302篇 |
1956年 | 294篇 |
1955年 | 239篇 |
1954年 | 258篇 |
1948年 | 242篇 |
排序方式: 共有10000条查询结果,搜索用时 738 毫秒
251.
252.
Systemic Practice and Action Research - This paper explores the usefulness of rich pictures as a method in Systemic Lean Intervention (SLI) process. It combines Lean and Systems Thinking analytical... 相似文献
253.
254.
D. J. Wildish S. R. Smith T. Loeza-Quintana A. E. Radulovici S. J. Adamowicz 《Journal of Natural History》2016,50(29-30):1911-1933
Five taxa of talitrid amphipods were found in the archipelago of Bermuda, of which three were recorded there for the first time. Four of these are supralittoral wrack generalists: Platorchestia monodi BOLD:AAB3402, (a unique Molecular Operational Taxonomic Unit according to the Barcode Index Number system), a related species recognized by molecular methods, Platorchestia platensis BOLD:AAA2949, Mexorchestia carpenteri carpenteri BOLD:AAC1491 and Tethorchestia antillensis; and one a terrestrial leaf-litter generalist: Talitroides alluaudi. A key is provided to discriminate between the formally described talitrids of Bermuda. Dispersal mechanisms from the American continent to Bermuda were considered for all taxa based on species distributions along the North American Atlantic coast and also investigated by molecular methods, using genetic population differentiation and haplotype network analysis based on the barcode region of cytochrome c oxidase subunit I gene. For P. monodi BOLD:AAB3402 the genetic results suggest that some dispersal events occurred before human colonization of Bermuda but are equivocal about the source population and therefore the direction of dispersal. Some very recent synanthropic dispersal is possible with this species. For the other two species studied genetically, P. platensis BOLD:AAA2949 and M. c. carpenteri BOLD:AAC1491, the small population samples analysed support dispersal to Bermuda from the American mainland, before human occupation of Bermuda, although the available sample size was limited for these species. The available limited direct, non-genetic evidence supports synanthropic transport for Talitroides alluaudi. Platorchestia monodi BOLD:AAB3402 is found in the same wrack habitat as P. platensis BOLD:AAA2949 on Bermuda, apparently without interbreeding. No evidence was found that driftwood specialist talitrids had become established in Bermuda. 相似文献
255.
256.
The positive impacts of managing projects as a portfolio are quantified by comparing the value of the integrated risk of a project portfolio and the aggregation of single project risks implemented separately. Firstly, the integrated risk is defined by proposing risky events based on set theory. Secondly, as projects interact with each other in a project portfolio, the integrated risk is evaluated by using a Bayesian network structure learning algorithm to construct an interdependent network of risks. Finally, the integrated risk of a practical case is assessed using this method, and the results show that the proposed method is an effective tool for calculating the extent of risk reduction of implementing a project portfolio and identifying the most risky project, so as to assist companies in making comprehensive decisions in the phase of portfolio selection and portfolio controlling. 相似文献
257.
Activation of stress signalling pathways enhances tolerance of fungi to chemical fungicides and antifungal proteins 总被引:1,自引:0,他引:1
Brigitte M. E. Hayes Marilyn A. Anderson Ana Traven Nicole L. van der Weerden Mark R. Bleackley 《Cellular and molecular life sciences : CMLS》2014,71(14):2651-2666
Fungal disease is an increasing problem in both agriculture and human health. Treatment of human fungal disease involves the use of chemical fungicides, which generally target the integrity of the fungal plasma membrane or cell wall. Chemical fungicides used for the treatment of plant disease, have more diverse mechanisms of action including inhibition of sterol biosynthesis, microtubule assembly and the mitochondrial respiratory chain. However, these treatments have limitations, including toxicity and the emergence of resistance. This has led to increased interest in the use of antimicrobial peptides for the treatment of fungal disease in both plants and humans. Antimicrobial peptides are a diverse group of molecules with differing mechanisms of action, many of which remain poorly understood. Furthermore, it is becoming increasingly apparent that stress response pathways are involved in the tolerance of fungi to both chemical fungicides and antimicrobial peptides. These signalling pathways such as the cell wall integrity and high-osmolarity glycerol pathway are triggered by stimuli, such as cell wall instability, changes in osmolarity and production of reactive oxygen species. Here we review stress signalling induced by treatment of fungi with chemical fungicides and antifungal peptides. Study of these pathways gives insight into how these molecules exert their antifungal effect and also into the mechanisms used by fungi to tolerate sub-lethal treatment by these molecules. Inactivation of stress response pathways represents a potential method of increasing the efficacy of antifungal molecules. 相似文献
258.
259.
Paloma Feyjoo Miguel Cabanellas-Reboredo Matías Calvo-Manazza Beatriz Morales-Nín Jorge Hernández-Urcera Manuel E. Garci 《Journal of Natural History》2016,50(9-10):543-555
The embryonic development of the squid Loligo vulgaris was observed from 183 egg masses collected from special devices deployed throughout Cabrera National Park (Baleares Islands, western Mediterranean Sea). Sequence alignment analysis of the cytochrome oxidase I gene revealed that all embryos belonged to L. vulgaris. In total, 549 egg capsules were examined. Viable egg capsules (n = 420) were classified into one of five maturation stages according to the primary external features. The length of the viable egg capsules varied between 40 and 170 mm, and increased with embryonic development. The non-viable capsules (n = 129) were categorized into four groups: I (Ginger root), non-viable II and III, and empty egg capsule (IV). The percentage of non-viable capsules (i.e. grades I, II and III) was 92.25%. Empty capsules accounted for 7.75% of the total non-viable egg capsules. Embryonic development was classified into a second scale of eight stages. Egg capsule stage and embryonic stage were significantly related (n = 420; p < 0.001), facilitating the determination of the embryo developmental phase based on the outward appearance of the egg capsules. The embryo development stage based on the external features of the egg capsules might constitute an innovative tool for in situ embryological data collection. This new method is neither time consuming nor invasive, and could be helpful in fishing cruises, for scuba diving visual census in natural habitats and for laboratory culture. Slight variability in the developmental embryonic stages within egg capsules from the same egg mass was identified. The origin of this asynchrony is discussed. Chronological appearance of organs was similar to that of the six loliginid species previously examined. However, some developmental changes in the timing or rate of events (heterochronies) were observed: Hoyle’s organ was formed earlier in L. vulgaris and the appearance of ventral chromatophores was slightly delayed (2 days) compared with the other species considered. 相似文献
260.
Gustavo E. Romero 《Foundations of Science》2016,21(3):455-460
I offer an analysis of the Principle of Sufficient Reason and its relevancy for the scientific endeavour. I submit that the world is not, and cannot be, rational—only some brained beings are. The Principle of Sufficient Reason is not a necessary truth nor a physical law. It is just a guiding metanomological hypothesis justified a posteriori by its success in helping us to unveil the mechanisms that operate in Nature. 相似文献